Electronic Repair Adventure- The Signal Tracer!

hi there and welcome to another episode of mr. Carlson's lab today we're going to troubleshoot repair and modify a signal tracer from way back when signal tracers are great tools to have on the bench and in many cases they can even replace an oscilloscope in many troubleshooting procedures oscilloscopes allow you to see what's going on inside the circuitry signal tracers allow you to hear what's going on inside the circuitry so it's just another device on the bench to bring you closer to success in your next repair or restoration now signal tracers like what I'm going to show you today can be had for really really cheap at local auctions or swap meets and usually you know because of their age they're in some state of disrepair so after some quick repairs usually they're ready to go now in this video we're gonna go a little bit further than that the line cord is cut off this thing and the case is looking pretty bad so chances are I'm gonna do a pretty extensive repair on this and as we encounter problems inside this we're gonna repair them and if the design needs any modification we're gonna modify it as well and I'll explain exactly my thought process as I'm going through these modifications and repairs so hopefully these modifications and repairs will pass on some information to you to help you in your next repair or restoration so we have a lot of territory to cover in this video and I'm gonna share some hints and tips with you about vacuum tubes and some other things along the way through the video so there's a lot of really neat stuff coming in this video so let's get started here you are sitting right beside me at the test bench so let's take a look at what we're up against today this is a stark signal tracer and it looks to be in pretty nice condition there is a probe that's missing on the coax you can see the exposed center conductor here aside from maybe the paint that's been ruined by somebody using a permanent marker and a few screws that are missing it looks to be pretty nice now one word of advice if you're ever going to try and sell something like this at a swap meet or online or anything like that don't write the price onto the paint with a permanent marker because when they say permanent it does not come off it goes directly into the paint somebody is taking the time to try and take the price off of here that looks like they've polished it or they've done something maybe used acetone or lacquer thinner and what that's done is that's taken the texture right off of the paint itself so we have kind of a like a a flat look over here and you can still see the price on the paint that's permanent that will never come off you'll have to strip the paint to get that off and this is what I'm going to end up doing with this this is not going to be a shelf Queen this is going to be a piece of test equipment that we are going to actively use together to troubleshoot other pieces of test equipment radios transmitters whatever we're going to be troubleshooting at the time on the bench so no shelf Queens this is going to be an active piece of test gear and I want it to fit into the theme of the old-time workbench so what I'm going to do is get rid of this ugly beige color and I'm gonna most likely paint this like a hammer tone black or something like that that would fit really nice into that old-time workbench so you'll notice a lot of older pieces of test gear have this somewhat a Bayesian nicotine e-type colored paint on it and the reason that they did that way back when is because it was really quite common to smoke in the lab everybody was smoking way back in the day so by using a beige colored paint on the case like this when they did get stained it kind of just blended in and there was a lot less cleaning hewlett packard used a color that was very very close to this very very close to what the stark is used here on its case as well so it really is an ugly color so I'm kind of looking forward to getting rid of this and the black hammer tone will work better with this white colored face and you know the black knobs and everything like that so kind of be in line with everything else here probably end up painting this grille here by the speaker as well because this is the same color as this paint other than that it's looking pretty good so I don't again I don't know what's inside this there could be a brick sitting inside this thing as I mentioned on all the other videos as we go through this and repair this I'm discovering the problems the same time you're seeing them so we're repairing this in real time now there is hardly any screws in the case there's one screw back here there's one missing here and you know the line cord is trimmed off the backside as well so it's obviously in some state of disrepair maybe they trim the line cord off for a reason who really knows so what I'll do is I'll turn this thing around we'll take a look at the backside and then what I'm going to do is remove this from the case and we'll take a look at the circuitry inside take a look at the design that Stark put together and talk about that a little bit here's the back side of the unit and as you can see it's in pretty nice condition you can see a screw missing here and there's also a screw missing on the side here and on this side so it really looks like the only screws holding this together are these three right here nice little sticker here it says special inspection series t approved by the hydroelectric power Commission of Ontario line cord has been trimmed back here so why it's been trimmed we'll find out here very soon so other than that now that's pretty much ready to be taken apart so let's do that right now take these three screws out see what happens they are pretty loose I can tell you that I'm not sure is loose Wow ready to fall in Wow look at that inside that sure is clean out of the way a little bit better light on the inside there yeah it's really nice and clean in there looks like it may have I can get the light on that see that from this angle here know what I'm going to do is I'll just take this out of the case now so it looks like this is just gonna come right out like that so I'll just leave this here like so just for a second we'll take a look at this and then what I'll do is I'll just get this off the bench you can have to work around the camera here so it looks like everything in here is intact look at how clean that is just dust on the chassis looks like on brand-new unit I really wonder how much time is on this thing so what I'll do is I'll pull this out of the casing get the the actual case out of the way and I'll move the camera around and we'll take a better look at this let's take a look at the design of this stark st2 and see what we can learn from what they've done now I can tell you this already I already see a bunch of flaws on the upper portion of this chassis design flaws really basic stuff too so if you want to guess at what the design flaws are what I'm gonna do is move the chassis around slowly and then when I'm done moving the chassis around you can pause the video and take a guess at what you think the problems are and then when you unpause the video I'll tell you what they are and we'll do the same thing for the bottom portion of the chassis so you want to get your arrow ready to pause the video so here we go I'll move the chassis around and you can take a guess at some of the design issues so you're gonna want to get ready to pause the video now if you want to take a guess at what the issues are on the upper portion of the chassis these are the issues with the upper portion of the chassis in the Stark St to signal tracer so the first thing that tells me that this thing is going to hum all the time so if I was to pull the tubes out and everything and just plug the unit in this thing is going to hum why is that well if we look at the audio transformer look at how close this audio transformer is to the power transformer and they are both situated the same way these two are going to couple the magnetic field from this power transformer is going to couple into that audio transformer and it's just going to make this speaker hum all the time this is really basic stuff and I don't know why stark did that this transformer should be out of the way of this transformer and it should be mounted 90 degrees so to stop that humming or to at least bring it down to a very minimal level the more you load the B+ or the filament winding on this of course that field is going to get stronger than what it's going to do is create more hum in that transformer the field is going to interfere with this transformer and it's going to impose hum into the speaker so really the only way to get rid of that hum at all in this design is going to be to shut this off now you're saying well how do you know this like this got some shielding on it and everything like that well when you work with audio design for a long period of time you get to know that yeah they put the shielding on this transformer to probably minimize that effect due to a bad design costs a lot of money to put these caps on transformers so this looks like a patch so they probably stamped the chassis he's put the whole thing together figured it was going to work and then realized oh there's an incredible amount of hum so they had to order transformers with caps or get caps put on them it's the reason they've done this no matter how much capping you put on a transformer like this when this transformer is open like this and pretty much almost touching it like look at the Clarence they're you know very very small amount of Clarence right there you know it's just going to hum in couple so we're gonna try this out here in just a little bit and we'll slowly go through this thing and repair this together and bring it back to life and see if we can minimize that at all really the only way to fix this design is to take this transformer and move it somewhere else on the chassis in order to get rid of that just basically I guess you could call it just static hum it's just gonna sit there and Hum for no reason now another problem on the upper portion of the chassis is the placement of this audio power tube again were in the area of the field of this transformer and it's a I can basically just stick a finger through there so what this is telling me now is that the grid the control grid of this tube might pick up that stray magnetic field in the audio output tube alone may introduce hum into this circuit so what we're gonna do is we're gonna pull this audio tube out and we'll see if that you know reduces any of the hum at all and we'll put the audio tube back in and you don't see the differences I can tell you already that this is just way too close this needs to be over here this can be closer to the front so you see this is up here but there's a lot of room underneath so this could be closer to the front that's how it should have been it gets the amplification chain closer to the cable where it's coming in and to the VR there's no need for it to be this far back so the closer you get to the front the tighter things are and then they could have taken this tube and placed it somewhere else in here maybe you move this transformer over to this area would have been the better idea they could shove all this kind of stuff over you know it's just it's silly this can could have been put right here keep the power supply on the other side of the chassis away from you know the whole amplification chain so there are a whole bunch of design errors in this thing we're still going to restore the thing and you know see how useful we can make this little signal tracer but if you wanted to take one of these things and completely make it quiet honestly what I would do is put an umbilical cord on the back of this thing and put the power supply in a separate box that would alleviate all the problems in this design so just get this thing out of here is it's just it's too close to everything like look at it's even this close to the speaker itself you know so like you mean that field this whole thing is right within the field here and you know we have a coil inside here as well there's just so many things you look at the top side and you see these problems just everywhere so this tells me that either a young designer put this thing together at Starke or somebody was just in a rush and really didn't care it's hard to say but you know the amount of flaws in this design are are really kind of over the top it looks really nice I have to admit it like the way they've laid it out and everything it looks nice nice and compact and everything but looking nice and working properly are two incredibly different things in the electronics world so let's take a look at the bottom portion of the chassis now and see what we could do to make that a little bit better and spot some of the problems there as well here's the bottom portion of the chassis and let's take a look around on the bottom portion and see what we can spot so again you can take a look at this just like this and you can get ready to pause the video I'll move this around just a little bit and then you can take a guess at what some of the issues are on the bottom side so that you know when to pause the video here so I take a look at it now a quick little glance at it the way it is this way here I'll grab this thing a little bit closer and I'll start moving it around again you can pause this at any time if you want to study something a little bit closer I think that's just about all angles there so I'll put this back down if you want to pause the video and take a guess at some of the issues on the bottom side do that now here are some of the issues on the bottom portion of the chassis of this stark SD to signal tracer so the very first thing that caught my attention was these two connections right here you take a look at those connections you can see that there's absolutely no solder on them so the wires have been wrapped but the connections have not been soldered whatsoever now keep in mind that this thing went through a special inspection remember that little sticker on the back side of the unit that's an very interesting special inspection so if there is any current demand on this outlet this would be very very noisy these would turn into a very very noisy connection so that definitely has to be fixed the next thing I noticed is they're using half wave rectification so we can see we have the two red leads which are the B+ leads one runs right to the chassis one goes through the diode and so we get DC at this point here and then there's no filter here they run it through 220 ohm resistor most likely to drop the voltage and then that runs into this capacitor right here so there's three filter caps in this can yet to discover where they go and what they do in fact I'll look for a schematic and when I find a schematic we'll take a look at that as well so they should have a filter here alone that's gonna make some more hum so well experiment with this design here in a little bit in you'll see what adding some filters and stuff does to this thing alright so we'll go through this piece by piece and try and improve their design to a point really what should be done is you know everything should be moved around and the things should be completely redesigned but at that point it's really not a stark st2 anymore than it's more like a Carlson ST - so I don't want to change it that much but we'll make the design you know a little bit more usable another big problem ceramic disc capacitors in a high gain audio stage is not a good idea because ceramic disc capacitors are microphonic you can tap ceramic disc capacitors and you'll hear the tapping quite clearly in a speaker whatever you're using to monitor what's going on so these should be a you know some type of a poly well back in the day they would have most likely had paper and foil style capacitors but it should have been a poly style capacitor or paper in foil or whatever you know way back when to slow down on that microphonic issue a little bit sometimes these things are so microphonic that they actually hear the speaker and they start us a feedback loops you start howling and whistling so here's the thing with ceramic capacitors they're great for RF bypass just general bypass capacitors these things very rarely fail so whenever you see these things in a you know whatever you're restoring if you see these ceramic caps again very very dependable capacitors they can be left alone most of the time just like mica capacitors so that's one nice thing about it is that you know that doesn't really need to be recapped at that point unless of course I wanted to change these all out so one nice thing about ceramic capacitors that's what they have over the old paper and foil style capacitors but again you know microphonic and an audio chain so some of the cheaper older style amplifiers with engineers again that didn't know what they were doing would put these in the signal chain of some you know nicer push-pull style audio amplifiers and I just look in there and just shake my head I did a video on how microphonic these things were actually tapped them and you can see it on the scope just by tapping it and there's some pretty large spikes just by tapping the ceramic capacitor so what I'll do is I will link that video below so just below the video is a description and below that description you'll see show more written in capitals if you click on that show more it'll expose some links underneath there and if you click on that video you can also go and watch that you can see how microphonic these caps are I've had lots of people that are skeptical about that they're like oh they're absolutely fine in an audio chain no they're not so those are just some of the issues on the bottom side you know again tube placement and everything I could keep going and going for my own use I don't want just a cable running out to a single probe what I really want on the bottom side is a BNC style connector so I can put whatever probe on the front I want and if I also want to you know say test a noisy resistor of some sort I don't need to be fiddling around with a probe with a little bit of a tip on the front and an alligator clip or something like that I can clip the resistor and put it off to the side so I don't have to hold anything because this thing routes high-voltage out of this probe lead right here so you can test for noisy components and I'll show you that here in a little bit as well so I want a BNC connector on the front here something that I can either put a probe on I can put an RF probe on I can just put a pair of alligator clips on a piece of shielded cable something like that so that's this is really what I want so I'm going to install a BNC here it's just a much better option something I definitely need to do because everything I have is good you know BNC connectors on a ring line cord could get fixed up here or anything we can see it just a line cord coming right in and nothing happening there so that needs to be fixed it was cut right off why I don't know we'll find that out here in just a little bit so that is just some of the problems on the bottom side here and you know as we're testing this we may find more some things maybe open resist or something I have who knows right we'll just go through this and take our time find out exactly where the issues are and try and solve what we can in this design you know the design is most likely going to be salvageable but it's you know it's gonna have some problems that are just gonna hang around I'm sure so let's get into that I may also actually just before that to is remove this and maybe get some black paint on this nice little grill right here because I've already got the cabinet off and I'm ready to repaint that cabinet and I kind of don't want to leave this that this ugly beige color so probably coat that black or silver or something like that we'll figure that out in a little bit so I'll remove the four screws and take this off it's just pinched between the speaker and the and the face of the unit here so I'll get that under way as well I got the grille out of the unit right here so it looks like somebody had even drilled the grille wrong at one point so these are the three holes here and then this one here is kind of elongated it was in here like this so to get the gorilla it was really quite easy all I had to do just remove the four screws on the face and just slide the grill out and a speaker itself is in very nice condition so what I'll do now is I'll put the screws back in just to hold the speaker in place and be very careful that I don't poke at it and I'll repaint this grille a nicer color it's still kind of undecided and something that'll work well with the face I'm ready to start glass bead blasting the case right now so right now it's inside of a big blasting cabinet and the noise that you hear in the background is a vacuum with a big filter in it what it does is it draws air through this case and then of course filters that arrow when your glass bead blasting in a case it tends to get really dusty in here so it draws all of that into a canister with a big filter in it and filters it out so it draws air on the back side of the cabinet so what I'll do is I'll just put my mask on here first I'll get a little bit Buffalo sounding and then what I'm going to do is grab the glass blaster and we'll start taking some paint off and I'll show you how that works okay let's see I'll start right about here [Music] [Music] so you can see how quickly it takes that down to bare steel that's just raw steel right there so the process is really really fast it'll clean this rake down and it's pretty much ready to paint directly after that with that hammer Tolan style paint it leaves a nice surface on top of the unit itself so that the paint really sticks well before I paint it though I'll use some brake cleaner things like that to clean the steel off to make sure that there's no wax or oils on it because I don't want any problems with the paint sticking to the metal and it is in this large cabinet here so that's the glass plaster so I'll keep on going and get the entire unit all blasted and painted that should make a really nice looking cabinet I'm looking forward to seeing what this looks like when it's all painted up in order to start the troubleshooting procedure on this little signal tracer the first thing that I'm going to do is get some of the basic stuff out of the way first so I'll solder up these terminals here I'll have to put something underneath here to press this wire away because it's actually touching this terminal here while I solder it or it'll melt into these wires here so I'll get these two terminals soldered up that was obviously forgotten during the assembly procedure and then what I'm going to do is cut out the remainder of this line cord and get rid of this little well this is a restraint a cord restraint on the backside here I'll get rid of that during the testing procedure I'm just going to put a cord in here temporarily while we're doing this reason being is the hole in the back panel that fits on this unit is not large enough to fit a plug through it so I actually have to put the panel on to the backside here and then put the cord through both so I'll show you the little hole in the back panel here again in just a little bit to the plug is pretty wide and of course the hole is pretty small so just for testing I'll get rid of all of this stuff here and feed a line cord in and we'll begin the procedure at that point I had to remove the watt meter socket from the front of the unit so I could clean up these incredibly oxidized contacts they would not take solder at all so I could apply our a flux feed solder in it would just ball up and roll off so after a little bit of time working with this in the chassis trying to make it except solder I just said how that's it I'm taking this thing out so I ended up cleaning these things up really well applying our a flux again and then working solder onto the surface so these are pre tinned so when I put this thing back in now it'll very easily accept solder these were very very stubborn so that's fixed up I'll reinstall this probably put some new screws in the front while I'm at it and I'll get to the line cord i Kemper airily installed a line cord into the signal tracer this line cord runs off to my current limited isolation transformer in variac supply that supply is going to allow me to safely bring up the line voltage slowly now if there are any issues in this unit any major issues that current limited supply is going to take care of that and I'm not going to cause any further damage to this little signal tracer if you recall they did cut the line cord off on the back so there might be something majorly wrong with that power transformer again we'll discover all of these issues together what I want to do is I want to bring you through the entire troubleshooting and repair process so when we discover a problem we're gonna take a look at that problem and see how that problem affects the rest of the circuitry that is absolutely crucial in troubleshooting not only just repairing the unit but actually understanding why the unit is acting the way it is in its failure point again very very crucial in troubleshooting so we're gonna do that together now if you're following along you're doing so at your own risk know that there is high voltage present all over vacuum-tube gear and in this unit there's AC line open basically just right here like this and up here in these older pieces of gear they didn't take the time to put heat shrink tubing over everything and make sure everything is majorly safe they just expected you to know that this is dangerous and this is very dangerous so all vacuum tube equipment has high voltage present all over the place aside from space charge circuits and I'll talk about them in another video but most vacuum to gear has high voltage and some of its got extremely high voltage present and of course we have you know the AC line present which is high voltage and high current there as well so if you're unfamiliar with working on a unit like this let me do the dangerous stuff you know stay away from a unit like this until you're familiar with the circuits until you're comfortable with that circuit if you are not comfortable with working on circuits like this don't work on them don't put yourself at risk there are lethal voltages and lethal currents available in this chassis so again if you're following along you're doing so at your own risk be very very careful I'm ready to apply a/c to this let's see what happens so turn on the supply and I'll slowly bring it up I can already hear the speaker humming that would be that coupling issue I'm pretty sure [Music] well there's definitely a hum there now the dim bulbs are the bulbs on the current limiting supply we're on very very dim so there wasn't really any excessive current draw there so it's just making a really bad racket so let's look into why it's making that racket so far we know that this very small signal tracer has a very big hum and we need to figure out where that hum is originating now in an audio amplifier circuit hum can be caused from a whole bunch of different things and that's really what this thing is it's just a very sensitive audio amplifier with a really high gain stage in the front end so the hum can be originating from the signal path it could be heater to cathode leakage in one of the tubes or both of the tubes or it could be main power supply hum so the main B plus supply might have ripple on it now the very first thing that you always want to check is the power supply and what really points me towards the power supply in this little unit is that the hum is so incredibly strong it's an incredible incredibly loud on this end so that tells me right there that I should be checking the power supply first now here's something to always keep in mind whenever you're working on an audio amplifier in this case this is the B+ supply and we're supposed to have nice clean filter DC at this point that's what this capacitor is here for it's to take the ripple off of the DC so that we have nice clean DC here's the thing with high gain audio stages and this applies to phono stages and any type of audio gain stage if there's any ripple or AC present on the power supply since the power supply is powering up the amplifier stage what it's going to do is it's going to take that ripple that's present and insert it into the amplifier stage and what it's going to do is get amplified all the way down the line is you get closer to the output so basically it's taking the noise from the power supply and amplifying it making it worse as it gets further and further down so they're more gain stages you have the more it's going to take that ripple that's kind of leaking into or mixing into the amplification chain it can take that make it really really bad so it's very important in very high gain stages and just amplifiers in generals to have a very very clean DC supply now that's another thing that brings me to the design of this thing I I look at this and I see okay to read leads coming out of the transformer so immediately when you see a transformer with green and red leads so green red and black we know green is the filament leads red is the V Plus leads and black is theme is the main AC line sometimes it's black and white anyway I just did a video on patreon explaining how to recognize leads on transformer so if you're interested in that you're gonna want to go to patron and check that out it's an entire video based upon that and also determining transformer impedance as well there's some equations up there math equation and stuff to help you figure out how to determine the impedance of audio transformers unknown audio transformers but any rate back to this so we know that we can see these two red leads these with v+ leads one of the red leads just ties directly to the chassis and we see the other red lead going right into this diode so one diode that means that we have half wave rectification so half wave rectification going through a resistor and then directly into a filter can and if I look at the filter can there's hardly any you know it's hardly any filtering there 50 micro farad or something like that I'm just looking at this just going what is going on with this design at any rate it just keeps getting more interesting as we go but it let's fix one problem at a time here so let's see how much ripple is going to be present at this point if there even is any you know maybe they have done something right so let's take a look at this point and if there is ripple here right there we know that this capacitor is most likely going to be at fault and we'll test that cap so let's do that right now so I'll take the black lead of my meter since we're going to be looking at AC this could be classified as a common lead and I'll take the red lead and put it right here onto the B+ so this is rate work comes out of the diode and it's right to the very first filter capacitor I'll turn this on to AC and I'll turn on the backlight like so now what I'm going to do is turn on my very act and isolation transform which is going to give this AC line voltage and I'm not going to leave this on for too long because I imagine you know the hum is so incredibly intense on this end it's probably really hard on that small audio transformer so what I'll do is I'll turn on the supply now I can hear it in the speaker I'll click the speaker off in a second when it starts to hum my fingers on the switch here [Music] [Laughter] wow that's loud look at the amount of a see there that's supposed to be clean DC there's almost nine volts there is nine volts nine volts AC and take a look at the ripple here nine volts AC at that point so I'm going to shut this off so there's only supposed to be DC present right here and there's AC on there so to give you an example here what I'll do is I'll just click this over here to DC so you can tell that we do have some form of filtering there because there is DC that's lingering at this point so we do have some form of filter capacitor happening over here and see how it's climbing so there is something working there so we do have some form of capacitance in this capacitor so it'll be very interesting to try this why do we have so much AC at that point look at it's climbing it's not even on this is one of the dangers with capacitors so and this is called dielectric absorption the capacitor there's nothing there's no nothing applied to this right now like the power supplies off there's no AC applied to this and as you can see that's just climbing up so this is the kind of stuff that gives technician shocks and this is why it's so incredibly important to safely discharge capacitors because that can keep climbing and climbing and climbing and in some of their very large capacitors that'll climb up to 100 volts sometimes even 200 volts that'll go right back up there again after sitting for a long period of time so really really dangerous you can see this is starting to settle off and go the opposite way no so what I'm gonna do now is I'm going to turn the AC supply back on now this is the same point where there was AC and as you can see there's DC there as well and we have about 130 volts DC there and if I click on the speaker again you're incredibly allowed that as it's probably pretty hard on that audio transformer so we have a lot of ripple present at this point right here and we need to figure out why there is ripple there so what we're going to do is check these capacitors what I'll do is I'll desalter the leads off of these capacitors here and we'll verify this and see how close they are to the actual spec I've disordered all the leads from the three capacitors in this one can so we can test each section individually so all three of these capacitors have one common ground which is attached to the can and the can is soldered to the chassis so we can just take the negative lead of the capacitor tester and attach it to the chassis and just test each capacitor here so I'll just turn on the capacitor tester right now now there should be 250 micro farad sections and 120 micro farad section so let's see what we get here well one section has half or just under half of a micro farad that's definitely very bad same thing over here on this section and we'll move over to this one here and same deal here they all look pretty close so that makes me wonder if the actual grounding has just come off from the cannon side usually what they do is they take the common to the ground from all three capacitors twist them together and then attach one wire to the can so let's just test between the actual capacitors themselves and yeah we have some capacitance there let's go over to this one here yeah so it's obviously damaging inside it looks like the negative has come off the the negative that attaches to the actual can so it's either corroded to offer maybe one time something shorted in it you know blew the common off inside so definitely a damaged capacitor so that's the reason we have so much hum here so what I need to do is replace all three of these capacitors now the capacitor itself isn't leaking it's absolutely fine just to leave this here what I'll end up doing is I'll put in a terminal tie strip somewhere on here and I'll put three modern capacitors in by putting those three modern capacitors in and the terminal tie strip will make it very easy to service down the road as well usually just take the caps out if something ever fails replace them in you're off and running again so that's what I'll do right now I'll put a tie strip in here and get all the capacitors in and we'll try it again actually you know what we should do first is I'll give you an example of how the transformers are coupling on the upper chassis that's just bad design right so I'll give you an example so the B+ is completely disconnected so this just runs nowhere right now this 220 ohm resistor is just completely open so what'll d'azyr's fold this over here like this and what I'll do is I'll get this attached to the isolation transformer current limited supply and what I'll do is I'll turn this thing on it'll bring this up to the microphone and I'll late to hear how loud the hum is in the speaker so I'll just get that all set up the B+ is unhooked on the bottom side and the two tubes are removed so there's nothing in circuit to drive that audio transformer just below the speaker so I'll turn on the isolation transformer in variac supply so basically what that's going to do is that's just going to apply AC to this transformer and you know just light the filaments in this one tube that's left over here so this is the indicator tube for the wattage section so what I'll do is I'll turn this on and we'll take a listen to the speaker and you'll see how badly this transformer is coupling with this other transformer here so the speaker is off right now so I'll bring this up to the microphone like so and watch this [Music] [Music] and that's just from these two transformers being too close to each other so really the only way to fix that would be to remove that audio transformer below the speaker and place it on this side of the chassis and that's gonna be pretty tight like this it'd probably have to be on an angle if I put it this way I might still get a bit of noise it would probably be better than what it is right now is so close just see the clearance they're thinking it's almost touching so yeah kind of a silly design chances are I'm just gonna live with that bit of hum and just leave that alone cuz it's quite a bit of work and reworking just to you know knock out that little bit of hum there and of course at that point I'm really changing the design quite a bit of this unit as well I have three new capacitors installed in this signal tracer so this capacitor this capacitor and this capacitor right here replace what's inside this can so the first capacitor that was in line with the B+ here so the very first filter is 50 micro farad at 200 volts that's what's inside this can that's been replaced by 82 micro farad at 450 volts so yes this is an overrated component and that's absolutely fine the second capacitor is the same as the first 150 micro farad at 200 volts this has been replaced with a 450 volt port at 47 micro farad that's absolutely fine in this portion of this circuit the third capacitor in here which was a cathode bypass capacitor is a 20 micro farad part at 25 volts what's inside this can that's been replaced with a 22 micro farad 35 volt part right here so this little teeny capacitor here has more capacitance and better voltage ratings than what's inside that big can right there I don't like modern technology can put these things really close to the tube socket as well I really like that really good temperature readings on these capacitors and everything very high quality components again when I do something I don't want to repair it again this thing is gonna be on the old-time workbench and we're gonna use this thing to troubleshoot other pieces of gear so I want this thing you know to stay together and be very very dependable so some very overrated components the 68 Kom resistor that was tied to this capacitor here with the orange lead is now jumping over here so I've installed a new 68 km resistor right here and it joins to the same area and that orange lead runs just right over here nice a little short run to this capacitor so this capacitor is replacing this one as you can see I've moved the resistor over and tied it to this point right here so this is a ground and this is a ground right here so the B+ runs to this side of the cap and then I've just bridged these two crowns to the chassis ground right here and again this is just a cathode bypass capacitor so this is across this cathode resistor right here and that's really all it takes to really calm this circuit down so we're gonna take a listen to the difference and then I'm going to show you a real common mistake that a lot of Tech's make so that you don't make the same mistake so I'll turn this thing on let's take a listen to it sure there's a bunch of things I'll tell you about here that caused Textus so-called chase their tail and audio circuits so I have the microphone really close to the chassis I moved it really close so we could really hear this so I don't really have to lift this thing up that's the difference now the little bit of hum that you hear there is a combination of that transformer coupling and open basically open line the hum so you could call that signal path hum it's listening to me sitting at the bench that's where the hums coming from watch what happens when I touch the chassis calms the hum right down that's because this is all unshielded right now so this is just picking up all sorts of noise when this is back in the metal case none of this will be an issue because you know it'll take care of all that it'll be completely shielded aside from you know the transformer coupling to the audio transformer so quite a bit of difference there quite a bit quieter as well so this is a real common mistake a lot of audio guys will chase this hum around liudmila where's that hum coming from where is it coming from when a lot of the times it's coming from them sitting at the bench or it's even coming from the line cord itself just being too close to other components inside the amplifier chassis and I'll talk a little bit more about that here in the future if I add one capacitor from one side of the line cord to the chassis it'll make this thing completely quiet as well right now so kind of interesting I'll touch on that a little bit more in the future when we're looking at audio amplifiers and things like that so what I'll do is I'll shut this thing off right now and I'll discharge the capacitors so discharge these caps make sure that they're completely drained off and they are this is the capacitor discharge device that I've designed and it's up on patreon as well I use this all the time to safely discharge these caps you don't want to go poking your fingers in here with one of these things charged up because you'll get a really nasty zap so this is a common mistake that a lot of texts make a lot of texts will go into a circuit like this and they'll take a brand new capacitor and they'll parallel it with the old one just to see if that's where the hum is coming from well you can't do that in a lot of cases and it'll cause you a test so called chase your tail in a circuit like this because you just won't find the hum if you do that so what I'll do is I'm going to parallel in just two sections not even three I'll just take two of those sections and I'll parallel them in so the old capacitor into the new ones here and watch what happens so I'll just attach this here and I will attach this here so this capacitor here originally attached this point so I'll just attach it back to that point and this capacitor originally attached to that orange wire and that's this capacitor now so we'll attach this to the exact same point again so technically I've paralleled in the new capacitors to the old ones let's see what happens when I turn this on [Music] I'll just shut that off is that's nasty that's the reason that you cannot parallel or use the old capacitor as tie points even if the capacitor seems to be open the old capacitor has to be completely disconnected from circuit will do is I'll just discharge these capacitors now keep it on these led see if there's any light here oh there is a little bit and this one here is well there you go just get this out a circuit so you can see the effect of paralleling these things up that can't be done the reason that this is happening is if you recall when we looked at this earlier with that other capacitor tester we could see that basically the negative or the common lead has been blown off corroded off or whatever it's just disconnected inside the can so these capacitors are connected together because all of those common leads are still connected together inside the can that's why we can measure capacitance from one section to the next so basically what I'm doing is I'm taking these capacitors here and I'm putting these two capacitors across this point here to this point here and that's what's causing the hum so I'm effectively bridging two capacitors in series inside this can over these two capacitors right here from this point here to this point here and that's what's causing that hum and that's why you absolutely cannot use the old capacitor as a tie point very very important thing to remember and even if you're troubleshooting you can't even bridge capacitors in because you just wouldn't find the hum in this case well now that we've gone this far with this little signal tracer you'd be kind of a shame just to put it back together and leave the hum in there because of the transformer way it's put into the chassis and things like that so as I say we've modified it to this point why don't we just keep on going and let's make this thing the best that we can make it so what I'm going to do is remove this capacitor here I'm going to take this transformer and move it over to this portion of the chassis so I can get it far away from the power to former you know what I'm gonna do is I'll grab a small speaker and I'll take this audio transformer and I'll to attach just the speaker leads to this external speaker and I'll hold it up to the microphone and we'll move the transformer around the power transformer and you'll see how badly they couple so even though this thing is shielded it's still gonna couple to that transformer there so let's get this thing out of the way and then there's another silly thing that they've done that I'm going to address as well but we'll tackle one thing at a time here I have both the audio transformer and the capacitor removed at this point I've installed a rubber grommet here and I'm pretty much ready to install the transformer on this portion of the chassis here's a quick look at the faulty capacitor for those of you that are interested so since I have the transformer removed what I'm going to do is move it close to this transformer and I'm gonna hold this speaker up to the microphone here and you can hear how badly this will couple with that transformer I'll just hold this up to the speaker like so [Music] [Music] so you can see how important component placement on the upper portion of the chassis is and that's so basic I have no idea why they mounted all these transformers like this so I say probably an inexperienced engineer or something put this thing together so what I'm gonna do is mount that transformer over here like this and we'll see how much that knocks the hum down and then we'll go on to the next thing that will be affecting them the transformer is now installed and that went pretty quick so let's take a look at the bottom portion of the chassis here so I installed the terminal tie strip here and just around the wires up to their original area on this portion of the chassis what I did is I just soldered the calman let's get this into the focus here common of the transformer to the chassis because the speaker itself just grounds to the chassis and then the green wire was the perfect length it led right back up to that switch again so that worked out very very well so I'll get this all positioned and plugged back in and we'll see the difference in the hum okay let's take a listen and see what a difference that made moving that transformer out of the way so I'll click on the isolation transformer and variac supply right now I don't know if you can hear the hum from just the transformer and the variac supply you might be able to hear that as well that made a huge difference reason I'm touching the chassis is because it's picking me up that made a big big difference and there's still something else we can do as well so that was definitely a worthwhile modification so the next thing we're gonna do is look into a hum balance control taking a look at this Stark Model S T - to signal trace or schematic they've done another very interesting thing in the filament circuit that we are going to correct so normally in a balanced filament system the center tap of the filament winding which this does not have would run to chassis ground or in a case like this where you don't have a center tap on a winding you run a separate hub balance control which is just a VR across the winding so from end to end on the winding and then the wiper would run off the ground and you adjust the V R for minimum hum that's a proper way to do things well what they've done here is they've used the highest gain stage vacuum tube and they've taken the filament center tap in there running that to ground so that's going to get corrected and I'm going to put this V R into the chassis this is a 2.5 K ohm V R and I'm going to adjust this for minimum hum so I'm gonna put that somewhere on the chassis where I can very easily get to it so right about here there's a little open spot under here so if I remove the back panel and I want to make a hum balance adjustment I can just take the back panel off and get right there put my screwdriver in and adjust it I really don't want to be putting it anywhere over and here or somewhere where I can't get to it and there is a little open area under here so I'll get that hum balance control installed and I'll show you how that works and we'll try and reduce the hum a little bit more all right let's adjust the hum balance control them so I'll turn the unit on let everything warm up right about there is its quietest point so it's working out very well so now all I need to do is put the BNC jack in the front of this thing change this capacitor right here put another point two micro farad capacitor in there and I'm ready to try the thing out the hum balance control VR is installed right here in the chassis and that's at the back side of the chassis you can see the twisted filament lines are running over here to the filament pins on this tube socket now you'll notice that these Brown leads are a little bit smaller than the green leads here and that's absolutely fine because these aren't really carrying any type of current the wiper for the hum balance control runs all the way along here you can see the black lead runs all the way over here up to the tube socket here to a low hum point on the chassis now when I say low hum point finding the best spot on the chassis for a star ground is a little bit of a job in itself in any design just because the chassis itself is carrying current through it and if you bring the ground on certain points of the signal path to different parts of the chassis you'll notice a hum and then just say alright you had a wiper then you were dragging say an alligator clip or something along the chassis you would hear the hum get lower and lower and lower as you get over here and then as you pass this spot the hum would start to pick up again and it gets louder and louder and louder and it's going to be louder by the transformer and as you get over here it would get pretty loud so then as you drag the alligator clip back over here again along the chassis you would find that the lowest hum point is at this point here and that's a real trick in amplifier design whenever you're designing an amplifier you want to find on the chassis where the best ground is and that's why a lot of amplifiers have a star grounding system that's quite a bit different than in an RF application because in an RF application having long lead length means inductance and you cause lots of issues with long lead length in audio application that's absolutely fine to have that longer lead length because of lower frequencies that the amplifier is dealing with it's not dealing with higher frequencies I'll talk a little bit more about this in the future in different types of designs and this applies to everything this applies to modern equipment this applies to circuit boards this applies to everything longer lead length means inductance and when you're dealing with RF circuitry that's your enemy again since this is audio circuitry and the frequencies are much lower you can get away with that and there is a balance point too so by having that longer inductance or those longer lead lengths in an audio circuit can actually be beneficial again I'll talk more about that in another video that's an entire topic within itself so finding the correct grounding area so anyways this is a part of the chassis where the hum is very low and this is far away from the transformer and this is where that 12ax7 but at 7:02 v is mounted right over here in this area and that's the reason that I didn't just ground this over here I ran the ground all the way over here right to this area so that's all done I've installed the BNC connector on the front here so I'll show you the front and I've also put the speaker grille back in that looks really nice so there's the speaker grille and that's got that hammertone finish on it's kind of hard to tell the hammer tone because the you know what's got so many holes and everything but it's a really nice color almost like a really dark black kind of like an ash smoky gray kind of color it turned out really really nice so B and C's in here everything's all ready to go so I can put all my different probes on this you know the face cleaned up really nice you know there's still some scratches around the border but that's where this actually meets the case itself it has that curl the edge on it and it meets this so all in all you know there's a few scuffs on the face but it's looking really really nice it cleaned up very well on little knobs cleaned up and everything so since I'm using stainless hardware I also changed the screws on the face with some nice screws the other screws were rusty and they were looks like somebody had stripped them you know they've been tightening things a little too tight and it kind of marred the screw so I put all stainless screws in the face here and all matching screws so that worked out very nice as well so this is cleaning up really really nice I really like the look of that girl much nicer than that horrible beige color and the case is the same color as this and you'll see that here in just a little bit that the case turned out just really really nice I'm very happy with the case so all in all it's coming together the next thing that we really have to test on this is the watt meter section and that has to do with the I tube so we want to make sure that this little area is working well what this thing does is you plug an appliance or whatever in here and you rotate this knob just until the shadow angle starts to open so there's a little gap in the shadow angle and then wherever it's pointing is how many watts it's drawing so it's using this is almost like a little CRT these are known as eye tubes so and at any rate we'll see how well that works I'll plug different loads in here and we'll turn this up and we'll see how accurate the the little watt meter is on the face it's handy to find out if you're gonna test an older radio or something like that you know you want to you can't really bring it up slow on this this is just kind of an all-out watt reading so if you plug anything into this and it is pulling excessive wattage you know there's not really anything unless you have a fuse in line or something like that that's gonna you know deal with that right so this is where a dim bulb tester would really really help so something like this and a dim bulb tester combined would be a really really good little design than I tube for a watt meter and a dim bulb tester so the bulbs light up really bright and you know you can tell if there is something wrong again this thing if you plug it in you know it's just gonna show extremely high wattage if something is really wrong and by that time usually by the time the you know you've sensed all that kind of stuff it's almost too late but still handy you know there's there are applications where the watt meter is really nice so we'll make sure that this portion of the circuit is working now everything is working in the hum is about as low as it's going to get at this point with all of these modifications unless of course I'm going to want to change this one capacitor and really up that capacitance you'll recall when I looked at the design I said you know this is you know it's kind of low and there is no filtering on this side here so you know this it's gonna drop the ripple you know down quite a bit by installing these new capacitors obviously you heard the difference in the hum it was pretty crazy I'll talk a little bit more about that and what I'm gonna do is I'm going to show you some examples of the results now that these capacitors are changed and what you can expect and I'll talk a little bit about what you can expect when servicing a half wave rectification design whether it be an all-american five all-americans six Radio one of these devices or something like this so this is something to keep in mind and this is something that really isn't talked about a whole lot so you'll get some first-hand examples of exactly what to look for if you're dealing with a design like this or you know as I say an older radio or something like that when the troubleshooting procedure began and this little signal tracer had a lot of hum that led me to check the very first capacitor in the B+ line and that was attached to this end of this resistor over here so this resistor was bent over here and attached to one of the legs on this can here as well as those red wires as well since I've installed newer capacitors I've put a terminal tie strip over here and I've installed the new capacitors like that excluding the cathode bypass capacitor which is attached directly to the cathode of the tube here right on the tube socket so all I've done now is I've taken this resistor and bent it over here to the new capacitor and attached everything over here on the terminal theis trip so everything is as it was over here it's just mounted in a different area with more modern capacitors now if you recall when we measured the ripple at this point it was incredibly high with nine volts AC or something up here so let's compare the new capacitors to the faulty capacitor let's see how much ripple is at this point now with these new capacitors and this capacitor here even has some more capacitance the original was rated at 50 micro farad this is now 82 so let's take a look this thing is on and it's warmed up what a difference nine volts down to 1.3 now some of you are saying that's still a lot there's 1.2 1.3 volts ac there and look at the that nice sawtooth looking waveform on the scope there I don't know what's even more interesting is this point here is what basically powers this tube right here so this is attached to this tube through the audio transformer why is the hum so low I'll answer that in a moment you can think about that let's test the ripple at the second capacitor and that's incredibly low why is the ripple lower at this capacitor than it is at this capacitor and what's between these two capacitors this capacitor here is the filter for the preamplifier tube so we would definitely need low ripple at that point so since this is attached to the preamplifier tube and we need low ripple for the preamplifier tube why is it okay to have 1.3 volts of ripple on the output tube again I'll answer that in a moment you can think about that and trying to answer that so now we have lots and lots of ripple on this very first capacitor I'll try and throw you a couple of curveballs here and see if you get the answers right so what I'm gonna do now is I'm going to shut this unit off and I'm going to take a 47 micro farad capacitor the same capacitor that's over here and I'm gonna put it in parallel with this one so I'm just gonna put it right across this one what do you think is gonna happen if I do that what do you think's gonna happen to that ripple make your guesses now so what I'm gonna do first is make sure that these capacitors are discharged very well and they are discharged another question you may be asking yourself is why are you continually going in there and discharging these capacitors are making sure that they're discharged when you know you can't see any glowing of these LEDs they always seem to be discharged when you go in there well the answer in short is the tubes themselves are discharging these capacitors because of the thermal mass and the cathodes these are indirectly heated vacuum tubes they're not directly heated so there's quite a bit of thermal mass in there but you can never be too safe you always want to make absolute sure if there's ever a Fault in the circuit these capacitors could remain charged so if you want me to get a little bit more into why these are discharging and what's happening with the thermal mass there I'll quickly touch on that so the heaters inside these vacuum tubes are inside of a cathode sleeve so there's a filament like what you see in a light bulb but it's tucked inside of a little pipe inside the vacuum tube in that filament when you turn the unit on it applies heater voltage to that filament the heater the filaments inside start to glow really hot and those filaments have to heat up that pipe that pipe is the cathode and it has a special coating on that cathode in the cathode is what emits electrons inside the vacuum tube so in order for the vacuum tube to work that pipe has to glow orange so that electrons are you know we have electron emission inside the vacuum tube right net goes for both of these tubes here now some tubes are directly heated and some tubes are indirectly heated these tubes are indirectly heated so it takes between 10 and 20 seconds for the filaments inside that pipe to heat that pipe up nice and orange that's why when you turn on an old radio it takes a while for the radio to come on or if you have a vacuum tube amplifier you turn on the vacuum tube amplifier and it takes a while for this sound to come out it's because those pipes have to heat up a little bit of thermal mass inside there well the same thing happens when you cut the power to the power supply so if I shut the AC line off so I shut it off either here or by the switch of course the whole power supply and everything is going to shut down and the heaters inside the tube are going to shut off so those filaments inside that cathode are going to get cold well since there's so much thermal mass inside that tube cathode is still glowing orange but it's slowly cooling off well as it's slowly cooling off because the power has been cut the tube is still working there's still electron emission happening so what's going to happen since it has no power supply powering up these capacitors what's gonna happen to the caps well these things are going to do the work now what's gonna happen is the tubes are gonna use up the charge inside these capacitors and it's gonna drain those caps off really really fast so these things are kind of acting like batteries for a very short period of time when you cut the power so what's happening really is the tubes are staying into emission longer then these things can supply a charge so it drains these things right off here's the thing if you had a very very large capacitor in the power supply the actual capacitor may supply enough current to those tubes as the cathode is slowly turning off to still end up with a lot of voltage so you always have to make sure that the capacitors are discharged or there could be you know quite a bit of voltage lurking in the chassis and in some cases that could be lethal just depending on what type of a situation you're in and what kind of amplifier some of these big vacuum tube amplifiers have huge capacitors inside them and you shut the amplifier off the it'll drain the capacitors so much but there's still a little bit left and then after that we get into another subject which is called dielectric absorption it almost appears like the capacitors are charging themselves back up again but they're not really what it is is it's again it's dielectric absorption and I'll get into that in a future video so it'll drain the caps off and then there's no more emission in the amplifier the caps are sitting there and it almost appears like they're charging up again so I'll touch on that in the future so what I'll do now is I'll grab that other capacitor there's so many different things I could explain I could make so many different videos on all of these topics and keep going and going and going here so I will make a video on all this kind of stuff here in the future it's just that if I keep talking about all of these small little aspects we'll never get through this video so 47 micro farad capacitor in parallel with this one here so where I'm taking this exact capacitor and putting it across this one here so now we have a lot more micro farad so we have 82 micro farad plus another 47 on top of that and the boots are far enough down here that nothing is gonna touch or short out so now I'm going to turn this thing back on again what do you think is gonna happen to the ripple take your guesses now here we go turn it back on and I have something that's gonna throw you for a loop again here in just a moment this will get a lot of the older tacks will probably be shaking their fists at me so that should be kind of fun in a moment let's find out so here we go now that I've got this cap installed what do you think's gonna happen what are we eating see on the display AC volts take your guesses now here we go so dropped it down to about 0.85 but as you can see that's a nice looking a little sawtooth on there that's still pretty high there's a lot of ripple at that point still reading even you know 60 Hertz right there so what only dropped it down a little bit so why is it so low at this capacitor and yet it's so high at this capacitor I answer that here in just a moment when you take a look at the schematic so I'll just shut this thing off I'm gonna do one more thing that's gonna really throw some of you for a loop put this here and I'll just make sure that this is all discharged you might also be asking what are you doing why are you doing this while I'm scrubbing the leads so if there's any flux or any kind of residue on the lead I want to make sure that I've got a very good contact so I scrub the leads and that little kind of a little Ridge that you see there the focus is over here that little Ridge that you see in the probe right here helps with that as well so basically what I do is when I go over to a lead on something I scrub it like this just to make sure I've got a very good contact because I want to make sure that these caps are very well discharged I do that a lot when I'm testing for voltage as well they'll scrub leads because sometimes you'll find flux or some residue will get under the probe and it doesn't make a good contact so by scrubbing you get a really good contact these caps are discharged what's Paul gonna do next well let's take a look here so here you have another capacitor that we're gonna put in line and we're gonna see what happens to the ripple if we install this capacitor right at the diode okay and I'm not gonna be shy with the ratings either so here we go we have 200 volts 200 volt ready cap same thing difference is Wow 820 micro farad were approaching a thousand micro farad here I'm gonna put this capacitor rate at the output of the diode here so right at the Reed at this where it's basically coming into this resistor I guess you could say there was never any capacitor here and that's not even on the schematic so I'm gonna put 820 micro farad right at the output of the DAAD now some of the texts are probably going you can't do that that's 820 it's gonna cause damage no it's not so here we go put it right here so rate at the output of that diode why this capacitor will not cause damage to this power supply in any way shape or form and this could actually be left in the chassis and it would work fine throughout its entire life I'll explain all of this stuff here in some future videos again I could just keep going and going and going about these little small details and there's just so many things to cover so a lot of the the techs out there think that you can't go about doing this kind of stuff what it's going to do is it's going to blow up the diode or it's going to harm the transformer transformer won't be able to deal with this I've heard it all all of that is absolutely false in this case right here it's absolutely fine and I'll even be a little bit cheeky and I'll show you something here in just a moment okay so here we go the capacitor is hooked up to the power supply power supply on so everything's on it's warming up now you're thinking oh that's got to be awfully hard on that diode or the power transformer right so what I'm gonna do now is you can you'll be able to tell by looking at that little orange glow in the pipe there if you don't believe I'm doing this so what I'm gonna do is I'm gonna shut the power supply on and off really really fast so if anything's gonna happen something's gonna happen in this time right so here we go I'm gonna shut it off you'll hear the switch clicking off on off on and off a bazillion times really really fast it's off right now on and I'll do it again so now it's on again nothing's happened why is that I'll explain that in a future video now this wouldn't be so incredibly tolerant if this had a vacuum tube rectifier this is a lot of capacitance and vacuum tube rectifiers have a bonding wire that run from the cathode so this is an indirectly heated tube okay so say we're talking about on All American five so a 35w four or something like that and say there's a bonding wire from the cathode to one of the lead out pins okay so what happens is is that bonding wire inside that tube acts as a fuse and so does the filament itself and that's why you see a lot of All American five radios with no fuse if anything goes wrong it just blows that bonding wire off and then of course the next weakest link is the filament itself so it's almost really no need for a fuse in there unless you want to be very very safe and add one I still do I still add fuses in there but technically that's why you never found a fuse in those older radios so there's your hint the bonding wire in that tube is the weak point and this thing right here would cause problems with that type of a vacuum tube especially fear to do what I just did this is a diode in here so it has absolutely no problem so understanding why nothing happened to this is really just it's a talked about the amount of current that the transformer itself can supply and the strength you know the current rating of the diode and things like that I'll get into all this stuff in the future again I could go on and on and on so here we go let's take a look at the ripple now at this point so this is what really nice filter capacitor rate at the output of the rectifier okay so here we go look at that how low that is what a difference from almost a volt down into you will say is hunting where you know it looks like a boat say we'll say 20 millivolts something like that now let's go look at the second one here so this is the one that's powering the preamp tube or being the filter on the preamp tube I should say look at how low it is over here it's just just hunting so there you go incredibly low with an added filter capacitor like this now why in the day wouldn't they put something like this in the chassis well a cost back in the day when this thing was made 824 had a 200 volts would be a monster-sized can and way back when capacitors like this were very very expensive so to you know purchase an eight hundred and twenty micro farad capacitor at 200 volts would be very cost prohibitive in a design like this so what they did is they got creative and they did this and they used the same kind of topology in all american v in all american six radios this topology here in excluding this large capacitor here so the original topology they used that in a-five and a-six radios and they could get away with it why could they get away with it with so much ripple and especially that ripple being tied directly into the audio output tube why could they get away with that well what I'll do is I'll grab a schematic here and I'll explain a whole lot of things to you why all of this works and it works not too bad of course leaving this in here right now would knock the hum down it would even take the hum down a little bit more but at this point it's so incredibly low and adding this capacitor into the circuit it's only gonna take it down just a little bit more than it already is and at this point it's a balance right at this point it's a balance if I wanted a very quiet circuit this is a signal tracer this thing isn't an audio amplifier it's not a radio that's gonna sit by the bedside and you're gonna be listening to it late at night to your to your favorite radio show or whatever it's a signal tracer it sits on the bench it usually sits afar a ways away from you and it's used as a test tool so that little bit of hum that you heard earlier before with these two capacitors is completely tolerable especially now that we've done all of these other modifications move the audio transformer and added the hum balance it's just so minimal so I'll get the schematic and I'll start explaining all of this stuff the modifications to the model st - to signal tracer are as follows the filaments enter top of the 12ax7 is now open it no longer ties to chassis ground so pin 9 is now floating a variable resistor has been installed across the filament winding and the wiper of that variable resistor now attaches to chassis ground the rating of that variable resistor is 2.5 k ohms so that gives us a hum balance control t2 has now been relocated originally t2 was too close to t1 and they were coupling getting quite a bit of hum at the speaker t2 has been relocated to where the original filter capacitor sat the value of c8 has been changed to a t2 micro farad the value of c9 has been changed to 47 micro farad and the value of C 7 has been changed to 22 micro farad a BNC connector or jack has been added to the face of the unit so it's been modified so that I can use different probes with BNC connections the line cord is still temporary I haven't installed the line cord permanently at this point because I need to put the back on the unit in the hole in the back panel was a little bit too small to fit the plug through the 68k resistor has been replaced with another 68k resistor just because I had to relocate it and the original resistor the lead was too short because it was installed into a different area so I just replaced the entire resistor other than that components have been moved around a little bit just moved around and wires have been moved not much has been changed now the watt meter section still hasn't been checked yet so this entire circuit we're gonna do that here in just a little bit we'll just get the amplification chain there basically the signal tracer out of the way first so if you recall at this point here we have one point three volts worth of ripple now this point here if we follow this path runs down here into this switch if we give this switch one click clockwise then four and five are going to connect to pin seven pin 7 is acting as a wiper so when we click this one click this larger area will contact this arrow these two are ganged so these two switches will do the same thing when I say gang they have one common shaft in the switch so now this is now connected to this point so technically this point here is now connected to here the B plus now goes through a 10k ohm resistor into this 47 micro farad capacitor up this path to the screen grid of the 12 C a5 tube and up this path here to the plates of the two sections of the 12 ax7 here if we also follow this path down here we'll also see this point here is connected to this point here at this point and it goes down here since we've clicked this one click clockwise again pin four and five will be connected to pin seven so now this point is now connected to this point here so we'll see a direct connection from this point with lots of ripple right directly into the audio transformer and to the plate of the 12 C a 5 2 now you're saying to yourself that's a lot of ripple to be on your how come this thing isn't humming a whole lot while they chosen the 12 C a 5 because of its power delivering capability this is designed to drive an audio transformer so there's not a whole lot of amplification happening here so that little bit of ripple really isn't affecting this circuit remember this transformer here is doing impedance conversion a 12 C a 5 tube has a very high impedance and we need to take that impedance and convert it or transform it to a lower impedance that will better match our speaker that's what the transformer does we're just basically taking this high impedance and converting it into some thing that will drive a speaker so not a whole lot of amplification here where it really matters is in the plate supply of these two tubes but you'll see that the B+ goes through this 10k resistor here and gets filtered again by 47 micro farad capacitor and then of course that feeds the screen grid and the two plates of these tubes over here right you see the path all the way up here to the two plates well why is there less ripple at this point then there is at this point well we have another capacitor here and we also have a 10k ohm resistor in the path so the strength of the ripple is greatly reduced at this point so by putting another capacitor here we can almost virtually eliminate it and there's so little ripple at that point that it's not really affecting the amplification chain all that incredibly much that's basically what's going on so if we look at this circuit here when I install that really large capacitor that 820 micro farad capacitor I put that at this point here to ground so basically right at the cathode of this diode here so what's really happening while we're adding a whole lot more filtering in at this point here now if we get rid of the ripple what's gonna happen to the DC the DC is going to shoot up this is gonna go up to about 160 volts or around that area and we're gonna have about a hundred and fifty volts over here well of course you know it's gonna up the voltage a whole lot to the circuit now it's really not gonna affect things too incredibly much other than take this out of its design parameters a little bit but again way back in the day they didn't have capacitors like that available so they may do with this particular circuit arrangement and as you can see it works very well still just a little bit of hum in there but it's it's livable it's not really worth adding that extra capacitor in there at this point and that's really what's going on in this circuit now if we take a look at paralleling the old capacitor so the old one that was faulty if we were going to parallel that into this circuit why would putting these new capacitors in the circuit not affect this why would the hum still be there if that old capacitor was there well this is the reason if I take a pen what's the hell over here just about lost my pen and all that stuff on the desk we have three capacitors in that old can technically okay so I'll just draw three capacitors quickly here I won't draw them as being polarized capacitors I'll just draw them as three capacitors so this goes off to the can or to ground and we have our three capacitors here so now since this is the can and this is tied to ground we know that this is open so this is blown open a rod at open inside there so now we have three capacitors just attached together and this is completely missing so if this was the 50 this is the 50 and this here was the the 20 micro farad capacitor so since we have 250 s we had one here and one here what's happening well we have a path like this going through here and also going through here but we'll just ignore this for now to keep it simple so we have basically two capacitors in series so if we're to look at this we would put a capacitor here and another capacitor here and we could just take that and tie that to this point right here not joining these two lines here but just attach it to this lead right here okay so we're now taking the ripple from this point and putting it through these two capacitors right here completely bypassing this component right here and we're taking that all of that ripple that was originally there and putting it here and that's the reason it made this entire circuit home because we have a path right through here like this and that's why you can't parallel these old capacitors so if something like this happens you're kind of almost fooling yourself into continually searching for that hum because you would put those new caps in there and the problem wouldn't get solved so that's really what's happening so if we wanted to really Orton this up we could just really do this that's really what's happening at this point completely bridging this out and taking the ripple from this point and putting it right here so that's the reason the home didn't disappear when we paralleled those capacitors originally okay let's check to see if the watt meter works I've got my light socket and a 40 watt light bulb so I'll screw this in here and now just before I completely connect this here what we want to do is move this just until the shadow angle just starts to open when that shadow angle just starts to open this should point to the appropriate wattage so I'll give you an example here so I'll just turn this on there it is let's get that out of the way so now what I want to do is I just want to rotate this until this opens and you can see you know it's starting to open about 30 to 35 watts so it's a ways off so I'll just remove this one here now a 200 watt light bulb I'll spare you from that on camera I don't think we'll be holding this for too long either so I'm gonna have to make this quick it's a big bright light bulb okay so here we go and it's not too incredibly far off this seems a little bit high just remove that it's not too bad not too bad at all let's seem like it's a little out doesn't it let's go through and check just make sure that everything is okay in that circuit let's take a look at the way the watt meter portion of this circuit works in this small signal tracer it really is quite a simple little circuit but it uses a very specific little component it's quite simple as well so I'll explain how it works on the schematic and we'll take a look at some of the components in the unit just to make sure it is working okay because it did look like it was off a little bit so first of all if we close switch four on the face of the unit which is the main power switch that closes this switch here and it takes this portion of the line cord and puts it at this point of the winding of t3 t3 is this little transformer here so this end of the winding is placed in series with the load so whatever we plug into the face of the unit and that's this outlet right here whatever we plug into that outlet is placed in series with this winding so what does that tell us about t3 well t3 has a pretty large primary winding it's a pretty special transformer and there won't be many turns in that transformer as well so what's going on the more load that we place on this out letter at this outlet here the more load it puts across this transformer the more we load the transformer the more voltage is going to be present on the secondary now you'll see that we have a 50 K ohm potentiometer across here with the wiper that goes to a diode so that 50 K ohm potentiometer is on the face of the unit here and they've written in calibrated steps in wattage all the way up to 500 watts and it actually exceeds it if we go right to the end there right so chances are this transformer will deal with more than 500 Watts so what happens is the more load that we place here the more voltage we're going to get on this portion of the transform what's happening here is you can see we have a diode with the cathode end that runs directly to the chassis so that means that we have a negative supply right here so it's creating a negative supply because if we're feeding the positive rate to the chassis and we're running the negative off this way we have a point 2 micro farad capacitor which is just acting as a little bit of filtering you can see we have a negative line that runs off here and it goes through the switching and it's going to end up at the grid of this 1629 I tube what's gonna happen is the more load that we draw at that outlet the higher the voltage is going to be so it's going to be more negative so when I say the higher the voltage so if we start at zero it'll be like negative 1 negative 2 negative 3 the more load we pull it's going to go to negative 4 and negative 5 in that direction the higher the negative voltage on this line here the more the shadow angle in this tube is going to deflect so what that means is the more load we pull here the more that shadow angle is going to close then what we do is we calibrate this until we can open that shadow angle again and when we get a little bit of a gap we look where that needle is pointing on the face or the pointer on the knob is pointing and it should point roughly to how much wattage we're drawing at this outlet and that's just how simple this circuit is nothing more than that so what we want to do is we want to check the validity of a few components and of course the major suspect here is going to be this one in 30 for a diode so let's check that out first so what I'll do is I'll move the eye tube out of the way it's just clipped in here like this there is no power or anything connected to the unit at this time you just take this out and lean this down here and we can get right to the VR and as we can see that little diode is right on the VR so what I'll do is I'll go get some test equipment get it placed on the bench here and we'll check some components the first component that I'm going to check is the diode and I'm going to check these components in circuit using a curve tracer one end of the diode is attached directly to the chassis and the other end of the diode is attached to the wiper so what I'll do is I'll just get the focus on there so you can see that a little clearer so you can see one end of the diode it's attached right here and the other end to the wiper so what I want to do is I want to make sure that the VR is roughly centered before I make any tests so I'll take the common lead of the curve tracer and put it to the chassis and I'll take the signal lead and put the signal lead right to the wiper so black lead is a common lead here and then what I'll do is I'll just move the focus over to the curve tracer and we'll take a look see if this looks like a diode and that does not look like a diode that looks like a resistor really low value resistor so chances are we already have a problem here so earlier on smooth the focus back over here again earlier on I replaced the capacitor that was right here it was a ceramic tubular style capacitor most likely paper and foil inside and a bunch of epoxy on each end so I replaced that just to be safe so I'll test this capacitor just make sure that it's okay so if the capacitor is okay I should get what looks like a circle on the curve tracer screen so I'll just move the focus back over to the curve tracer and here we go and that capacitor is fine so now if I want to check to make sure that this VR is working okay so it's working through its travel now we looked at the i2 but earlier and we saw that the i2 moved open and closed pretty easily nice and smooth I guess you should say so most likely Vieira's okay but I'll show you what it looks like on the curve tracer so I'll go from one end to the center here so I'll move the focus back on to the curve tracer so I'll go from this end here to the center of the VR and I'll rotate this so as I rotate this you'll see how the trace is twisting and it'll go right up to the top and that's a short circuit now you'll notice how it kind of popped into place so I'll just rotate this VR on the front of the unit here very slowly you'll see that nothing seems to happen it remains like a dead short for a short period and then all of a sudden boom it just pops into resistance at a very small spot that's very common for a lot of VRS so if you ever see that when you're using your curve tracer don't worry about that and you can see it moves smoothly throughout its travel there is no problems with this VR and of course we verified that by looking at the actual eye tube on the faces I rotate the VR you could see that the eye tube is slowly closing and opening so no problems there so so far it looks like this diode is toast so what I'm gonna do is D solder one end of this diode in fact I might be able to just do that right now see if I can just lift that lift one end of this thing here I can that's nice it was a wire wrapped okay or they had didn't curl the lead under I'm thinking to myself oh this is not gonna turn out good but that was actually pretty smooth it's surprising sometimes things go like that so let's take a look at the diode again so the wiper end is open so I'll just you know that die was lookin pretty ugly this diode has changed its form to a resistor it's changed its state it no longer wants to be a diode anymore it wants to be resistor this diode has resistor NV so I'm gonna remove this diode here and I'll show you what a proper diode looks like so I'll just get that underway and I'll be right back let's take a look at the original diode the diode out of the watt meter on the curve tracer so this is a little one right here so what you see here is I've got the curve tracer attached to my digital voltmeter so when I designed the circuit board inside this curve tracer I also gave it a sleep circuit as you can see when I designed the circuit board inside this curve tracer I wanted to give it the option of having an external digital display to take a look at the forward of diodes and the reverse of diodes Zener diodes and you know check out transistors and all sorts of things like that so that's all built into this you basically just attach this to any old digital voltmeter or anything you have and it gives this old oscilloscope or which is now converted into a curve tracer a digital display it's kind of handy so at any rate we can look at the forward of the diodes here and if we were looking at a Zener we could look at both forward and the reverse so I'll attach this into here and we'll wake up so there we go and as you can see the original diode is looking pretty nasty diagonal line indicating resistance if it was a dead short it would be straight up and down so it's slightly resistive so the one in 34a is a germanium diode and i've got lots of glass diodes tons and tons of glass diodes you can see here of all sorts of different makes and sizes and shapes the this one here's a really high quality it's got gold leads on it and this is another one that I picked out of my junk bin so if I test this one here with the gold leads first so I'll put this on the scope here so on the curve tracer we can see that this is the forward here so if I turn the voltage down and keep an eye on this here we should take the corner at around point six of a volt so you can see we're at point five of a volt and it'll start to take the corner right there you can see how it's just starting to take that corner so that's the forward of this diode right now about point six so I know that this is just a standard silicon diode so germanium diodes have a really low forward of around about 0.2 or 0.3 of a volt so I had to go through my junk bin and I found another glass dial long leads on it like this they're not gold but that's okay so you can put this in the thing any which way you want and as you can see I'm at 0.6 of a volt but I'm well into the turn so if I back this off and bring that right down to where it just about starts making the turn you can see it's making the turn about 2.24 of a volt so you can see that this is definitely a germanium diode right down there right see it's making the turn in there and it's well into the turn about 0.3 so you can see definitely germanium so this is what I want to replace this nicely shorted device with now the specs of this diode aren't really too incredibly important as long as it is a germanium diode reason being is the voltage on the side is so incredibly low you only need maybe negative five negative six bolts something like that so you can see here wattmeter negative five point six volts at zero Degree shadow angle right so we're dealing with really low voltage on this side so pretty much any germanium diode is going to be just fine to replace this one in 34a and that's what we have right here so you can see it's coming into the turn it you know 0.24 it's really low as well under the turn at point six of a volt you see that so definitely germanium this is going to go into this circuit i'll get that installed and we'll check the watt meter again new diode installed let's take a look and what the forty watt light bulb does here we go okay just tell it opens right there look at that that's what I want to see what do you want bulb okay let's try that two hundred watt bulbs yet the incredibly hot light bulb whoa that's bright okay so here we go pointing right at 200 it works great here's an example of how the paint turned out pardon the fingerprints here and there for me holding it that finish turned out really really nice I really liked that hammer tone finish see the case here it's not sure gonna look a lot nicer than that ugly beige color that it was issued in so I'm looking forward to getting this thing all back together and seeing how it looks all assembled so the next step for me is to get the line cord into the unit with the back of the case on and I should be ready to assemble this thing I'm not gonna put the handle back on I don't like the handle it probably just put some screws in here to fill these holes so that it looks like something's happening here but I really don't like those handles and that allows me to put other pieces of equipment on top of this as well when this is on the old-time workbench so I'll get this all assembled and we'll try it out the signal tracer is reassembled and I think it looks absolutely great in that case the color of the case and the grille really sets off the face and even the colors of the binding posts it just looks so good I don't know why they didn't issue these things in a darker color like this instead of that horrible beige color they put on these things at any rate it works incredibly well give you an example so the gain is right off so it's right at the stop I'll just put a standard cable on here that's what they gain off I'll give it just a little bit of gain that's just a bit of game that's picking me up so if I touch the case it gets quiet this is shielded right up to this point and it's still hearing me so you know what's gonna happen if I touch this right it's just gonna get incredibly loud whoa I couldn't even imagine if I had this pointing at 12 o'clock it would probably destroy the speaker incredibly sensitive unit so this will be great for signal tracing I'll put on an RF probe that I have just right here is a very sensitive RF probe I designed some time back highly sensitive so this is for sniffing noise crackly components and things in circuit so let's turn this up so it's just picking up switch-mode noise right now so I'll bring this close to the microphone that noises the camera so that's a switch mode or a bunch of stuff going on inside the camera maybe the switch mode supply that's powering it it's bringing close to another one it's another lamp incredibly sensitive know what else I have here that makes noise anyways RF probe very sensitive you can see if I I'm holding the the ground of the RF probe just to keep it quiet but if I shield it with my hand it's just picking up random switch mode noise gets louder when I open this towards the microphone in the camera so very very sensitive the gain is hardly even just moved there look at that so the noise works well there's a little bit of a thing that you need to know about the noise circuit whenever you click these things on to noise it puts about a hundred and some-odd volts on the probe so if you don't know that don't use the noise section in one of these noise tracers and definitely your signal tracers you got to be very very careful with that because you can get a pretty decent zap now as a high value resistor in line with this so that you can listen too noisy components and I just happen to have when I saved one here else to reach over here and grab it and I'll do a comparison for you so there's two resistors here high-quality allen bradley style and even 2% tall or it's and then there's an old round e here so well like yeah take a listen to both of these things so it's not on noise you can see that I have it on a trace only you need to remember this with these signal tracers or you'll buzz yourself so this is a just F a newer style allen-bradley resist a very good resistor so turn the gain rate down that's me so I'm gonna hold the negative here just so it keeps me quiet I'm not touching that portion of the probe because that's live right now and you can hear that's pretty quiet very quiet look at how much gain I'm giving this thing so I'll turn this back down I'll turn it back to trace because I don't want to touch this so I'll put this resistor in and this one's like the ocean so let's take a listen to this one put this back on noise listen to that can use this thing as a static generator so just think if that was in an audio circuit very very noisy component quite the difference so the noise circuit works very well and the nice thing about the watt meter circuit on this is when you click in at a watt meter this gets nice and bright see that it makes everything go quiet switches a bunch of things out of circuit inside and you know I'm ready to use the watt meter and I demonstrated that earlier that works just perfect very very well so all in all I'm extremely happy with this and I'm glad to add this to the old-time workbench it is coming together and the Vita Lizer is getting close to completion that is such an incredible project that I'm putting smaller projects in the middle so that I can you know get some videos up for you guys and of course it gives me a break from that thing because that thing has just been all-consuming so yeah that's been quite the project but this one is done and it looks fantastic and works fantastic and I'm looking forward to using this with you guys and showing you how to troubleshoot circuits with this signal tracer very very happy with the results I'll share some vacuum tube knowledge with you that not a whole lot of people know I'll start with the filament system here so we noticed in the signal tracer that the filament lines were twisted together like this and pressed close to the chassis and the reason that's done is to stop the filament system from interfering with other parts of the circuitry to keep the hum down this is pretty common a lot of people do this and this is you know pretty well known so usually this is part of a balanced AC filament system you know a transformer with a center tap running to the chassis or just a hum balance control so the filament lines have to go up to a vacuum tube to light the filaments so if we take a look at a really high quality vacuum tube and we look at the filaments inside so this is a really sought after vacuum tube you can even see the little Telefunken diamond on the bottom there so if we look at the filaments inside you can see those little white wires they run up inside cathodes sleeve turn around and come back out again and it's the same on the other side because there's two triodes in here you can see the little filaments and they're running up into the cathode sleeve and then coming back out again so again a very high quality tube so if we want to take that to the next level go to something with extreme quality we can go to this 5751 five star vacuum tube made my General Electric it doesn't get a whole lot better than this so you can see the filaments here they run up inside the cathode sleeve and come back down same what this one runs inside the cathode sleeve right up to the top and then comes back down again right again to triodes now when we talk about high quality tubes suited for audio we look at a 12 ax7 everybody knows what the 12 ax7 is but there is the 702 v version you can see that 702 v a what's the difference in this vacuum tube take a look at the filament inside that vacuum tube what's happening with that filament in there notice on that side right there what have they done with a filament that's how you spot a very high quality audio tube and if you're ever building an audio amplifier you want to look for the filaments that do that inside top-quality tube right here 56 91 look at the filaments inside that tube another really good audio tube designed specifically for audio you can see the cathode rate in the center of that tube right there look what they've done and that's the difference between a high quality audio tube and just a high quality tube so whenever you're designing an audio amplifier and whenever you're wanting to search out some really good quality tubes those are the things you want to look for you want to look for those twisted filaments for the lowest hum possible I'll talk more about this in a future video and we'll actually do some experimentations with hum and all sorts of things so there you go some vacuum tube knowledge that isn't normally talked about or discussed thanks for stopping by the lab today I hope you're enjoying me sharing this information with you and the repair of this st - to signal tracer if you are enjoying my videos you can let me know by giving me a big thumbs up and hang around there'll be many more videos coming in the near future we'll be taking a look at vacuum tube and solid-state equipment alike so there will be a lot of troubleshooting repairs restorations and even some circuitry design on this channel so if you haven't subscribed now would be a good time to do that as well if you're interested in taking your electronics knowledge to the next level and learning electronics in a very different and effective way you're going to want to check out my ongoing electronics course on patreon I'll put the link just below the video description so just below the video description is a show more tab written in capitals if you click on that it will expose the link you can click on the link and it'll take you right to patreon I'm also sharing many of my designs and inventions up there as well so there's a lot of custom pieces of test gear that you just won't find anywhere else lots of really neat things going on on patreon alright until next time take care bye for now you

Channel: Mr Carlson's Lab

Views: 485,715

Keywords: learn electronics, fix electronics, repair electronics, Vacuum tubes, radio repair, troubleshoot electronics, test equipment, Mr Carlson's Lab, tube audio, electronic lab, audio repair, amplifier repair, electronic restorations, curve tracer

Id: uGXQc-zanWg

Length: 113min 32sec (6812 seconds)

Published: Tue Mar 05 2019