Audio Signal Generator Restoration EICO 377

hi there and welcome to another episode lab today we're going to repair and restore audio signal generator from the 1950s era so it's about time that we start populating the old-time workbench with some test gear so that we can start repairing modern and antique gear so this will be one of the pieces that we're going to use to do that now I've got lots of old test equipment and what I'm going to do is repair all of the stuff and we'll cycle through a lot of this older test gear on that bench so you can get an idea of what this older test gear works like and how to get familiar with this kind of stuff so there's lots and lots of repairs and restorations on the way so let's get started on this one and get the ball rolling this is the ico model 377 audio signal generator that we're going to repair restore and bring back to life today now this audio generator is from the earlier days of Aiko it still has the blue color scheme on it and I have some other tests Kure made by Aiko with this blue color scheme as well so we'll go through all of this stuff and it'll be kind of a neat looking set up for the old-time workbench now we're not going to just stick with Aiko stuff will cycle through Heath kits stuff and all sorts of different types of test gear but we'll go through these ones first just because they have the same look and we'll start with the 377 here now this is classified as an audio signal generator but it does go to 200,000 cycles so if you're working on some older radio gear that has an if' frequency that's under 200k you could very well use this audio signal generator to align it now of course 200k is well beyond the audio able region most you know young people can hear just below 20 kilohertz somewhere that would be on the top scale right here so you can see how far beyond 20 kilohertz that goes so it's uh it definitely exceeds the audio signal generator area and I guess that would have been a selling feature for way back when and if that's not good enough it does sine waves and square waves as well so I imagine the circuit in here is pretty heavily coupled to this output section and we'll take a look at the design here in just a moment when we open up the unit and go inside they were nice enough to stagger the colors for the bands just to keep things nice and easy to read so you can see the first bottom scale here is just a logging scale we have a B C and D so a is red so you would follow a which is this one here and then B you can see right here is the blue and then see again is red and then D is blue so they kind of staggered it set the scale is easy to track now the leader machines they didn't do that it was just all one standard black color kind of a black and silver design so you could kind of see maybe they were trying to keep up with times and they're just trying to you know amend the way that the unit looks to make it look a little bit more modern or who knows maybe even they were just cheeping out so that's a pretty standard thing for a lot of companies as they move along so I really don't know if you have an echo model 377 with the silver face you can compare it to what we see inside this thing and you can determine whether that is the issue or not so we have an on and off switch here and a jewel light here a ruby light on the top which will look really nice when it's lit up some nice binding posts here and a gain control so that's pretty much it things are smooth and feels like everything's working I imagine there's some large variable capacitor in there in a moment we'll find out when we get inside this thing now inside this there really is no line cord or anything so we can see that it you know it's been clipped off at some time why it's been clipped off I really don't know but we'll find out here quite shortly we'll get inside and check it out the case itself is in pretty nice condition so I'm hoping it looks the same inside so let's open this thing up and find out to get inside here it's relatively simple I just have to remove these screws around the face and there's one screw on the rear and this whole thing should come right out of the front I have all the screws removed out of the front of the unit except for one and I've also removed the screw on the rear so I'll remove this screw just like this probably gonna fall stuck to the screwdriver that's nice and this should just fall forward like this if we look at it from the side you can see it pretty much just slides out like this so what I'm gonna do is just move this back yes and get rid of the case down there and as you can see inside it's not looking too bad it's not looking great either you see here the tubes in the variable capacitor some sort of interesting thing going on over here and if we look at the other side yeah we can see that there's something very interesting happening with the chassis here the coating looks like something's happened with the coating here it doesn't seem like it's rubbing off or anything so what's black and the coating is blackened which usually means a bunch of things and there are some rust on here so what I'm gonna do so we can get a better look at this is I'm gonna mount the camera in a different position we'll take a look down at it and we'll have a better overview of what's going on here here's a closer view of the top side of the chassis and the first thing I notice is it's been tested June 4th 1950 some other so that looks like either a three or an eight I'll call it a three because of the blue color scheme on the face of the unit and this is you know an earlier version so we'll call it June 4th 1953 you see it has one two three four and five vacuum tubes way more tubes and I was expecting to find in this little echo audio signal generator five tubes means it's bound to be good you see that we have a four gang variable capacitor here really nice capacitor can be used for a lot of neat projects Teno tuners and things like that but it's staying in here and we're gonna restore this unit and bring it back to its tappy old self again this little coupler here a bunch of capacitors here this is obviously the power supply area and they've got a shield here power transformer rectifier tube filter and reactor down here now one of the things that makes this restoration kind of interesting as you can see this is this is a cadmium coda chassis you can see that it's rusted through over here and you can already see what's happening here it gets even more ugly over here you can see there's blackened over here I don't know what's been happening over here and as we look at this side it gets really bad of course now this is the area that gets really hot and it's gonna you know pull air through this area and as you can see it's definitely done that this is a blackened cadmium coating now there's a bunch of things that can cause this to happen first of all the prep to the chassis alone can cause this to go black if it's you know contaminated actual the chemicals that they're using but what tells me that this happened later on is there's shadowing happening so look at the transformer you really can't see it'll remove the shield there's a shadow of the transformer here and you can see that it's kind of shadowed with the capacitor here now way back in the day it was really common for these things to be on you know service benches and they would service Auto radios so in order to have the power supply for an auto radio you need a 12 volt battery usually and a charger and if the battery is off gassing or charging beside this what's gonna end up happening is those fumes are gonna go in here and it's going to black in the cadmium now you can see that this does not come off see there's some surface dirt there but this will not come off you can see it's just part of the coating getting this blackening off of the cadmium and you can see it's even gone right through the cadmium and it's rusted here right down to the metal the only way to get rid of this is it has to be completely removed now the thing with cadmium is this very very toxic if you want to remove cadmium you need to have the proper mask on it needs to be done in a well-ventilated area and it's kind of looking like in order to restore this thing you know and make it look decent and most likely function properly I am going to have to remove that cadmium look at how bad it is on the bottom it almost looks like a transformer is has smoked up here you can see this on here you know and look at the chassis it's all black but again it was really common on service benches to have a 12 volt battery for servicing those radios and it was probably sitting right next to this thing now the it really gives out of ways you can see the studs of the screws are rusted like this yet the chassis is just spotty so yeah it just it just screams car battery has been sitting beside this thing sit down here it's gone right through the cadmium coating even if you were to try and clean this off and if you were to you know get it kind of an okay result you'd still have all this rust and everything so what that means is most likely I'm gonna take everything off this chassis and I'm gonna probably sandblast your glass blast this chassis and put a coat of cold galvanizing or something like that on this just to protect the chassis now again you know if you're gonna ever sandblast or glass blast any kind of a cadmium chassis it needs to be done in a well-ventilated area and you need to wear the proper you know respiratory protection you need to have a mask on because this stuff is extremely toxic you definitely never want to breathe any of this stuff in it sure be nice if that entire chassis look like this down here you know would make things that you know a lot easier on my end but you know what I'll bring you through the entire cleaning process and the and all that kind of stuff as well so it just gives us more stuff to do in this video you see how that's all over the place and it's most likely under the controls here and things like that you can see that it's you know surrounding tube sockets down here so it might even be affecting connections so it definitely you know even if you were to try and polish this off yes chances are this is underneath here so yeah it really just needs to be everything needs to be removed and it just needs to be cleaned properly now looking at the actual circuit design itself you can see that we have an oscillator here this is an an audio oscillator and we have a light bulb in this audio oscillator circuit now I know everybody from patron is yelling at the circuit I recognize the circuit already we built the solid-state version of this oscillator with an op-amp over there and this would be the tube version of that oscillator and this is a pure example of circuit recognition we see a light bulb we know that we have an oscillator in here we know the name for the oscillator right now and I'm sure some of you are yelling it at the screen if not typing it right now so we'll talk about this circuit and the name of the oscillator here in just a little bit when I get the schematic out you can see here that there's a lot of caps that are going to need to be replaced these are definitely going to be shot all of these caps these are you know all waxey's this one here looks like it has gotten really really hot and in fact the wax is gone in this area right here there isn't a wax here at all so it's gotten so hot that the wax is rolled off to this end because you see the capacitors on a bit of an angle you see that it's just rolled right off here so chances are this is probably close to being a jumper so you know lots of issues here need to be solved there's a lot of Roundy's resistors in here so we'll go through and check those another capacitor here that's gonna have to go so lots of things this light bulb is gonna be a special light bulb because it's in the oscillator circuit obviously this is going to be used as something with a positive temperature coefficient again when we take a look at the schematic I'll explain a little bit more about that oscillator there so quite a bit of work ahead of me to remove all the wires from the transformer you see the line cords been cut it looks like you've been twisted up here or something it's pointing lower this way great so see over here yeah he's hooked up the switch looks like it's an okay condition there's a bulb in there this is running up over here this capacitor on here so looks like a 10k resistor across the jacks so these cans are gonna have to be substituted as well might do that with those little ceramic standoffs that I've done in the past here so quite a bit of work to be done here down here is an adjustment that somebody's comfortably plastered a bunch of red paint over it looks like that's they have to carve off of that control so lots of work so that's what I'm gonna do now basically in order to remove everything from this chassis what I'm gonna do is get a drill and drill out the rivets on the bottom side here so don't damage the chassis or the tube sockets so I'll drill out all the rivets on the tube sockets and as you see there's nothing really soldered to the chassis itself everything goes to a terminal tie strip soldered to that and that's popper riveted or you can screw to the chassis or something like that so in order to get all of this out of here I basically have to remove the the reactor the transformer dassault of the wires and whenever you're doing this it's a really good idea just to take a picture of this so if you have a phone or if you have a basically a notebook or something like that snap a picture of this and then that way take a bunch of different angles and when you want to put it back together it goes back together really easy for example if you wanted to remove this capacitor here you wanted to remember where the wires go clip it just a little a ways away from the terminal here and leave a little bit of the wire color on so basically this will run right up there and tells you right where they used to go you just skinned a little bit of the wire take some of the insulation off and away you go now again because everything is basically riveted to this chassis there's nothing really solder to the chassis when I drill out the the rivets here on the tube sockets and everything and remove the controls off the face all of this stuff that you see here is just gonna lift right out so it looks like an incredible amount of work but really it's not gonna be that you know that incredibly bad so a little bit of work ahead of me here so what I'm gonna do is I'm gonna get started on this and all basically get the chassis bared and we'll take a look at that I'll blast it we'll take a look at that and I'll put some cold galvanized coating on the on the chassis here and get everything ready to reinstall here and then we'll take a look at the schematic and move on from that point in order to remove the face on this ICO 377 it's relatively simple first we gotta get rid of the chickenhead knobs they're called chicken head knobs because they look like chicken heads now usually these are pretty easy to get off unless they're seized up that's pretty loose this one here is a little bit tougher to get off there it is usually what ends up happening with knobs that are on switches way for switches is sometimes they slip so what they do is they score the shaft and then people tighten the screw up tighter and then sometimes they loosen off again and they score the shaft and then it puts a ridge on the knob and then sometimes it harder to get off so you loosen the screw up and just turn it until it kind of smoothes the ridge then they pop off so that was relatively easy now the next thing that I want to do is loosen up this switch and I'm gonna also need to remove this well the dual light itself will probably come out when I remove the nut on the backside we'll see here in a moment so you want to be very careful with your pair of pliers you don't score the face of the unit so I just set it down and very gently give the ring a pinch and it should just loosen up I can feel the switch itself on the other side there we go it'll switch moving so sometimes it also helps to just hold the ring and then move the switch on the other side and then let go of the ring and then put the pliers on again and then move the switch so you're kind of making a ratchet motion and that way you don't score the face of the unit this is nice and clean and I want to keep it like that so on the backside of the unit here we have a incandescent dial light I'll just take that out and we'll take a look at this here there we go 9/16 take that off pretty standard usually always 9/16 that's why I have all the tools ready it's always a good idea to put the nut back on the backside of the little jewel lamp so that you don't lose the nut it's a really nice-looking little jewel lamp there you can see it's got a really nice color to it when the incandescent bulb was glowing behind that it'll really nice and on this side you can see here that there's two with a little quarter-inch nuts that need to be undone so we'll do those get this over here like so and once you're done with these again when you take these off you're gonna also want to get all little pieces together keep them together you can see that there and then put this on the side like so just tighten it on there like that good enough just so you don't lose the pieces and the bottom side here is tightened right here kind of the same way except there is no spacer on the back side because this has just got a somewhat of a locking tab here move that a little bit of a lock washer on the back side with a connection on it like a terminal tie lug here right here so that's all loosen then the last thing to do is remove these right here and those look to be 1/2 inch I'll see if I can this here is close enough on here and just like that they're loose so one there is a washer here as well it's handy to have a little bucket to put these in so that way you don't go losing any of the parts and pieces and this here like so I think after this we're probably ready to remove the face let's see yes there's something else holding this thing on doesn't look like it there we go and it comes off just that simple [Music] so we're just about there so the next thing that we're going to do is I guess I could probably remove this shield here that shield will most likely come off a quarter-inch driver as you can see I'm really prepared to do this I wasn't planning on taking that off here but do that right now anyways so there's a screw here or not I should say on the bottom side I'll remove that there's a magnet in there so it'll hold it in and another one over here that looks like there's one more down here and this one is really rusty yikes so it'll all be stuck in the end of this thing nice thing about magnets is it holds them in they're able to get that out with a small screwdriver so allow me to show you that yeah you can see that they're how its kind of created that shadow with the transformer here that I was talking about earlier it's definitely some some form of nasty corrosion there so just get this out of the way you can see how quickly this is basically stripping down so the next thing I'm going to do is remove this I'll do this off camera I have two desalter the wires on the bottom side it mark everything at any rate so take the screws out for this transformer remove that there's two here for this as well again marking the wires to where they go and it'll be the same for the capacitor because the capacitor will have to come off of here and at that point it's pretty much ready just to drill the rivets out on the bottom side of the sockets here and that's what I'm gonna do and once I have everything ready pretty much just to come out I'll show you how that comes out it'll just be like a big wiry mess that'll come out and when I want to put it back in the chassis the big wiry mess will just fit back in now I'm not gonna change any components in here until I get everything mounted back in the chassis here again just because that's just gonna lessen confusion so everything else you'll be attached everything will still be hooked up and then once everything is basically screwed back down to the chassis again then I'll start going in here and start replacing all the components that'll make things much easier the last thing I would want to do is remove all this and then start changing out parts and pieces with this you know why are we mess on the bench that would you know definitely create a whole lot of confusion this just keeps it nice and simple so we can you know keep track of what we're doing kind of piece by piece the power transformer inductor reactor and all that has all been removed is the capacitor here you see this Hardware all over the bench here I'm just gonna replace all the screws cuz a lot of these were really rusty so there's the capacitor there I've also drilled out all of the rivets on all of the tube sockets then I've loosened up the nut on this VR here so I've also drilled the rivets out on the terminal theis trips pretty much everything that's been holding something down on the chassis here has been loosened up this was held down by a screw and a nut this is the the bulb on the bottom side here so as you can see I just have to gently kind of pull everything through I play this we'll come through that does but I might have to be sold to that there we go there it is pretty much just the way it was in there but now I'm left with a bear chassis that I can clean up so I still have to get some of the rivets out of the top here I just push them through the center punch from the bottom side they've all been drilled and I'm pretty much ready to sandblast the chassis now again this has a cadmium coating on it so I have to wear protection and of course the outlet for my sound blaster is going to be blowing outside far away from where I am so you can never be too incredibly careful so you need the the proper mask and all the proper precautions to work on something like this you definitely don't want to be inhaling any of this dust very very toxic the chassis has now been glass bead blasted and it's pretty much ready for the next coat now before I go about putting the next coating on this I'm going to use brake cleaner make sure that there's no wax or oils on the surface even me handling this is going to put some oils on the surface and down the road that can cause the next coating to come off so I want to make sure that this is nice and clean and as you can see the shield for the power supply has also been done so it's nice and clean as well so well on the way to getting all the components put back on the underside here again sure be nice if I could leave the chassis like this it sure looks nice like this but unfortunately in short order this will start to rust in fact it's actually kind of amazing how fast a glass bead blasted metal surface will rust this will rust in no time in some cases when you sandblast or glass bead blast cast iron you can glass blast a thing 20 minutes later it's brown so it really just depends on the environment you're in and everything like that so this is staying nice and clean looking this is done about 15 minutes ago so it's still nice and clean I'll get through the rest of the process here and when I have the thing coded I'll be back and I'll show you what that looks like here's a look at the chassis with the cold galvanizing compound on it everything turned out really nice at this point I'm pretty much ready to put all the components back in the chassis and mount everything back on the top here so what I'm going to do is start with mounting the transformer and the inductor of what they could pass it around here and I'll get all the larger components on the top side here and then I'll mount all that wire a mess on the underside back in and then I'll basically fold the tube sockets back down and I'll use 632 screws to mount the tube sock gets back so I've got a bunch of stainless hardware and I'm gonna use the stainless screws nuts and star washers to hold everything to the chassis so it should look really nice when it's all done well here's the the shield for the power supply as well I turned out really nice the chassis reassembled with ease and it looks great with all the stainless hardware holding everything to this chassis while all the components were off the chassis I used that opportunity to clean them all up with a mild detergent as well that includes the face in the backside here here's a catch that gets quite a few people some very mild household detergents that you would never suspect attack aluminum and I mean attack it for example you spray it on the back side five seconds it looks like this chassis so always test the mild household detergent that you're going to use in a very inconspicuous area before you go above basically putting it all over it or you could have a very bad day so always keep that in mind another thing that should be mentioned about this coating that I've used on the chassis here is it gets extremely hard in about three days now while this coating is still soft you want to make sure that you remove that coating where there needs to be a good ground connection to the chassis or any type of connection to the chassis for example this tube retainer ring that you see right here has a little ground lug on the side of it here and that needs to make really good connection to this chassis this coating that is on the chassis is an excellent insulating barrier you need to make sure that it's cleared off any kind of area on this chassis where there needs to be a good connection to the chassis so you want bare steel under all of these vacuum tube socket retainer rings under these little lugs over here that need to make good contact ground as you can see the capacitor over here is not going to be in service anymore and this one isn't either they're basically just dummy capacitors that are there for the looks on the bottom side here I have some standoffs that have been put on the screws that have been used now on the chassis here underneath this capacitor retainer ring all of that has been cleaned off as well so all the coating is cleaned off and as you can see I have a nice little ground lug here for the new capacitors that are going to be installed on the bottom side of the chassis now all of this wiry mess that you saw that was on the bench here looks like a big pile of spaghetti basically just fit right back in and very comfortably settled right down all the tube sockets lined right up with their holes all I needed to do is just put in the new mounting hardware and everything fastened in just like you see right here so if you ever need to basically strip the chassis of any radio or any other piece of test gear or an older television or something like that you always want to keep this method in mind because it's just that quick to slide the whole thing back in and fasten it back in and if you've taken really good pictures now you have the map to basically go and recap and if you want to rewire and change the resistors and everything like that you definitely don't want to remove any components before you take this step because it is really important if you remove all the capacitors and things like that not only will a lot of the tube sockets not be supported properly and they could move around and things like that but when you put the whole thing back together now it's basically like it was before I took it apart aside from you know disconnected caps and things like that so it makes the rebuild very easy just a quick little tip to help you get through that step if your chassis is in really bad condition like this one was now if I knew more about what had actually happened to the coating on this chassis and I know that it wasn't going to you know basically continue to degrade I could have possibly left the chassis alone but I really didn't know what that blackening on the chassis was from and we saw in some areas that the rust had you know it was rusting so had gone right through the Academy encoding so you know ten years down the road something like that who knows what's gonna happen how far that's going to go and of course it could affect the performance of the unit especially if that that blackening starts to go under grounding areas and things like that I could lose grounds and things like that so the best option is in a situation like that when you really don't know what had happened or what kind of chemical was on there it's just a strip it all off and make sure that you're dealing with a good foundation again now you can see that I've tacked the wires to this resistor here and I've done that in other places I know that this resistor goes to a lug on this capacitor and by tacking the wire to the resistor like this I know that they both fastened to the same place you're gonna want to do that in certain areas where you know that things need to attach like over here I've got a resistor just tacked to these other resistors here and I know that the area over here will go to a lug on these capacitors so basically this resistor will go across these lugs here you can see these isolated standoffs here are going to support the new capacitors and I'll show you that when they're all installed things like that just prepare yourself I've twisted these wires together I know that they both go to the same terminal on the tube socket and again you know I've taken very good pictures of this so if I need to find out where anything goes say have lost place or anything like that instead of having to follow a schematic you can just take a look at a picture and you know just very quickly fix the issue now of course if you want to follow the schematic and you know completely redo this at that point that's absolutely fine as well whatever you're into basically when I do something like this I just want to get it back together exactly the way it was before I took the thing apart within limits and then I can proceed on the restoration and it just makes it that much quicker and efficient you can get through the recapping and the rewiring process or whatever you have to do at that point so what I'm going to do now is start going through here and I'll remove all of these questionable components I'm not leaving any like this in here these are all going to be in some particular point of failure rate now we'll take a look at them later on a capacitor checker but you know all of these are going to be leaking and all you know all that kind of stuff so they all gotta go and the resistors all the Roundy's I'll end up checking and if I find any bad ones I'll show you what I mean and we'll take a look at the mono meter and things like that so I'll get through the recapping process right now and all that kind of stuff then I got this all cleaned up and pretty much ready to test I'll be back for those of you that are subscribed to my channel you'll know that I talked about this particular style of resistor here and this particular style of resistor here in some of my videos for those of you that are new to my channel this particular piece of equipment is such a good example of the failure point in this particular style of resistor that I'm just going to go over that again and give you a really good example of why these resistors fail and what they do when they fail so again if you are new to my channel and you're into restoring this older type of gear or even modern solid-state stuff I have a huge list of videos and in each video I try to improve your troubleshooting techniques and show you the failure points in different components whether again it's solid-state or vacuum tube gear so I strongly suggest you go down that list and check out some of my videos I'm sure there's something there that you're going to enjoy there's all sorts of repairs and restorations there so again for those of you that haven't been subscribed in that you're new to the channel this particular style of resistor here is known as a round II style of resistor this is known as a generic carbon composition style resistor this is known also as a carbon composition style resistor but it is an allen bradley style of resistor how you tell the difference between the two this kind of looks like the shape of a hot dog leaner with the lead sticking out of each end these particular styles of resistors have a squared off end on each end now they come in all sorts of different sizes you can see this would be known as an allen bradley style of resistor and again this resistor on the bottom is known as a roundy you can see it's rounded on this end down here where this is nice and square now here's the thing these particular styles of resistors are very susceptible to the environment that they've been stored in over the years and we know through this video that this thing has not been in the optimal environment that chassis was looking pretty bad and it was rusty and that black discoloration all over the place here so we know that this thing has been stored in a pretty rough place now as I've mentioned in some of my other videos these particular resistors here are very very susceptible to the area that they've been stored in if this has been stored in a shed in a damp location or maybe beside a battery like this particular you know piece of tester was over the years these things here are going to get contaminated and usually what ends up happening is they end up getting closer to being open now when I say closer to being open I mean that they're climbing in resistance so here's a really good example of that so this is 100 K ohm resistor here we got brown black yellow it's 100 K 104 okay so if I put my lead here and my lead to this other end here we can see that this is 226 K so that's over double what the resistance value of this is supposed to be this should measure 100 right this is a 68 km resistor we got blue gray and then we have orange here so that's 68 K it's 89 almost 90 K so this is a really good example of these particular resistors here being in a very bad environment now as I've said before this particular style of resistor this allen-bradley style of resistor they're very stable no matter where they've been kept it's always good to check all of them though because again these capacitors here they leak and now when I say they leak that I mean that they're electrically leaking of course this thing looks like it's been leaking wax but they leak electrically so what they do is they pass direct current through them so basically these things themselves are turning two resistors so these things are trying to turn into those things basically what's happening inside these things is they're degrading inside and they're just getting closer to being a short they're turning into resistors and over time they start leaking and then when the electrolytic caps fail they end up shorting and what they do when they short is they put a lot of load on these resistors and the resistors themselves go open so we know that this particular resistor was across this cap here so we would also want to check this now again this is an allen bradley style resistor so these are very stable throughout time so let's check this square it off version here that's a 10k ohm resistor and it's reading 9.5 K so that's great in fact it's actually gone the other way so very good this here is a 10k ohm resistor here again so we got brown black and orange 103 so look at that 10.0 okay so now on the underside here we have orange orange orange which is three three and three zeros so that's 33 K ohms and again this is a roundy style resistor let's see how accurate this larger Roundy style resistor is so that lead runs up to this terminal right here so that's over double its value so you can see the trend here and this is a really good example of what happens with these Roundy's now here's the thing you can find a chassis like this that's been stored in a nice warm attic for years or maybe in a heated basement or something like that and all the Roundy's will test fine if they test fine it's perfectly fine to leave them in if you want but if you feel more comfortable and replacing them it's your restoration you do what makes you feel comfortable if they're fine at that point and you start using the radio again and it's still kept in a controlled environment chances are those resistors are gonna be just fine and I've experienced this time and time again they just remain the same so again you know that this has been kept in a very rough environment so you can see the movement in all the Roundy's here's like here's another one here let's just measure this one so this is 2.7 K let's see how much this one's moved if it has yeah its 3.2 so it's moved up quite a bit as well so you can really see the trend in this chassis here so we would know that since we've tested a couple of these things and we see that they've moved up in value using the best piece of test here that we have and that piece of test gear is right between our two ears we're gonna know that okay we've tested say 1 2 3 & 4 so if we were to test the other Roundy's and they're ok I would still want to replace them just because I know the environment that this has been in and if any of that contaminant has soaked into the body of these resistors because they are kind of a porous resistor there's a chance that it's going to fail so again I'll go over this in many of my future videos again and I'll give you a lot of future references to the failure points and older test gear whether it's you know the higher end techtronic stuff or just a you know a standard piece of test gear like this we'll go over it and I'll explain schematics and everything to you and show you all the different failure points so my next job is to not only get rid of these capacitors but I'm gonna have to get rid of all of these round e style resistors as well so I've got a bit of a job ahead of me these ones here are sealed and they're going to be absolutely fine I'll go over them and test them and just make sure that they are absolutely fine I'm pretty confident that they are so I'll go over these and testis here in just a little bit individually but you know these don't very commonly fail and quite some time later the unit has been recapped and all the resistors have been changed and these are all the components down here we'll take a look at some of the faulty components here on a tester quite shortly so I wanted to clean this up the best that I could and as you know the tube sockets are here and a lot of the components have to go up to this wafer switch here into this tube socket so they were on an angle so I just figured why not mount the new capacitors all on an angle as well here so it kind of keeps everything in this kind of direction I just clean it up a little bit do what you can with the layout that's provided right so everything is all solid and back into place and I'm pretty much ready to try this thing out but before we go and try this thing out let's take a look at some of these components here first and see how badly they're failing all right let's test some of the capacitors that have been removed from this little signal generator and see how badly they're failing in order to do that we're going to need to stare deep into the magic eye so this is a Heathkit model i t11 capacitor checker and this here is a tuning indicator vacuum tube so it really it's a small CRT and over the years they've been used in many antique radios as either a tuning indicator or signal strength indicator and over the years they've also been used in pieces of test equipment like this and various other pieces of test gear as well so nice little tube so what I'm gonna do is turn on the unit and when that vacuum tube here warms up you'll see a green glow in there and you'll see a little opening in the bottom now the way that this particular device works is it feeds a voltage out of these test clips here so there's two little banana plugs at the bottom here and then I've got some test leads plugged in and what happens is is I can source between 3 and 600 volts across the capacitors here now of course when you're testing a capacitor you don't want to exceed the maximum voltage of the capacitor what this thing does is it detects for leakage in these capacitors so what happens with these capacitors is over time they deteriorate inside and they start to turn into resistors when they start to turn into resistors they start to pass DC through them and if this particular capacitor or any capacitor like this is in an audio amplifier anything usually what happens is you have one side that's connected to the plate this is the outside foil and he sees as outside foil so this end is usually attached to the plate and then this goes to the grid of a next stage if this particular capacitor starts to leak inside what happens is the plate voltage that's applied at this side say there's for argument's sake 250 volts on this side if this is turning into a form of a resistor well it's going to pass some of that voltage on to the grid of the vacuum tube in the next stage changing its bias point causing distortion and all sorts of things driving the next stage you know into heavy class-a things like that so it's very important to have good capacitors in whatever you own whether the old caps like this or even modern caps capacitors are a failure point in many pieces of equipment so this device here is designed to detect failure in paper and foil type capacitors you can see it says here outside foil so it's paper and foil in here and really what's going on right now is the paper inside is deteriorating and as it deteriorates it turns this thing into a resistor so it's at some particular resistive stage at this point and it'll keep deteriorating eventually until this thing just short so usually how they do short is of course you have high voltage on one side and this side is you know quite a bit lower and they start to arc inside and then you know they just end up shorting like that and you can tell this one here has probably been warm you can see the wax is starting to pop out of this end of the capacitor see how it's in here and it's popping out it's probably vented at some particular time so at any rate let's start testing these things now since this particular device can source up to 600 volts if you're unfamiliar with the Heathkit model i t11 capacitor checker or any other capacitor checker that can source 600 volts you have to be very very careful with these things because you can really hurt yourself now one of the things with these older capacitor checkers is they really rely on you knowing what you're doing you can see it says discharge and leakage so right now it's in leakage so now there's 3 volts across those two alligator clips that you see down there now there's a hundred and fifty volts on those alligator clips and now there's 600 volts across those alligator clips and as you see it's not spring-loaded it's not popping into the discharge position so a lot of people like these particular testers because they like to reform capacitors and I'll explain that in just a little bit as well so this device can be extremely dangerous especially if it's left in the leakage position and you forgot that this is at the 600 volts because if you were to grab those clips right now you would be in for a pretty nasty surprise so if you're following along you're doing so at your own risk take extreme caution if you own any capacitor tester like this that will source any type of voltage and of course there is some current supplied there as well so be very very careful so one really good rule of thumb if you own one of these things is after you test your capacitor of course you always want to click it to discharge to discharge the capacitor and you want to turn this right back down to the minimum voltage especially if you put this thing away for a while now way it's safe you have it in the discharge and it's at a very low voltage just a couple things to keep in mind so let's take a look at this capacitor first because it's kind of the ugly one so this here is a wax coating that they put on top of the capacitor and over years of this thing being inside an older piece of equipment you know wires laying on it it's getting hot and cold and hot cold and people have been poking at it with their screwdrivers and things and and they end up looking pretty ugly after a while just like this one here so if this capacitor is leaking this eye is going to remain closed so when I click on to leakage it will close for a minute as it's closing it's charging this capacitor when the eye opens it means that the capacitor has charged up and as I increase the voltage it'll keep doing that until the eye will not open anymore if the eye does not open it means that there is excessive leakage current so here's an example so I'm going to take my clips here I'll put this this end here and this on this end here and I'll just move this out of the way cuz I don't feel like holding this put that down there get this cap out of the way so I'll just lean it right there and kind of see that auto focus but on the side this is the important part we want to make sure this is in focus so what I'm going to do is make sure this is on paper Myka etc because it's a paper capacitor and I'm going to click this onto leakage now you can see the eye closing it was charging and now it's open at 3 volts so now I'm going to move up to 6 wait for it to charge and it opens again 10 getting slower it's ok at 10 and that's getting pretty bad it's probably gonna open here it's gonna take a while there we go so you know 25 to 50 volts this capacitor is going to be bad now the capacitor should charge a lot faster than it is because it's taking so long to charge what does that tell us well if you wanted to look at this you could picture a resistor across this that's really what's happening it's stopping this thing from charging because it's basically self draining through its own internal resistance so at 25 volts that eye is not opening so I'll click discharge put this back down to 3 volts now if we look at this capacitor here you can see that it is rated at 400 volts and it's failing you know 25 volts is really bad now of course it's failing before this because we can see how long it's taking for that particular eye to open so I'll give you an example of a new capacitor so this is a new one this is 0.47 micro farad so very close to that almost 0.5 this is 0.4 and this is rated at 630 volts so I'll put this on the clips here just put this off to the side like this so again this capacitor is basically the equivalent to the one that we've just tested so we're at 3 volts clicking on leakage look at how fast that is so what I'm going to do is move this up 250 volts 250 will say wherever it lands and I'll just bring it right up to 600 look at that that's how quickly it opens so discharge and I'll click it on to leakage look at that how fast that opens that's a brand new capacitor with extremely low leakage and that's how all capacitor should be now any leakage in a capacitor is not a good thing so if you say you found this thing it was working up to say 350 volts or even 300 volts this is rated for 400 so you can tell that okay if the eye isn't opening at say 300 but it's opening at 250 you know that this thing is in some form of deterioration and it should go if you leave that in a piece of equipment it is bound to fail now rule of thumb is nowadays anything with a wax coating on it you find in an old radio is just automatic get it out of there because these things are just neither prone to failure and as you can see this unit here let's move this out of the way I'm going to turn this back down to 3 volts and I'll move this out of the way here even though it's still on discharge I want to move that back down to 3 so you can see here this is another wax capacitor right sealed tight sealed tight to keep the leakage in so made by solar and this is going to be very leaky - I really don't even need to test this I'll just give you an example here here's another one I'll put this off to the side another paper and foil type capacitor so leakage see I was jumping around like that you know what that's telling us inside it's sparking away it's all back down to 50 there's a good chance that the capacitor could even be open and by me turning it up to that point there what ended up happening is it's basically trying to reconnect so you can see how bad that is you can almost use this as an oscillator look at that was a homestead of a steady rate there so you can really see that this is just you know it's really bad look at this thing so yeah anyways it's just it's incredibly faulty so you know what that would be doing if this was left in there it would be you know open and trying to make connection again you'd have a staticky noise and if it was in an audio circuit your audio will be disappearing and coming back and doing all sorts of bad things if that did short and was connected to the grid of another tube I can you know destroy stages and I have sections and things like that as well so just depending where on the circuit it is a FA F or RF wherever it is it can you know really cause damage especially if it causes the vacuum tube in the next stage to pull a lot of current so whatever the next stage is attached to if it's attached to say a transformer or a resistor it's a good chance that it's gonna open or burn out those components so I'll click this to discharge here this back down so we definitely know that this capacitor is definitely toast you see 600 volts 0.25 micro farad sealed tight paper capacitor see that mark the outside foil and on this one as well so this is known as a paper and foil style capacitor so we have some electrolytic capacitors here now these are a catch-22 here's the catch with testing these capacitors with a device like this the first time you put these things on this particular device it's gonna look like they're failing and you'll wait a while and all of a sudden the eyeball opens so you bump the voltage up a little bit more it looks like it's failing and then the eyeball opens up here and you're like what's going on well what you're doing is you're reforming an old capacitor and reforming an old capacitor is not a good idea capacitors are cheap enough nowadays whenever you find an electrolytic like this do not take the chance and leave it in a unit because what's going to end up happening is you're gonna go grab yourself a cup of coffee you're gonna be listening to your favorite radio program you're not gonna pay attention you're gonna come back into the room no radio program is playing and the room is full of smoke and the power transformer in your unit has burnt up where you fried the rectifier tube so do not take any chances with any type of old electrolytic capacitor because after all it's only guarantee for one year so these are definitely gonna be bad so let's see if I can display this so we'll make sure that this is on electrolytic here we have to go to electrolytic it's an electrolytic cap I'll attach the leads see if I can display what's happening so hopefully this will do that particular reforming thing and I can display that to you so we'll start at leakage here and we're down to three volts so of course that capacitor is rated for much more than that so we'll keep going up you can see getting slower so that's at 300 volts and you can see that it's slowly opening there we go let's go to 350 and as you can see on these capacitors here they are rated for 450 volts eight mics 450 volts so you can see we're at 350 and the thing doesn't want to open but if we leave this thing attached like this for a long period of time there's a good chance that that eyeball up you can see it now it's trying so it'll sit here and you'll see that I start to open and basically what you're doing at this point is you're reforming these capacitors now a lot of people think that reforming again is an okay thing to do because hey look it looks like it's opening right well sometimes that eyeball will close just as quickly as it opens meaning that the thing turned into a dead so it's never a good idea I can't stress that enough I see so many people repair older pieces of test equipment reforming capacitors and they say oh it's absolutely fine I've been doing it for years yeah so at any rate so you can see at 350 it's opened up so if I do this at 400 now it's going to do the same thing now after this point after I you know reform the capacitor here hopefully this one here will behave through this whole procedure you'll find that I can bring this up to 400 volts very quickly and it'll open very fast now because it has reformed somewhat so this is you know time-consuming procedure because this is current limited it's not going to be passing a ton of current into these capacitors and the reason that they do that is if something does go wrong you don't want the capacitor exploding right so the whole thing is current limited now being current limited is it is still sourcing some current and there still is a chance that you know the capacitor could pop at any time but you know being limited it's you know quite a bit safer so this might have to be left in this position for a very long time I'm not really too sure so or this could even just be you know showing up as leakage right now and it might not even change it looks like I can see the little little partition in the eye and it looks like it's very slowly moving backwards because the the shadow angle overlaps a little bit and it is starting to move back now we might actually see it open here in a little bit it's getting closer and closer to opening so there it is oh that's trying it's trying you can see that we just about have a little crack in the middle there we go so it is reforming there we go now some people will leave these attached to capacitors for an extremely long time and that's why these units are desirable is because many people want to try to do this to see the old capacitors again not worth it so you get the idea so if I click discharge here alright so right now it's discharging the cap that's why you saw that I go closed for a moment and now it's discharged so if I get say bring this back to 300 volts now and click it on to leakage look at how quickly that opened because it's technically reformed so there is an example of reforming capacitors now this can be very tricky if you're testing for a leakage because you're thinking the things leaking on the first go-around and technically when you're waiting for this thing to that you know to indicate the leakage is going away basically you're actually reforming it during your test so you're kind of defeating your test so lately I've been seeing the prices of these types of capacitor testers absolutely skyrocket it wasn't long ago that I could go to my local ham radio swap meet and pick one of these things up for between 10 and 50 dollars and if I got the fifty dollar version the thing looked like it was meant out of the box of the bright green glowing eye now I'm seeing these things sell for three hundred and fifty four hundred dollars plus that's getting ridiculous so I've had quite a few patrons ask me Paul can you design a device that's more cost-effective let's face it I don't want to design anything that's going to source six hundred or in this case five hundred volts so I've been burning the midnight hours down here in the lab and I've come up with a solid design that tests these capacitors at a low voltage a much safer voltage and gets the same results these capacitor testers get at 600 volts so for example say I was to test this capacitor on this capacitor tester and it was showing leakage at 500 volts you can see this is rated for 600 volts the capacitor tester that I've designed will show that leakage at just over 20 volts so a much much safer design now I've showed this to some people and they say Paul you're crazy you should be patenting this thing works that incredibly well let's face it I'm all about safety and I'm just gonna release it to my patrons and it's going to be released very soon with schematic and printed circuit board layouts and everything if you wanted to build the basic version you don't even need to put it on a circuit board you can build it point to point that's how incredibly simple this circuit is if I was to show you the bucket of capacitors that I have gone through doing comparative tests you would think I'm absolutely crazy there are so many caps I have tested and of course spending time working everything else out as well so I've shown this to some of my engineer friends and they are quite amazed as well and I think you will be amazed when I show you how well this thing works in some upcoming videos so you'll be seeing less of these things and a lot more of that little low voltage tester and it'll all sit in a very small little box and it'll have a nice little LED display on it and everything so if you're interested in building something like this for yourself head on over to patron it'll be there very very soon there's a public video on patron right now with a parts list for the basic version of this and there's a bit of a challenge going on I'm seeing if any of the patrons can guess the schematic that I've designed and people are leaving their entries right now so feel free to become a part of that and enter your schematic if you'd like as well so this is all going to be coming very very soon the unit rate now basically it's ready to go I'm just giving it more bells and whistles I'm making it a little bit more basically creature friendly that said I guess you could say so some pretty neat stuff coming here very very soon so if you design guitar amplifiers or fixed guitar amplifiers or radios or televisions or older test gear and you want a device that's much safer to use that you know tests capacitors around the 25 volt mark you know it's not testing them up at 600 volts anymore you're gonna want this device much much safer to use than these things here let's take a closer look at Aiko's design using their schematic this really is an minimalist approach to an audio signal generator and you know what ICO is really good for that back in the day ICO would design affordable pieces of tester so say you wanted to try your hand at repairing televisions radios or audio amplifiers or something electrical you could go out and purchase one of these signal generators for a very reasonable and affordable price now one of the things that gave the model 377 the edge on the competition and ICO was known for this is this oscillator circuit right here many other pieces of test gear of this caliber back in the day did not use this oscillator and this really is the secret sauce in fact this oscillator design is so good it's still used today except of course it's solid-state now what I'm going to do is explain this here in just a moment we'll move this way down the schematic for you patrons out there this is a really good example of your circuit recognition skills now now on patron we've designed the solid-state version of this oscillator the reason that we've designed the solid-state version of this is because we're designing our own test gear up there for some future projects that we're doing the reason that I chose this design but the solid-state version is because of its low distortion and that's why it's still used today in fact I have a signal generator right now on my bench that uses this design again of course being solid-state the key to recognizing this circuit is this right here there's a light bulb in the oscillator circuit if that isn't enough to recognize this circuit you can see that we have a selector switch over here with four resistors here that are placed in series with these variable capacitors and on this half here we have four resistors that are selectable again that are placed in parallel with these capacitors here this switch is ganged together now when you hear the word ganging that means that these are connected together but there is no real connection to the chef they've ganged them or connected them together so when you turn one shaft it's moving these along like this now ganging doesn't mean complete isolation these capacitors here are ganged together the capacitors themselves are separated but there is one common section to the capacitors here so many of these capacitors that you see will have one common section that's tied together and you'll see that that's basically the frame of the capacitor because there's a bunch of plates that are connected to the rotor or to the center portion the shaft of the capacitor so those all have to be connected because they're kind of pressed into the center connection so you can see the common connection between all four of these capacitors now that worked out very well for that error variable on the top side of the chassis these four capacitors are the four sections in that air variable since the outer the outer case of that capacitor has to be lifted from the chassis it has to be isolated the case itself runs over to the grid of this tube right here this is the control grid inside this tube all the other portions of the capacitors are running out to little they're like little porcelain tabs or little ceramic tabs that are pressed into the capacitor and when we take a look at the design again here and try it out I'll point all this stuff open I'll show you how this is gang together and everything this here is just a small trimmer capacitor and this is designed to align the scale on the face so there's that pointer on the face so we want to align that so when it you know say points the two kilohertz we want it to be at two kilohertz right so just a trimmer capacitor here these are all ganged together these are gang together and this switch over here is ganged together as well again this is usually denoted by a dotted line they don't have a dotted line on here I guess they're just taking for granted that you should know that they're gang together I really don't know some schematics they they do different things in schematics ie for example you'll see a suppressor grid in this tube and it's not hooked to anything why haven't they hooked it to anything because they take for granted that you should know the suppressor grid in the 6k six is tied to cathode same with this six case six tube here you see that there's no connection it's just tied to the cathode internally little things like that when you start looking at these schematics more often you you pick these things up so at any rate back to the oscillator design here this oscillator design is known as a vane bridge or Wien bridge oscillator if you prefer again the thing that really gives it away is the light bulb that's in the oscillator circuit now here's a catch for a lot of people that find these oscillator circuits and they see a light bulb and they power the thing up and the light bulb doesn't glow well it's not supposed to this light bulb is just acting as a resistor with a positive temperature coefficient all right so to explain that a little better if you have a light bulb and you measure the resistance across a filament style light bulb that's known as an incandescent style light bulb when the filament is cold the resistance is low if you heat the filament up inside that light bulb the resistance will go high so you can see in the circuit when there's current on the cathode line here what's going to happen the filament inside this light bulb is going to get warm and it's the resistance is going to go higher what is that doing it's acting as an automatic gain control like an automatic leveling control in this circuit so kind of neat that's just a feature of an incandescent light bulb now this design again is still widely used today with op amps and you can get surprisingly low distortion out of a very very simple circuit now I've designed a really low distortion and I've even given the distortion specs and everything up on patreon so if you're interested in designing an audio oscillator for testing purposes or something like that you're gonna want to check that out and I also even provide the math and show you how to work out the resistor and capacitor combinations to determine your frequency it seems like everybody on the internet wants to complicate that math procedure so I've simplified and make it very very simple you can just work it out on any regular calculator very quick so at any rate that's all up on patreon so I could spend a lot of time in the circuit here and explain this circuit but it wiII be here for a very long time here's another little neat thing about incandescent lightbulbs okay so say you had an incandescent light bulb and you were to break the glass on that incandescent light bulb and you were to apply a little bit of current across the filament so the film went really isn't glowing but you know it's it's getting a little bit warm okay so if you blow air across that filament that of that broken light bulb the resistance is going to go low if you reduce the airflow passing that filament the resistance is going to climb again because the the filament is going to try and heat up again what do we have we have the design for the mass airflow sensor in your car that's how a mass airflow sensor works in in pretty much every car they've got little filaments that are strung in the air path and what happens is they're supplied with some constant current so it's pretty much a regulated voltage source across the little filaments what happens is they get nice and warm when you start your engine up air starts to run pass it the resistance changes and they have you know an op-amp and a bunch of things in there that basically take that resistance change and turn it into a voltage change in your ECU is programmed to recognize basically the different voltages that are developed through that resistance change it's very simple circuit so anyways that's what the this is kind of the sealed version of that I guess you could say there's no air blowing across it but you know if you let the thing alone and because it's not really glowing and it cools off very quickly just because that the actual filament itself is sunk to the lead in wires and things like that everything inside that light bulb plays a role in how this will work in this circuit so believe it or not when you're designing a circuit like this a lot of thought goes into that because of things like thermal inertia and anyways we'll get into all that stuff here in the future over here we see the negative feedback path this is that V R on the top side of the chassis that had paint plastered all over it and to chip all that stuff off was kind of a gummy substance anyways it cleaned all that off this is the negative feedback path goes through that variable resistor and back down to the cathode here light bulb is attached from this vacuum tube right here so anyways moving on we have a nice clean sine wave at this point right here the switch is in the sign position not in the square position so you can see it bypasses this wave form shaping Network right here so you can see it goes up here over here we have a nice sine wave present at this VR this VR is the gain control on the face of the unit the wiper of that gain control goes through a blocking capacitor and into the control grid of this 6k six pentode vacuum tube again this grid not being attached to anything here internally it's attached to the cathode so the signal is going into the control grid you'll notice that we have the plate of the vacuum tube or the anode of the vacuum tube tied to the screen grid here this is known as triode connecting a pentode and you'll see this in a lot of high-end amplifier applications and in designs you'll see that we have the cathode here and the signal is coming out of the cathode and it's tied to the jack on the front of the unit through a twenty micro farad capacitor now there's a bunch of reasons that they're using this design in the circuit one is because of this and another thing is we don't want any iron in the path so in other words we don't want any transformers in the path anytime you introduce a transformer into the path of any kind of a signal you're adding a bandwidth limiting device now of course this thing has to be really broad we're dealing with you know from the low Hertz way up into the high kilohertz so the last thing we want is an audio transformer or something like that in here even a very broad audio transformer is still going to be limited at the top end or at the bottom end so what's the best way to keep the signal integrity to keep that sine wave nice and clean and to keep that square wave nice and square don't have any iron in there use a cathode follower circuit so this is acting as a buffer circuit and have the signal go through this capacitor which is you know 20 micro Fred that's some pretty stout coupling to this output here so this is a really low impedance circuit and we're gonna keep that signal integrity at this point here again there's no bandwidth limiting iron or anything in this circuit what you see here is just a biasing Network for the control grid here this resistor that you see on this side of the capacitor is acting to discharge the capacitor so you can see that it will discharge through this path right here if we didn't have this resistor that's acting as a discharge and somewhat of a load what would end up happening is this capacitor would charge up if the jacks were left open and you'd be zapping your equipment so that's the reason that this thing is here 20 micro farad will hold quite a charge at this voltage that you see right here so all in all this is known as a triode connected pentode cathode follower circuit there you go that's a multiple now this here is a waveform shaping network so if we move this switch remember this is gang together if we move this switch from sign to square wave we're gonna take our nice sine wave and put it down this path right here is somewhat of a divider we can have a nice sine wave here because we have grid current present present at this point here if we take a look at the sine wave we're gonna notice that the top portion is starting to flatten off and that's because we have grid current present at this point so just from this point of the resistor to this point of the resistor right here we're already starting to form a square wave now from my vacuum tube explanations earlier if you've seen my earlier videos you're going to know that the signal is going to be 180 degrees out of phase at the plate and because of this network down here we're gonna start to square off the bottom portion of that school of that now kind of modified sine-wave so now we're we've got a pretty square top and they were starting to square off the bottom half once it goes through this portion of the vacuum tube that's enough to finish squaring everything off and we have a nice square wave present at this point because this is coupled to this portion here again remember this arrow is going to be pointed down here we're going to be feeding a square wave into this vacuum tube here and again we want to keep the integrity of that square wave and that's why we have a buffer circuit here and a cathode follower you can see if we were just to take this signal and basically if we didn't have the circuit feed it right to the output Jack any load on this point would change the oscillator as you can see we have resistors right over here and we also have our feedback network right here so we need to isolate this from the output jack and that's another reason we have this cathode follower who this is acting somewhat like a buffer as well so if we move down here we have a pretty standard power supply topology we have a full wave rectifier set up here the center tap is grounded the outer ends of the windings here go to the plates and the 6x5 rectifier this is known as an indirectly heated rectifier tube here there's lots of different types of vacuum tube rectifiers now when I say indirectly heated that means that there's a filament that goes up inside a pipe inside of this rectifier tube so the filament isn't directly heated the filament itself isn't the cathode there's a cathode pipe inside here now 6x 5s are known to be troublesome there's an X plate and a wing plate type of 6 X 5 that's been in use for many years the x plate is a little safer than the wing plate design the wing plate design was known to destroy many zenith radios and things like that just because of heater to cathode shorts so if you're uncomfortable using the 6 X 5 you could replace this with just a bunch of you know 1 & 4 double-oh-seven s or something like that in the circuit and you'd be absolutely fine I'm gonna leave the 6 X 5 and there because I'm really not too worried about it so we have 300 volts at this point we have a PI filter Network here so if you want to understand a PI Network if you were to take the two grounds and just tie them together through this coil or through the this choke right here or reactor you'd see that we have something that looks like a PI symbol that's really what it is one easy way to remember it so we have CL and C here so this is the filter rate at the cathode we have a reactor or choke if you'd like to call it that here and then we have another filter over here so we have nice clean DC here and this is applied to this entire circuit now if we wanted to improve this design even more we could add a regulator tube over here you know something like a you know an oc-3 or if you want it to be fancy ob3 or - or something like that you put a you know a regulator tube Oh a - or is it yeah it's a no way - there's also a no way three tube and all sorts of different kinds of tubes that they use for something for rectification in one for regulation Oh said for if I remember correctly ism is a rectifier tube at any rate so we could regulate this and that would make this oscillator even more stable than it is so we could keep improving this design but again this is a minimalist approach and you know what it works very very well these particular designs anything that I co put together always worked very well just from what I can see on the schematic already I know that this is going to be a very solid design one other thing now because of the nonlinear characteristics of the six sn7 it is a really good choice to use this as a you know a tube for you know squaring off that sine wave to say you were to add a 6s l7 in here or something like that it wouldn't work as well right even though the pin note is the same now here's the thing you're not gonna ever get a really really perfect square wave out of this and you know you're not gonna have really fast rising and you know falling you for to look at this thing on a scope so again it's a minimalist design it'll produce a decent square wave but if you're looking for you know a really nice clean fast square wave you know this really isn't the circuit for this now we're gonna experiment with the square wave here down the road this is going to go on to the antique workbench or the old-time workbench and this is going to be a piece of the test gear there as well as an IQ oscilloscope and as well as an echo VT VM and all sorts of things we have a lot of really neat projects coming up here on this channel so at any rate I'm going to use this thing as it's designed because that's the fun of using this older test gear we want to restore it and use this stuff like the way it was intended to be used way back in the day so I'll show you how to use this square wave the square wave in audio signal generators like this was used to test audio amplifiers so you would feed a square wave into the audio amplifier and look at the signal integrity at the output jack of the audio amplifier and determine how well your amplifier is performing and again we'll look at all of that here in the future I would have to say 99% of the time this is going to stay in the sine-wave position we're going to be using the sine waves out of this thing and I think for you know in most applications unless you're you know doing something pretty specialized it's pretty much going to live in the sine wave position so there you go all in all it looks like a really solid design and I'm looking forward to using this thing in the future now keep in mind if you're working on a piece of equipment like this there is high voltages present all over this thing as you can see there's 300 volts DC coming out of this rectifier and we have 275 volts ac at this end and we have 275 volts AC at this end so we have a lot of voltage from end to end here to the center tap it's 275 so it's 275 from here to here and 275 from here to here all right so a lot of voltage from end to end if we're to avoid this Center tap you get to be very very careful around circuits like this if you haven't worked on any type of vacuum tube gear before I strongly suggest that you research this stuff before we get into it just because you really don't want to get zapped it really is unpleasant and you know it can be very very dangerous so if you're following along you're working on one of these things you're definitely doing so at your own risk take extreme caution when you're inside these things at this point I'm pretty much ready to try out this little audio signal generator here's the thing when I replaced all the components on the underside of the chassis I've just put the new components to where the old components were I haven't compared anything to the schematic so I'm basically trusting the guy before me to have repaired or assembled this thing correctly so there's a good chance that it might not fire up it might not be an audio signal generator but it might be a great smoke generator so let's find out here shortly at any rate no big deal if the thing doesn't work we'll just make it work it really doesn't have a choice so before we get into that let's check out this air variable that I was explaining to you earlier I'll just zoom on into this here and I'll explain what I mean by ganging and ever thing so this here is that air variable for section air variable and when I turn the shaft here you can see that these four sets of plates move in and out of these ones here so these ones here move and these ones here are stationary now the ones that move are ganged together by a common shaft this common shaft is isolated from this common shaft by this little flex coupler right here the reason that this has to be isolated from this shaft here is because the frame of the capacitor is tied to the grid of this tube now that it's fastened to the chassis here if I back out on the focus here just a little bit you can see on the chassis that there's some little spacers right so this is standing the capacitor off the chassis this cannot make contact with the chassis if it does of course it's going to short the grid to ground and the thing isn't going to work so the entire frame of this capacitor is attached to these right here because you can see the shaft comes through a ball bearing and there's little fingers on each one of these little separators here that make contact to this to make sure that there's really good contact in the capacitor or it could act up at higher frequencies so you can see the precision of this device I'll just really zoom on into this here and there we go now if I rotate this like this you can see those plates are going inside of these plates right here now throughout the entire travel here these sets of plates here can never touch these so these can never touch these in each section but they can get very very close and because they're very very close and they don't touch they're forming a capacitor these are these little ceramic buttons that you see that are pressed into this steel frame you can see how they've pressed them in right around here so I'll just move the camera just a little bit so it'll really focus in on that for us you can see how they've got little press marks here from some form of a press pressing the ceramic button into the steel frame that alone would be a pretty precision job command just a little bit too much pressure and this thing would burst or crack now these things here go through and they're soldered to this right here and it holds the stationary portion of this capacitor in place and that again is a you know a real precision setup if that's over just a touch these plates will touch so and that would make it short so you can see the precision involved in way back in the day this was a very pricey piece and in pretty much any radio receiver that was always a very pricey piece now if we move over here down just a little bit that capacitor that you see is a trimmer capacitor in that is what's used back this out just a little bit that's what's used in aligning the dial scale on the face here so you can see this little pointer here this has to point to the correct frequencies so basically what this is doing is that's trimming this big one up so that when I say I want to point this to 2 kilohertz if this is pointing to 2 kilohertz we want 2 kilohertz present on the output jacks over here another thing I should mention is down here you see right down there you'll notice that I've mounted the new capacitor on the underside of the chassis down here just a little bit and move this over so you can see right there I've just run a wire from the new capacitor I've put a standoff here so this is an isolated standoff the reason I've done this is the old capacitor sat right alongside this 6 k6 tube and it is a maraca let's see where did I put that thing if I've got it over here I think I may have there is so what I'm gonna do is try and shake this thing by the microphone so this is the old capacitor and it was sitting right alongside through that hole there let's see if I can make this make any noise in the microphone here you can hear how to I hope this thing is you're not that's the basically in the internals of this capacitor you know going around now it is already you know a dry electrolytic capacitor but now it's extremely dry like this is you know just absolutely faulty at this point so at any rate the new capacitor is on the underside of the chassis and it's below the vacuum tube and there's just a wire that runs out from here so I'm keeping it away from the heat so you know a couple of little design modifications there so what we have to do is on line if the thing works to begin with we have to on line this here and then on the other side is over I see that right down there we have to adjust that VR on the bottom and we're gonna use an oscilloscope to do this we don't need to do it the way that they suggest in the manual this you know their original alignment procedure was taking into you know account that most people didn't have frequency counters back in the day and we have that now so we can really simplify the alignment procedure for this thing so so that's what we're going to tackle next well get in here and first of all we'll see if this thing basically starts up so what I'm gonna do is get the you know this thing hooked up to my variac an isolation transformer and I'll bring this thing up very slowly and very carefully watch the light bulbs to make sure that they don't glow brightly all right let's find out if this thing is a signal generator or a smoke generator so I've got the oscilloscope set to where it should be and I've got the dial pointer just pointing straight up and down here it's on sine wave I got about the amplitude controls about halfway up on the face here and I'm on Bansi so that should correspond with where I am on the time base oscilloscope here so what I'm going to do is turn on the isolation transformer and variac supply so we'll just move you over here and we'll take a look at this here so the very act is down it's on current limit and I'm gonna turn this thing on right now so what we'll do is keep an eye on the bulbs on the top so I'm going to advance the very act now I'll bring it to 85 volts that's 85 volts right there and we should see a slight glow that's fine that means that you know thing is drawing just a little bit here you can see you see the light is lighting up in the unit itself so that's a really good sign if that light wasn't coming on there definitely be something wrong so now as I advance the variac you see these things light up some more so I'll just bring this thing right up to the top and the glow of those light bulbs is fine no problems if these things were blazing bright there would be an issue they look a little brighter in the camera than they actually are time for me to do some dusting here Wow at any rate that's the old look and earring so now what I'm going to do is just bypass this because it looks like the signal generator is fine I don't see any smoke and hey look at that on the screen there's a sine wave well that's a good sign wave all right zoom on in here look at that no problems so if i bypass this obviously the voltage is gonna go higher and I should probably see an amplitude increase let's see here it is now through and everything is warming up yeah there we go that's looking like a really nice sine wave - so we're off to a good start let's try the square waves here we go they should be interesting Hey look at that those are some really nice square waves not bad at all so let's move to different ranges here and put this back to the sign we'll go down to range B make sure all the ranges are working and it looks like they are no problems go down to range a go no problems let's go to the highest range here this up here there it is no problems of a 54 kilohertz right now or something like that let's move the amplitude control around hey move the variable cap all looking good move it right through it's throw to make sure there's no shorts and no shorts boy we're really off to a good start here so the next thing we need to do is align this thing now everything is working great this is the simplified alignment procedure for this echo model 377 audio signal generator remember this unit has high-voltage all over it so if you're following along you're doing so at your own risk if you're unsure of what you're doing do not do this make sure the unit is switched off make sure the band switch is in the band B position make sure that the waveform switch is in the sign position make sure the amplitude is at its maximum put a 1k ohm resistor across the output jacks now if you removed the knob for the main tuning capacitor here you want to make sure that it's aligned properly mechanically so first of all when you put the knob back on you want the actual little black line in the pointer to align up with this line here when it's fully meshed when the capacitor is fully open the black line on the pointer should line up perfectly with this line right here once that's done it's mechanically aligned at that point you want to take this and point it right at the 200 so you want it right in the middle of the 200 on the band B scale here so once that's done we're ready to turn the unit on and take a look at an oscilloscope the next thing to do is put your oscilloscope probe across this 1k ohm resistor right here I've got my oscilloscope probe attached on the backside it was just an easy place to attach it but it is across that 1k ohm resistor the first adjustment we're gonna make is the dial scale tracking so we know the dial pointer on the face of the unit is pointing at 200 cycles so we want 200 cycles present at the output jack if there is a difference between the two we will make an adjustment to this capacitor right here and make them the same now you don't need to get incredibly accurate with this you know if you're at you know 201 or 199 cycles it's absolutely fine so what I'm gonna do now is turn the unit on wait for it to warm up and I'm going to use this insulated screwdriver to make the adjustment here so we'll take a look at the oscilloscope screen again it is a good idea to let your unit warm up for a while and I'll just zoom on into this so now I want to adjust this so the frequency here reads 200 Hertz and as you can see it is off just a little bit so we'll also put my insulated screwdriver into the adjustment here and I will adjust this right now I'm tightening it up so you can see I'm at 199 or 200 it's a little bit of movement they're not bad so 200 point for as I say you don't need to get too incredibly accurate I'll move this down just a little bit more give it a touch jumping around a little bit because of the screwdriver and me moving that around get it down just a little bit more about there I'm getting really close I'm just being finicky right now again when this thing warms up that's close enough when this thing warms up it's going to move around just a little bit anyways so that's adjusted just that simple the next thing we want to do is adjust that potentiometer that's on the top side of the chassis there so in the instructions or the alignment procedure it says to align that to between 10 and 11 volts RMS so what I'm going to do is put my screwdriver insulated screwdriver down into that VR right here and I'm going to just adjust that now remember the amplitude control has to be maximum on the face so I'll just adjust this up you can see it jumps around a little bit so between 10 and 11 volts so let's save 10.5 let's adjust it for there it is a really finicky adjustment see here I'm just touching the control to make it do this right now that's close enough ten point five they say between ten and eleven volts and there it is that's it it's aligned it's ready to use here's the completed eco model 377 audio signal generator back in its case and I have to say I'm very pleased with the results in fact they even gave the case another coat of paint and it turned out great oh wait a minute I need to have the red jewel light glowing so this was a real time-consuming restoration this one here kind of went above and beyond having to strip the chassis and everything like that but it's worth it because it's gonna have a permanent home on the old-time workbench so now I'm on to restoring the next piece for that bench thanks for stopping by the lab today hope you enjoyed this episode involving this audio signal generator if you did enjoy the video you can let me know by giving me a big thumbs up and hang around there'll be many more videos coming like this in the near future there's lots and lots of test gear to repair yet if you're interested in taking your electronics knowledge to the next level and learning electronics in a different and very effective way you might want to check out my ongoing electronics course on patreon I'll have the link just below this video in the description and I may pin it at the top of the comments section as well so until next time take care bye for now you

Channel: Mr Carlson's Lab

Views: 124,532

Rating: 4.9498062 out of 5

Keywords: Tube Audio, audio test equipment, audio repair, signal generator, sine waves, square waves, vacuum tubes, schematic explanations, oscilloscope uses, learn electronics fast, capacitor leakage tester, cap checker, invention release, old test equipment, new test equipment, EICO 377

Id: 5tDCQDdilCo

Length: 98min 31sec (5911 seconds)

Published: Fri Mar 23 2018