Electronic Circuitry Diagnosis And Fault Finding! [Repair]

hi everyone and welcome to another episode of mr. Carlson's lab today we're going to do something just a little bit different I have a radio from the late 30s era and what we're going to do is just replace the components that absolutely need to be replaced under the chassis so we're just gonna replace the components that are stopping the radio from working just the failure points so what I'm going to do is bring the radio up slowly on a current limited isolation transformer and variac supply just to make sure everything is nice and stable and after that point we'll start the diagnosis and troubleshooting procedure together so it should be a lot of fun let's get started here's the radio that we're going to diagnose and get working today so one by one will replace all the components that's stopping this thing from working so we'll trace down all the faulty components and slowly bring this thing back to life and we'll see just how many components we have to replace to actually make this thing work now I have noticed that somebody has been in here at some time and they've replaced the line cord I have a schematic for this thing and I'm pretty sure I know why they replaced that line cord because some people probably got a pretty good shock off of this thing over time and I'll show you exactly why the build of this radio is just a little bit different than all American five and all Americans six radios to the point to where they've actually tied one part of the line cord directly to the chassis and it's a direct wire rate inside right through the switch directly to the chassis so if you were to plug this thing in backwards with the original non-polarised cord the chassis itself would be live arenal with 120 right on it and of course having 120 on it if you were to you know come across that touch just the chassis of this thing that would be I guess you could say a very enlightening experience definitely nothing that you would ever want to come across so we'll take a look at that here as well so what I'll do is I'll turn the radio around and we'll take a look at the back side here in just a moment it looks like the dial moves so there's no problems with the dial the on/off switch clicks and it feels pretty smooth now there are two bands on this radio there's a broadcast band and on the bottom it looks like it goes from the top end of the broadcast band through the hundred and sixty meter band right to the top end of the 80 meter 75 meter band rate up to four mega cycles or four mega Hertz and today's speak so it has a bit of a shortwave area in it and it also has a broadcast band so when we're testing it we'll just use a broadcast band because that's the most used one and there is going to be quite a few active stations on the broadcast band around here so that'll help us troubleshoot this so what we want to do is we want to troubleshoot this just the easiest way possible want to see if we can get this thing working with you know a minimal amount of test equipment and I'll show you multiple ways of finding the problem we'll use different pieces of test gear and we'll work our way to the problem with different pieces of gear so I'll get the radio turned around we'll take a look at the back side see what we're up against there I'll show you that line cord and then we'll take the thing apart and bring the thing up very slowly on a very AK with current-limiting so whenever you get a radio like this you never want to directly plug it into anything especially of this age because the capacitors inside they they go bad they short out and then what happens is you plug the thing and turn it on and then you destroy a tube so the tubes are getting kind of pricey for these older radios now and a little bit harder to find so you don't want to take that chance and that's why I'm putting this on a current limited isolation transformer and variac supply because again one end of the line cord is tied to the chassis so I want to be as safe as possible here's a closer look at the back side of the radio and one of the things that I immediately noticed is somebody's name is written right on the if' transformer here now this was common way back when when the person brought this thing in to the repair shop the owner of the repair shop would usually write their name on the chassis so they'd write the owner's name on the chassis especially if there is more than one of these radios in there so that way the the actual owner of the radio got their radio back instead of say maybe a radio that looks so it was really common back then to see this and it's so clean still nobody's taking it off it's just kind of nice now obviously as mentioned earlier this has been in so somebody's been inside this thing at some time because they've replaced the line cord with a polarized line cord now I have the schematic for this thing and replacing this with a polarized line cord without rewiring a portion of this radio doesn't make it all that much more safe then I'll explain that here when we take a look at the schematic so oddly enough if they've attached the line cord correctly when the radio is on so when the switch is turned on it's a lot safer than when it's turned off kind of interesting so I'll explain why this is here in just a little bit so keep in mind way back in the day polarized line cords were not a thing so it you could plug your cord in this way or you could twist it around and plug it in this way it could go in either way which gives you a 50/50 chance of putting the hot side of your line directly on the chassis which makes this thing pretty dangerous so there are ways to make these radios a whole lot safer and I'll talk about that again as we start looking at the schematic here so we can see that somebody has replaced the antenna line with some form of it looks like a piece of line cord it might actually even be a piece of the original line cord I would think that this maybe has a cloth line cord on it when it was original so this just might be patched in there so we'll find this all out together so I've only taken a look at the schematic we together are going to discover what is wrong with this radio you know at this point as much as I know about this radio other than me just having a quick peek at the schematic so it should be interesting to look under the chassis and see what's actually been done whether they've replaced any components with this line cord or whatever whatever it takes we'll get this thing working piece by piece so we'll replace only the components that are necessary to make this radio operate so we'll troubleshoot this thing maybe we only have to replace one component maybe two maybe five and to make this thing run we'll find out contrary to popular belief usually the tubes are the most dependable part of the radio a lot of people think that whenever something goes wrong with a radio like this first of all Oh a tube is dead actually usually not usually it's something underneath the chassis the youtubes are usually very strong so again we'll find this out together now this radio was around before the standardized All American five and all Americans six style tube radios were out this is 1937 or about is when this radio was made now in those years they use tubes with filament voltages that don't add up to the standard line voltage okay so in the All American five and All American six radios if you add up all the filament voltages they would add up very close to you know the standard line voltage well since this is going to be low so if we add all of these filaments up it's gonna be quite low it's gonna be way lower than say 120 115 volts maybe even 110 so what they did way back when is they installed a ballast tube and this really is just acting as a big resistor making up for all that filament voltage that's missing because this is shy quite a bit so in other words okay so what happens is is way back when the Christmas light strings that you used to buy from the store all the Christmas lights were attached in series okay so if one filament burnt out in say you had a Christmas light string with twenty lights in it okay and one light bulb burnt out the whole string goes out that's not like the modern ones where they have that little wire in the bottom that shorts the the burnt out bulb back in the day it wasn't that easy all the lights would go out so you'd have 20 say 20 light bulbs to test out to see if you know which one is dead so you could make your string light up again okay well this is set up in much the same way all the filaments in here are attached in series so if one of the filaments in one of these tubes burns out all of the tubes go out so the next would be to go to your local drugstore yes the drugstore so what would happen is most all drugstores they had a tube tester in them and you brought all the tubes from your radio to the drugstore you would take the tubes and test each individual tube so you'd have to set the tube tester up for each individual tube test that tube if it works you put it back in the bag and then you go on to the next tube and you keep testing all of the vacuum tubes until you find the one with the open filament so let's just say your radio has six tubes in it alright and you had you tested all five and you're down to your last tube and finally you find out AHA this is the one with the open filament you would call the owner of the drugstore he would come over and underneath the tube tester they had a little lockup area they would open up the lock-up area and say the dead tube was a six Delta six or six D six okay he would look through the stock oh here's a sixty six you would pay for that tube you would bring the tube home plug it into the radio and into the area that had that dead tube and your radio works again you know putting all the rest of the tubes in of course it was very common for radio owners and even television owners way back when to repair their own televisions and radios it was just a standard thing that people knew how to do way back when kind of interesting so of course nowadays everything is solid-state and there's no vacuum tubes in it nothing is hooked in series like that so you know usually when something goes bad you know the component has to be D soldered but they made it easy you could just pull the tubes out of the socket so you say this was the bad tube alright so I'm just gonna rock this gently so I'm you never want to pull straight up like this and then you know pull a lot of force cuz what you're gonna end up doing is the sockets gonna let go the tube and you're going to smash the tube and in the top side of the radio so here as you can see this is one of the tubes this is the rectifier tube right here right then you just take this open you would head off to your local drug store with all the rest of them plug it in and test the tube out and that was a very common thing to do way back when so again all the tube filaments in here do not add up to 120 110 you know 115 volts anything like that so they put this large resistor tube in here to make up for that missing filament voltage and as you can see this one here it has seen quite a tough life it looks like there's almost rust inside the bulb so hopefully the ballast tube this is known as a ballast tube it still works if it doesn't no big deal we'll put a bunch of resistors in there and and I get the rest of the string working so we can continue troubleshooting this thing but hopefully it does work that would be nice so now recognising these tubes is really simple just by the way that they're put in I should say recognizing their function okay so a ballast tube has filaments that are just strung up and down like this inside so this again is just acting as a big resistor to make up for it say the missing filaments okay so very easy to recognize that this is a ballast when we look inside here we see two sections in here and we see two cathodes with the filaments poking up out of the center of the tube there so what is this simply it's just two diodes inside of one tube so this is the rectifier tubes that's taking the line all right and it's rectifying that and we get the correct voltage for the rest of the the tubes in here and it's rectified so it's direct current then underneath the chassis we have some filters to filter that off so it's nice clean direct current for our radio so if we wanted to if we put a resistor across the filaments of this tube so that you know we still have a series connection in here we could replace both of those diodes with something as simple as a 1n for double-oh-seven teeny little diode would replace this big tube and of course we would need the resistor to make up for the missing filaments again so rectifier tube right here so taking the line changing the line to DC for all the rest of the tubes okay aside from this this is the resistor that's in series with all of them now they usually place the oscillator modulator or oscillator mixer depending on who's designed it and how they classified it right close to the tuning capacitor this is the tuning capacitor here and there's two sections on that tuning capacitor and they're gay together so that means that both sections move their can in conjunction with each other so they're the same one portion of this capacitor toons the oscillator and the other portion is in making an attempt to try to tune the antenna okay so the long wire or whatever type of antenna it's going to try and tune that so that you get maximum sensitivity out of this radio so oscillator and mixer or modulator now the if' amplifier tube is very easy to spot because 99% of the time it sits between the if' transformers I have transformer AF transformer I have amplifier 2 pretty simple just that easy so this is the if' amp and then after that we need to detect and we also need to amplify that signal so the next one down the string would be the detector and audio amplifier tube and that would be this one right here so you can see the path already is going this way and the only tube left is the audio output tube the tube that drives the audio output transformer which drives a speaker and that would be that one way back in the corner over there like so now this radio here has again it's not the standardized All American five style tubes or octaves or anything like that these are old glass tubes and whenever you see a tube that has this little dip on the side of the bulb these are known as shoulder type tubes or st style tubes now before Rogers had you know brilli gotten known for their spray shields they used to put a shield on the outside of these glass tubes known as a goat shield like billy goat goat shields so these are commonly known as now this tube here is underneath this metal shield so that this tube does not see that tube and this tube does not see this tube they shield the tubes so that they don't oscillate with one another so for example if you have a guitar amplifier and you take you plug a microphone into that guitar amplifier and you bring the microphone close to the speaker so you bring its own microphone close to the speaker what's gonna happen well it's gonna start to squeal and make all sorts of noise that's because it's oscillating what's happening is it's causing feedback it's squealing in there while tubes do the same thing when they see each other they'll start to squeal especially if they're you know a high-gain stage so the way to stop that is to stop one tube from seeing the other tube and by doing that you shield the tube okay now when this tube would oscillate with this tube it's much higher than the audio frequencies that you would normally hear so if you put a microphone close to a speaker on a guitar amp it's gonna squeal and it's gonna be audible so or audible so you just squeal is really loud right you can hear that well this is gonna be oscillating at a much higher frequency so you're not gonna hear it or anything but what's gonna end up happening is these tubes are just gonna cause all sorts of problems with one another because they're feeding back but in a much higher maybe at an RF frequency or something like that so in order to stop that they put metal shields on the tube to isolate the stages a little bit better now as time went on into the 1940s they actually put in metal shields on the tubes themselves and we'll take a look at those particular types of tubes down the road here in the future when we service another radio that has those metal tubes and it'll explain that then so these tube shields here have a little cap on the top which also shields the grid cap so the signal goes in the top of the tube here and then it comes out on the bottom side of the chassis and that's with this tube and this tube here and you can see here we also have a grid cap on the top of that tube there now one thing you have to be very very careful with is if you're working on something like a transmitter or an old transceiver say like a Yaesu Kenwood EF Johnson Heathkit whatever like that not all of the caps on the top side of the tube our grid caps some of them are the anode caps so if you're working on a transmitter some caps on the top of these tubes can have between say 700 all the way to 4,000 volts on them and the caps are exposed inside these transmitters and things like that so you need to know your vacuum tubes so say there was a large ham radio amplifier okay and it had big tube inside hitting you and there's a cap on the top of the tube that is not a grid cap that is a very high voltage point a high voltage point that would kill you instantly if you were to touch it so these are all things to remember now in this particular radio these are all grid caps so there's just signal coming in here there's no you know crazy high voltage unless something really goes wrong inside the radio there's no high voltage really present on them but again you always have to be careful especially with any exposed connections inside of an old radio like this because you don't know what anybody's done somebody could have been inside this thing and butchered it up really bad and who knows what's happening again we're gonna discover all this together so I'm gonna go over all of that safety stuff and show you how to recognize tubes that have plate caps and grid caps and how to know the difference we'll do all that here in the near future I have a bunch of really big transmitters that we're all gonna go through together and we'll get them running as well so that should be a lot of fun down the road here so really what we need to do is attach this line cord to an isolation transformer and current limited variac supply and I'm going to do that what I'm going to do is I'm going to troubleshoot this thing just the way it sits right here we'll see what we can learn by just troubleshooting it just like this so I'll get everything hooked up then I'll explain that here in just a moment the radio receiver is attached to an isolation transformer and current limited variac supply so basically what that does is it limits the amount of current that's supplied to this radio receiver so if this radio receiver say tries to draw a little bit too much current that device will limit that so the radio cannot be damaged for example say there's a shorted filter capacitor or something in here I don't want that to destroy the rectifier tube so that supply will limit the amount of current that's available to this radio receiver it's also attached to an isolation transformer the isolation transformer eliminates the chance of this chassis being attached to the AC line so basically it's like a one-to-one transformer that isolates the AC line from the chassis a very very important thing to have for safety whenever you're working on any type of radio were one side of the AC line could be attached to the chassis I've talked about the dangers of this in other videos I'll quickly go over this again say this didn't have a polarized line cord and you plug this in so that the chassis was hot if you took an oscilloscope probe and attached the common from a normal oscilloscope probe right to the chassis it would blow the oscilloscope probe right off you would get a huge flash a lot of the times it damages the oscilloscope things like that so isolation transformers are very very important to have whenever a radio like this is on the service bench extremely important to remember all right so what I'm gonna do is I'm just gonna turn this thing on and I'll let it warm up and we'll see if there's any sound coming out of this I have this attached to an antenna right here this red lead runs out to an antenna and we'll see if there's any sound or anything that comes on so that's a good sign the dial light comes on in there and the bulbs on the dim bulb tester did light up just a little bit when I turned that on so we give it just a moment here to warm up since this has a ballast tube in here it'll take just a little bit longer for this to warm up and not only that the line is reduced so the line voltage is reduced down just a little bit as well as being current limited just to be really safe with this thing again I don't want to damage this rectifier tube and have to go and hunt one down and find another one to replace it because you know I kind of want plug the thing right in and tried it so very dangerous thing to do again with something like this is you never ever want to plug something like this in directly into the wall without it being properly serviced because again if anything is wrong on the underside of the chassis you could ruin a very expensive tube ie the rectifier here so it's glowing I can see it glowing what'll do is I'll just zoom on into that bear with me here alright see the filaments glowing in the top there so it's on and it's normal not to really see any type of a glow in a ballast tube but the rest of the tubes the cathodes would be glowing in them as well so I'll just back the lens out here a little bit right to about there and give it some volume and the thing is just silent moving around absolutely nothing try the band switch it's not even humming or anything it's just so quiet dead silent so at this point I don't know if there's any B+ or anything happening cuz I don't have this out of the cabinet and I'm not using a voltmeter at this point so this has a field coil speaker in it basically an electromagnet speaker so this is before permanent magnet speakers were around they actually had a coil wrapped around it and that would magnetize the speaker now if there's no B+ there'll be no magnetism there will be no B pluses the positive that comes out of here by the way so there'll be no magnetism on the speaker anything like that and it's sitting silent like I mean there is just no life here whatsoever so what I'll do is I will grab the Carlson super probe and we'll just sweep around in here just with it just like this and we'll see if there's any signals or any oscillator anything's happening with the thing just in the case just to get an idea real quick and we'll see if there's any hiding life happening inside here that we can't tell boy that's just dead silent I have the Carlson super probe on the bench here so this is just an amplifier the probe itself has got a bunch of circuitry inside and this is just an external speaker that's all it is so there's some batteries in here and this is the power for this probe as well now I release this on YouTube quite a while back if you look down my videos list you'll see Carlson super Pro you can see the schematics and how this thing is built and everything like that if you want to put one of these things together for yourself if you're really serious and you want to build the actual circuit boards the actual circuit boards for the amplifier and the parts lists and everything and everything is already sized and everything that's all up on patron so all of the layouts are already sized you can just print off your own circuit boards and you know make an exact copy of this pretty much if you'd like in fact many people have and they've shared their projects up there so this is on right now everything is on and what I want to do first of all is see if there's any B+ or anything happening in here and the easiest way to do that is to use this thing to see if the oscillator is running if the oscillator is running this little LED on the backside of the probe will light up really bright when I put this close to the oscillator so I'll turn this thing on like so I have the game turned down there's a different switch setting for RF and audio on here as well so I'll attach this here unless I'll just slide this in here and see if the oscillator is running so the oscillator is running inside this thing that means that there is B plus there is DC flowing in here right now there's just nothing's coming out of the speaker which is kind of interesting so the oscillator is working which means that we might actually be receiving radio signals so the first thing I want to do is make sure my actual antenna is receiving something so this thing is so sensitive that it will just in fact I could just leave it on this side and just go close to the antenna wire so I'll turn this up really sensitive and [Music] so you can hear its receiving stuff on the antenna so no problems there so just holding this close to the antenna wires all you need to do to see if the antenna is actually receiving anything you get your Morse code in there and everything so the next thing I want to do is I want to bring this close to the grid cap of the modulator oscillator sometimes known as a converter mixer whatever I want to see if I can hear any radio stations with this thing so I'll put this here and I do so I'll give us a bit more audio [Music] and I'll just tune the dial around [Music] the whole band is active so what that's telling us so far now what that's telling us is that the signal is getting into here through the antenna coil and into this and it's actually tuning it so we've verified a whole bunch of things just by doing this first we know there's B plus here so the filter capacitors must be doing some form of work because this I don't really hear all that much Hummer anything present I'm hearing radio stations which means the signals getting in from the antenna into here and this is tuning it so next thing I want to do is I want to see if that signals getting right into the if' tube alright so there probably be just a little bit of signal reduction here so let's see if we can hear anything here just listen to the grid lead here I'll move this around [Music] you can see the LED kind of get a little bit brighter when I hit a an actual radio station [Music] not incredibly strong so you can hear anything like this no that's more of an audio setting there is signal there you hear that [Music] those are both the strongest signals I think right up about this end here you can see get bright when I get on to the station so there is signal getting through the if' transformer into that first if' tube right here and then that would be amplified and then run out to the detector now I don't think we're gonna hear anything at the detector let's find out [Music] oops ground clip popped off there [Music] I didn't think so because the signal is pretty faint at this point rate here as well so we already know that the thing is trying to work just nothing's getting out of the speaker so most likely the problem is dwelling in this area right here because it's working up to this point it got a little bit weaker here and nothing here so I would say I have audio section somewhere in here is a problem now since the speaker is dead silent it's that you know raises concern of the actual audio section itself so the only thing to do now is to remove this thing and let's really get into it let's you know probe around with this thing and see where the signal goes missing and then we'll also take a look with an oscilloscope as well show you how to do that with an oscilloscope and find everything out now now don't get me wrong finding the signals in here and locating this stuff with an oscilloscope at this point wouldn't be possible it's not as sensitive as this device right here this device is actually listening to the antenna line as you heard so we could hear just random signals on the antenna line right so if I go back to the antenna here you'll hear this [Music] so you can actually hear signals coming in on the antenna line with this thing that's how incredibly sensitive this thing is right now it's so sensitive it's just it's lit right up because of the sensitivity here right so an oscilloscope is good for you know troubleshooting if you actually want to tie in two points or get very close and couple in like if you bring the probe very close to a high signal point sometimes it'll couple into the oscilloscope probe so that works fine for that so you know two different you know tools for the job but I'll show you the difference between the two here and we'll get this thing running it looks like it's just a book there honestly you mean oh there's you know radio stations in here you know [Music] this year so we're well on our way at this point already so it's working at this point just not the radio it's needing this thing to make noise out of this speaker right here the radio was unplugged from the supply and it's been sitting for a while to let the tubes cool down getting inside this radio it looks relatively easy so the first thing we need to do is take the knobs off of the radio because the chassis comes in from the backside pull out one off those come off really easy I'm really lucky these things didn't get lost they almost feel like they could just fall right off and then the next thing to do is to remove the feet the feet have little screws right in the center of them so I'll remove these feet and this thing should probably just come right out so I'll take the two bottom ones out first you can already feel the chassis letting off there so what I'll do is I'll just put my hand on the backside of the chassis so it doesn't want to fall right out and this one right here and I think we are in like at that mess on the bottom side that's quite the mixture of old and newer components put that over there on the floor behind me the case so I already see a mixture of different parts and lots of original parts so very few parts have been changed in this thing looks like this this and this have been changed in the line cord so far and I think that's yet so these are the filter capacitors this is a filter and a filter here somebody's put in in these orange dip style capacitors and of original parts Wow okay what I'm gonna do is reposition the camera zoom on in on this and we'll take a closer look in fact you know what let's take a closer look at the radial first look at that speaker that looks like it's just pristine I'll move it make sure it's not grinding no grinding no anything that's nice nice and fluid no problems there this looks like the factory filter can overhear and it may actually still even be hooked up who knows here are the tubes that let's say 25 l6 that's a nice-looking little tube there nice compact little chassis nice and solid audio transformer field coil speaker you can see the electromagnet wound around the speaker here lots of whines of wire there and the filter I can right there is poking up through the top wow that's really quite a large filter can and just lots of original looking capacitors coils and such well it's nice to know that it hasn't been tampered with all that much aside from a line cord and maybe a few capacitors the majority of this looks original so this will be a lot of fun to see how many parts we actually need to replace to make this thing make noise should be a lot of fun alright I'll move that camera it'll be right back here's the closer look at the bottom side of the chassis and immediately I can see that they've disconnected one of the filter capacitors on the top side of the chassis here so that red leads got some tape on it they even stalled some other capacitors here so these may have been installed at different times this one here is older than this one right here so the underside of the chassis may have been visited a bunch of times including the line cord so the orange dip is here but all the rest of the capacitors and resistors these are known as dog-bone style resistors are all looking pretty factor so this here has the factory coating on it all these capacitors so you can see c1 0.4 c1 0.5 and if we look at the schematic you can see one zero point five one zero point five one zero point four so those are definitely all factory components which is kind of nice so there's not a whole lot of tampering that's gone on on the underside here so they definitely give us something neat to troubleshoot so looking at the schematic here we can see this ballast here and as you can see it's in line with the filaments of all these tubes so again if one of these filaments was to open up right all of them would not glow they've put the dial lamp across the 15 ohm portion of the ballast there's 130 homes worth of resistance right here so this is just in line with the actual filaments of the vacuum tubes so another thing that's really hard on these ballast tubes is if you ever find one of these tubes on the top side of the chassis and you'll notice that most of the tubes I'll just turn this back over most of the tubes have this kind of a Cromie look inside so some of them have a chrome gutter you can see this is the gutter on the top of this one here it's kind of a chrome look on the inside if you ever see that that's gone a powder white color you definitely don't want to be plugging this thing in because what's happened is that tube has come to air so the seal on the tube has broken and it's really hard on the other tubes and the other components if that has come to air so you definitely don't want to be powering anything up if you see any tubes with a white powdery coating inside so I was talking earlier about the safety of this particular radio and as you can see what they do is they have one portion of the AC line through a switch directly to the chassis now this is this ground is the chassis there are no chassis symbols on here and if we look what comes off the switch right here I don't know if you can see that this wire right here goes directly right over here and solders directly to the chassis right here right there you can see that so one portion of the AC line is definitely connected to this seeing this most likely the reason that they put this polarized plug on but here's the thing so if you were to make this the chassis actually positive it would hot I should say it would make this thing very unsafe so you would you know if just say that this was the hot side and this was the neutral side all right if this was the hot side you'd be running the hot side directly to the chassis but the switch would be in the hot side so you when you open the switch the hot would be broken and everything would be okay that way but you can see that the neutral is intended to go to this side so this radio will accept either AC or DC so you can feed this with direct current as well so if you fed this with direct current and you made this positive and this negative it wouldn't work because the diodes or this rectifier here would be backwards right so it wouldn't do its job so it's intended that this side here is positive or the hot side and this here is the negative and neutral side so what ends up happening here is if you open the switch here alright to the ground on this side here and this side is hot and this side is neutral you're opening the neutral line right to the chassis well what happens is is you have this which goes directly through all of these tubes so if this is hot the hot isn't broken right because we have the neutral side here with the switch in it that's broken here so if this is hot you can see that if you break this connection to the chassis the entire chassis will float as being hot right because only the hot connection will be connected so it'll go through all of these resistors and the tubes right to the chassis which makes us a dangerous situation so in the event that this was to be restored what you would want to do in a case like this is make the neutral solid to the chassis so that it's safe and use an indexed plug or a polarized plug so that the neutral is always connected to the chassis and then move the switch to the hot side and in most cases I'll even add a fuse in a case like this so I'll put a an absolute value fuse in here so this here would remain solid to the chassis and the switch would move up to this side so the hot gets broken that would make this a lot safer than it is so kind of an interesting thing you open the switch and the radio or you turn the radio off and the radio becomes more dangerous in this situation right here which is kind of interesting so one of the things that has to be fixed so over here we can see the field on the speaker is an interesting field connection it takes the B+ through the field directly to ground so this is a 3000 ohm field right here and that's a you know a load for this rectifier for sure you can see that they've taken both plates and both diodes and attached them in parallel so there's obviously going to be a more current passage through this diode less drop across the diode and it just goes right to this field directly to ground in a lot of old radios what they do is they actually run the B+ through the field on the speaker and then the rest of the radio draws current through that field and it magnetizes the speaker in this case the field connection itself is basically just a coil right to ground kind of an interesting setup and you can see here they have this 400 ohm you can see that we have a choke right here and this runs through this choke so we have two capacitors right here one on each side and then it runs up and feeds the rest of everything else so if we look here we can see we have an audio output transformer and we can see we have a choke down on the bottom 40.1 vi is it's part number so let's see if that is the same on the parts list there it is 40.1 sex so this is the choke so all in all when you really look at this radio this is a very well built radio most radios of this era they won't be doing this they won't be doing this again they'll be running things through you know the current through the field and using the field itself as somewhat of a filter so they're you know there's I'll put it this way there's a lot of iron in this radio receiver would have a lot of money to put this together and that's why these fighter radios were more expensive than the rest back then so we have a choke here to again get rid of some of the hum so we have a filter on this side we have a choke and then it runs off to this side through the choke it goes up here into the audio transformer and then of course it feeds all the B+ to all the tubes right here so the most common problem in a radio receiver like this after it's been sitting for an incredible amount of time is these capacitors here both go bad one or each or both and they'll either open or they'll short and if either one of these capacitors goes bad what it does is that takes this rectifier tube out it'll destroy the 2505 so what it'll do is it'll blow the bonding wires off the cathodes if I can take this tube out here maybe I can show you what the bonding wires are inside the tube let's take that out again see if I can see the bonding wires see the filaments on the bottom so I'll try and zoom on into this a little bit here see if we can see the bonding wires okay you see that little strip right there that's running off we'll try and get my little tool right here you see that little strip that's running off right over to there and see if I can point out the other one see this one here with a little hook in it kind of hooks up and then runs over here those are the bonding wires to the cathodes and if one of those filter capacitors are bad these things act like little fuses they just blow right off inside the tube and then the tube doesn't work anymore it'll still glow the tube itself the filaments that we looked at earlier will still operate but it won't work anymore because it's just you know you've actually destroyed portion of the circuit it blows this portion right here blows these little cap these are the cathodes here so it blows a little bonding wire off the part from the bottom portion of the seat the outer portion of the bulb here blows those little bonding wires off so there's no connection from the cathode itself to this line right here again it'll still glow because the filaments are on a separate circuit it's just that the rectifier won't do anything anymore we'll just zoom on out here it's all back into view again so again these capacitors are what causes that problem all the time now when capacitors like this short out this tube here will dry heavy current usually these capacitors end up going bad and what they do is they pass the plate voltage from this stage into this stage and basically what that does is it turns this tube on really really heavy this tube draws out a lot of current and sometimes what it'll do is it will open the audio transformer or cause other damage here so these coupling capacitors are always a troubled spot and we can see these decoupling capacitors here between the two resistors right you can see these capacitors here and here and here and we see all these capacitors all those capacitors are real sore spots in this you'll notice on the antenna circuit here we have a capacitor that's in line with us on tentacle right this a capacitor here is in line with the antenna coil to the actual antenna itself if this capacitor here goes bad and way back one if you had a non polarized plug and you had it plugged in the wrong way and the chassis was hot there's a really good chance that you'll destroy this antenna coil if it's attached to something outside you know or if there's any leakage from your antenna to ground some people had some pretty goofy ideas way back when in order you know to make the antenna work people would do things like you know ground these things to their radiators and sometimes even make the radiator and their host the antenna real goofy things like that and what it would do is it would absolutely annihilate this coil right here on the antenna coil so this is a coil here that kind of sticks out above the rest you see that one right there just end up blowing that coil right off so they open up so it's always important to make sure that this capacitor is good I usually replace this capacitor here with a safety capacitor as well so an X Y rated cap just to be even more safe so this is a real sore spot I've taken some hammer land radios apart the coil on so many of the best has just been blown up so what happens is they try and listen to say the 80 meter Bend and they move the band switch to 80 meters and then boom it blows the coil off and they move it to say 40 meters and then it takes out the 40 meter coil and they've moved this along so a lot of people have done some pretty goofy things with their antenna systems so now what I usually do is put a capacitor in line with the antenna connection to the antenna coil and if something ever does go wrong it protects these coils because these things are really hard to get no and if something like this was open it would have to be rewound in order to make the antenna circuit in this regular receiver perform like it should these are the if' transformers that you saw on the upper portion of the chassis right here this is the if' tube right here in the center so CIF tube right in the center so they call this a converter and this one here sometimes they're called oscillators modulators mixers in this case it's a converter just depends on what the manufacturer wants to call it so there's an oscillator in here that will oscillate see what is the if' of this radio receiver because out here this is peak equals 4 56 kilocycles so the oscillator in here will always be 456 KC's above what you're listening that's how it works and then these transformers are tuned to 456 KC's or kilocycles and then what you're listening to or what's at what the antennas receiving is present at this point right here so what happens is is say we wanted to listen to 1000 a.m. all right so one mega cycle 1000 kill the cycles however you want to call that the oscillator here if the if' chain is tuned correctly so these two transformers here if they're tuned correctly the oscillator would be sitting at one point four five six mega cycles in order to be receiving 1000 a.m. so that's how this works now I explain a little bit more the way the if' section works in some of my other videos i have an eddy stone is it in any stone no it is a echo phone I have an echo phone video you see one video where I really get into explaining how this works so if you want to understand more of how the if' and the converter and oscillator work in a radio receiver that's a really good video I go into detail right there so again this is always when you're tuning the dial the oscillator is always 456 kilocycles above what you're listening to so this is oscillating all the time and you hear those stories about those trucks running around way back in the day with loops on the top to see who had radios while this is what they would listen to they would listen to this oscillator tube here because a lot of the times the oscillator or converter mixer whatever you want to call it inside one of these radios is strong enough that will back feed the signal out of the antenna so your antenna is actually transmitting the same time it's receiving so again this is the reason that they used to have thing called pre selectors and in some cases they'd even put one RF stage in front of this to isolate the oscillator from the antenna just by one more stage that's another story altogether so way back in the day that's what happened so they can even tell what depending if they you know could make the correct guess this is close enough to 455 so it's 456 they could even make it guess at you know what frequency you're listening to if they could get close enough to your antenna you know and pick up the oscillator right again if they were if they're receiving you at 1456 they know you're listening to 1 mega cycle kind of interesting again the need for pre selectors in one RF amplifier in the front end of most of these old receivers that was quite common in military receivers so what we need to do is we need to see where the signals going missing we see that the converter is working so the oscillator portion of this converter this is doing two things it's an oscillator and it's a mixer that's what this one little tube does here it's doing two things so we know that this thing is oscillating because we saw that with Karlsson super probe so we want to follow the signal all the way down and see where the signals going missing and we also want to see if this actual field is working because the radio is really quiet and we can do that very easily even without a voltmeter just by touching a piece of metal to the center portion of the speaker right here and we can see if this is magnetized or not and that'll tell us whether that field coil is actually working so what I'll do is I'll get some test equipment hooked up and let's find out where the problem is in this radio receiver I have the radio attached to my isolation transformer and current limited variac supply and it's on right now so I have to warn you working on something like this is dangerous especially if you don't know what you're doing because there's high voltage present all over the place in these radios and of course the risk of the chassis itself being hot again this is on an isolation transformer so this is much safer to work on but it still has lots of dangers so if you're following along you're doing so at your own risk so what I want to do is I want to look for the signals in here now that I'm on the underside of the chassis I can get the non-contact probe a little bit closer to some of the pins and we can see where the signals are so first of all again we can look to see if we can hear a signal on the antenna I'll turn on the super probe here so we'll listen to the antenna [Music] so we can hear that there's a signal at the antenna so I can go over to the antenna coil itself so I can tell that the antenna coil is working so what I'll do now is I'll go close to the oscillator and I should see the green LED come on nice and bright see how it gets quiet and it gets bright it's a sign that the oscillator is working so let's go close to the if' tube and see if we're receiving any radio no problems there we can see that we have signal on the if2 I can even tune this [Music] so now let's go over towards the detector tube which is this tube right here so if we look at the schematic I want to go close to the detector plate so you can see that there's two plates tied together inside here and if we look here we can see that there's two leads tied together right here and that is the detector plate so I'll just move this over to this side over here clip this on here like so let's go towards those detector plates and see if the signal is getting scrab the schematic here again see if the signal is getting all the way over to this point right here let's move that out of the way so turn this back up no problems fact it's very strong probably turn it here here that no problems lots of signal over there so lots of signal present at the detector plates over there yet we still have nothing here I can turn the volume all the way to its max which it actually is right now and this thing is just sitting dead silent let's turn that off so what I want to do is I want to actually see if this is magnetized if the actual field is magnetized because if that if this thing isn't magnetic you know it's not going to be working no you can see it's definitely magnetic so it's sticking to it so there's enough magnetism there for the speaker to work yet there is nothing now I told you that I'd show you how this works with other equipment as well we can see if the oscillator is working now I'll show you with this one here quickly again so I'll put this back on the chassis here and this is the oscillator coil up here so oscillator and antenna coil so I'll turn this back up you can see here if I get close to the oscillator coil and see the little green LED coming on and it gets quiet so we can determine that the oscillator is working just like that I turn this down okay so if I turn that down that's what the sensitivity of the probe turned down so that way it's a little bit more positive [Music] very easily we can tell that the oscillator is working with a super probe now we can also do that with an oscilloscope and what I'll do is I'll go grab a scope and I'll show you how that works and a very easy way to check the oscillator with an oscilloscope without really making any connection here's an example of how to use an oscilloscope to see if the oscillator is operating it's really quite simple the common lead of the oscilloscope probe attaches to the chassis and you can take the probe itself and insert it right into the oscillator coil just like that and look what happens we can see that the oscillator is operating and give it just a little bit more vertical sensitivity here so the oscillator is operating right now 1.96 megahertz so keep in mind that the radio is receiving 456 kilohertz lower than this right now so again the oscillator is always 456 kilohertz higher than what you're listening to and that's how easy it is to see if the oscillator is operating now keep in mind that if you take an oscilloscope probe like this and you put it inside of an oscillator coil like this it's going to drag the circuit this is a really high impedance circuit so it'll drag the oscillator off frequency but it'll still give you an idea if the oscillator is operating and we can definitely see that it is operating we just have absolutely no audio coming out of this radio receiver that seems to be the issue so let's find out why we have no audio the radio is just sitting silent and the speaker itself is magnetized so the field is working so there's obviously something wrong in the audio section over here and that's what we're for now by using the carlson super probe we found that the signal is getting all the way to the detector plates but the whole audio section is quiet so this tube here is a detector and audio amplifier so if we take a look at the schematic here the signal that we listen to is getting all the way to these plates right here so this is the detector portion of this tube and the triode that you see in here is the audio amplifier portion the next thing I want to do is check the plate voltage on the six q7 and see what that's at and then I also want to check the plate voltage on the 25 l6 here so the plates in both of these tubes are pin three consequently the screen grid in this tube is pin four and the control grid it which is where the signal goes into is pin five and the cathode in this tube is pin eight we're going to be taking a look at all of that stuff here so I'm going to check out the plate voltage here on the six q7 which is pin three so what I want to do is put this to the DC meter and I'm gonna go over here now pin three if we look at the index here we always count clockwise so this is the index this is pin one pin two and pin three so this is where I want to check the DC voltage and there's 12 point two volts on the plate which is extremely low all right so now I want to go and check the plate voltage of the audio output tube again this is the index over here so we have pin 1 pin 2 and pin 3 it's very low on this ten point six let's take a look at the screen voltage 17 volts so there's actually quite a bit of a drop across that audio transformer let's take a look at that so there's quite a bit of drop even at that low voltage across this audio transformer here which is very interesting so again pin three okay right here you have ten point six volts and then if we go over to the screen the screen is high so this here is the screen so we're getting a drop across this audio transformer quite a drop across that so let's check the voltage at the cathode of this tube here so the cathode again is pin 8 so that's this one right down here this last pin right before the index wow that's incredibly high on the cathode for a plate voltage of 10 volts so that would either be a shorted tube or something so this is the cathode and this here is the plate there's pretty much no difference between the two so now if we look at the cathode right here and we look at the plate what's connecting these two we have this capacitor right here which is connecting these two so again we look here the plate all right 10.6 1 which is at this point right here and if we go to the cathode which is pin 8 you can get this down in here there it is basically the same ten point five nine ten point six this a nice polished cap here is keeping me from hitting my probe into that area down there so there's a really good chance that this capacitor here is shorted and that capacitor is running from the plate which is here to the cathode and that's this cap right here so why don't we try and open it and see what happens I'll just clip it open Wow [Music] well I would definitely say that that is the problem now that the capacitor is removed and the radio is operating let's take a look at the plate voltages so this is the plate voltage on the output tube and this is with the current limited supply in line right now so you can see there's quite a jump from about 10 volts all the way up to 51 let's take a look at the plate of the amplifier tube quiet a bit higher 34 now what I'm going to do is I am going to up the supply here to its max right there and as you can see the sensitivity is picking up if I move the band switch that's a little bit better so make things nice and quiet so we can test this out 95 volts on the plate of the audio tube the audio output tube and almost 50 volts on the plate of the audio amplifier and that is looking a lot better so this capacitor here is acting as a bit of a snubber in the circuit here so get this here so this is acting as a little bit of a snubber and it also knocks out some of the high frequencies so it tones down the sound of the receiver just a little bit so this capacitor here should be replaced with the same value and it looks like it says it's point zero three on here in order to you know bring this radio you know bring the tone of the receiver back down again will affect the tone just a little bit so let's see what's actually wrong with this capacitor here what I'll do is I'll shut the radio off and let's go over here to the ohms meter actually let's go to continuity because I've got a really good feeling that this is shorted so they'll enter and I will put one lead over here on this end of the cap and let's test this end that capacitor is a dead short so that's definitely the problem and the radio seems to be working just fine without this cap so this will definitely need to get replaced now for right now it's not one of those crucial components I can just take this thing right out of here I'll turn the who is quite a nice little burn on that side as well that might be the reason you see that right here quite a little bit of a burn there that'd be one of the reasons why this thing actually shorted or the short inside the cap might have actually caused that burn in the paper right there you can see it's melted the wax away from that spot kind of interesting that was hidden on the other side of the capacitor so what I'll do is I'll just turn this back on right now I'm just gonna turn it on first in current limit mode and I'll let everything warm up here I want to be nice and easy on this because this still has all the original caps in it at this point just move this out of the way get this out of the way just shut this off and we can cruise around the dial a little bit and just see how well this works that full line voltage all right so here we go turn this back over to here and here's a little bit noisy and I'll turn this up to full line you can hear how sensitive that is it's not on a frequency right now that's as low right here but as low as the volume goes at this point so I'll just leave the volume right down [Music] married and divorced calm today it's actually working extremely well for all of these other original components one of the shortwave band works definitely tell that switches and stuff need to be cleaned on here it's very temperamental some CW that must be rate at the top end of the broadcast band again oh there is the time signal wrong way that's the WWV Wow so it's working very well even on the shortwave bands so Toulouse I'll just shut this off so the whole radio is working just that one capacitor stop this thing from working and that really is the problem with leaving all these old components in here is because really it's basically just guess when the next one's gonna go pretty much spin the wheel and you know take your best guess at this point so there's a lot of other wax capacitors in here that could fail at any time and some of these are looking like they are just about ready to go you can see looking at this one down here the line cord right now is disconnected by the way and take a look down in here right some sort of a black stuff it looks like it's almost coming out of the capacitor there so in order to make this radio dependable and operate properly again it's going to require an entire restoration so basically all the capacitors that you see on the bottom end will need to be removed including these capacitors here this capacitor here you know is of and all the capacitors in this section here this waxy and this waxy up here they all need to get replaced the ones that can stay are move the chassis over you can see that there's these mica capacitors here so I'm just gonna move the focus over here we can see this so like these capacitors here are usually okay so this one here can stay in the capacitors that are like that but anything that's a waxy like that waxy down in there this one up here all of these have to go as in any of my standard restorations so these dog-bone resistors all look to be in really good condition and at the time of restoration if this thing is actually going to get restored I'll go about replacing all the capacitors at that time and testing all the resistors and we'll do that all together so that's all it took to get this working we found our way all the way to the problem and eliminated the problem and the radio came back to life so do you think we should actually go through and completely restore this radio and make it work like the day it rolled off the factory floor you can let me know in the comments below and there's enough of you that want to see this thing get restored we'll go through the entire process together and bring this thing back to life make it work the way it did way back in 1937 thanks for stopping by the lab today I hope you enjoyed this diagnosis and troubleshooting procedure if you are enjoying my videos you can let me know by giving me a big thumbs up then hang around there'll be more videos like this coming in the very near future we'll be taking a look at vacuum tube and solid-state electronic devices alike so if you haven't subscribed now would be a good time to do that as well the official mr. Carlson's lab 2020 calendar has been released and it's photograph quality just below the video's description is a show more tab written in capitals if you click on that show more tab it'll expose the link you can click on the link and it'll take you right to where the calendar is you can flip through the pages and check out the entire calendar lots of really nice pictures in there from many of the projects that have been on the bench here 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 definitely going to want to check out my ongoing electronics course on patreon I'm also sharing many of my personal electronic designs there as well the circuit board designs and all sorts of information for the super probe that you saw here on the bench is up there and many other pieces of custom test equipment to make your troubleshooting procedures all that much easier definitely check it out I'll put the link just below the video's description as well under the show more tab so I'll put that just below the link to the calendar and I'll also pin that link at the top of the comments section so it's nice and easy to find it alright until next time take care bye for now you

Channel: Mr Carlson's Lab

Views: 845,860

Keywords: Repair electronics, test equipment, amplifier repair, radio repair, learn electronics fast, restorations, electronic restorations, receiver repair, test equipment repair, oscilloscope, fix electronics

Id: LC7OjuYAOQE

Length: 74min 50sec (4490 seconds)

Published: Sun Dec 29 2019

Reddit Comments

[u/fermat1432]

Mr. Carlson is so much fun to watch. And he is a licensed ham!

[u/Firewire_1394]

I just found his channel a couple weeks ago. His knowledge level is through the roof.