Fix A Radio Receiver!

hi everyone and welcome to another episode of mr carlson's lab today we're going to fix a radio receiver together the story goes like this the person that had this radio receiver tried to do a restoration on it and it never worked again so usually this goes one of two ways usually it's really really bad when you open it up and parts and pieces are hanging everywhere or it's a really good restoration and something minor went wrong at any rate we're going to find this out together so let's discover what went wrong in this radio and bring it back to life again let's get started here's the radio that we are going to get working today very simple little radio rca victor and everything feels good the needle moves and it turns on and off makes the switch clicks anyways i haven't plugged it in at this time on the back side it's got a screw here and a screw here they're both different hopefully they're not threaded into a cracked bakelite case inside so usually they put a schematic or diagram on the bottom and as you can see that's long since gone and really it looks like the only thing that's holding this thing together is these two screws right here and the knobs on the front so what i'll do is i will pull these off they come off just that easy and i'll have to get two different screwdrivers here the rear side let's see this one will work for this one down here look at that you can see that kind of a star washer on that i'm glad that didn't tear this up there's a little piece of it missing on the end maybe it did and let's see this should work for this one here all right and if all goes well hopefully we are in so put this down like this and that way if it wants to come out it's nice and easy to get everything off just like so so put this behind my chair very carefully speaker's in very nice condition look at that speaker looks brand new this radio looks like it's in very nice condition i can see why you wanted to restore this look at this thing almost looks like it came out of the box well already i see a problem i already see a problem here so what i'm going to do is i'm going to reposition the camera here and we'll take a look at this problem that i see i'll give you a quick view of this right here and you can guess at the problem if you don't already see it and i'll reposition the camera and i'll be right back the first thing i noticed when i removed this radio from the case is the vacuum tube placement so i'll start over here with this variable capacitor so when i move the tuning around as you can see the needle moves and the capacitor moves at the same time because they're all attached by a dial string which doesn't feel all that incredibly smooth but it works anyway what happens here is there's two sections in this capacitor so this tunes the antenna which is on the back here you can see this wire that's on the back side and this portion of the capacitor tunes the oscillator so what's happening here is this is known as ganged so these two capacitors are ganged together by one shaft and the reason that they are ganged together is so that they track now what all this means is is when you're moving the dial on the radio receiver what's happening is there's a frequency change because this capacitor is adjusting the oscillator all the time well this portion of the capacitor is attempting to keep the sensitivity of the radio at its maximum as the oscillator is shifting because you know they're both moving at the same time so what they've done is they've attached the oscillator and the antenna tuning section together so that as you're tuning this you can look at it as it's automatically continually keeping this antenna tuned to the radio so you have maximum sensitivity throughout the entire band or throughout the entire dial range of the radio now you'll notice that in some radio receivers they'll have you know two three four sections of capacitors all ganged together with one shaft and they're all tuning different stages at the same time so one might be tuning the front end one might be tuning the rf stage may have a second rf stage and then an oscillator stage so they'll have capacitors with many more sections in them and then of course they're tailored to the design itself so when they're putting these things together the engineer is saying okay well the capacitance needs to shift this much so we're going to give it this many plates so that it moves in this many picofarad and you know there's a lot of thought that goes into all of this stuff and these here are the adjustments for it so this will trim up this capacitor here so that you can get rid of the you know the variability i guess you could call it what's going to happen here is that you can tune this for a certain area and then it will track from that area on and that's the reason that they have this right here and then of course this is the adjustment for the oscillator and then there's another adjustment down here for the actual oscillator coil where does all of that bring us what flaw or what problem do i see on the top end well immediately it's tube placement so this glass tube right here is a rectifier tube all right and then what this does is this takes the the line cord so you have 120 on the line and that takes that and changes it to direct current for the radio that's what this one does here so the entire radio inside runs on dc so it takes whatever is coming out of the wall and changes it to dc and on the underside there's going to be some filters we'll take a look at this here in just a moment and then what happens is that powers up the entire radio so this is a very important tube in this radio right it's changing everything to dc the problem lies is this here is an audio output tube this big glass tube is an audio output tube usually a 50 l6 and what happens is is this needs to be attached to the audio output transformer we'll take a look at that here in just a moment now whenever you have an oscillator section in a radio like this the oscillator tube is right next to it because you don't want any excessive lead length well the audio output tube is right next to the oscillator section so that means that there's a tube placement issue they wouldn't put the oscillator tubes over here and then run all the leads all the way over to the oscillator coil in any oscillator section you want to keep lead length as short as possible because you deal with stray capacitance and you know in leads and things like that you also get inductance in the length of the leads which causes issues that will cause oscillations and everything so i can already tell you that this tube right here is in the wrong spot let's remove this this should be a 50 l6 this is a very standard common aa5 there it is 50 l6 right there all right gt is glass tube and then g is glass all right made in canada don't see that very often anymore okay so we know that we have a problem there now the 12 sa7 will be the oscillator tube that should be in here a 12 sk7 is known as an if amplifier and the 12 sq7 is an audio amplifier and a detector tube okay so let's see what they plugged in right at the opposite end of the radio there it is 12sa7 this is the oscillator tube so this is going to need to be in this socket right here yet to verify this okay so know that that already is wrong so i'll just leave this out because i don't know the placement over here i have a really good idea usually after the if transformer right after the if transformer is the 12s k7 and then it moves down the chain to the audio amplifier detector and then the audio output so i imagine that's most likely the way that this is going to be so let's take a look at the underside and as you can see the fella that worked on this did a pretty nice job honestly don't really agree with the uh capacitor choice i stick to rubycon nishikon united chemicon i'll stick to those because i know that they're very good um yeah at any rate this is what he's put in here and he's usually used the old little bracket to hold the old capacitor and he's just put them in there all in all the job that's done under here is a really nice job even though you know he's used the j-hook method right there's a little bit of the j-hooking method going on in here which is fine a lot of the components were soldered directly to the socket lead length here is a little bit excessive and as you can see the lead length over here is a little bit excessive as well but all in all for somebody that's doing this really as a beginner this is a very nice job it really is put it this way i've seen a lot worse so all in all it's looking really good on the bottom so now we want to know if the rectifier is in the right spot so this is the rectifier here okay and as we can see the line cord runs over here right over to here one way to find out if it's in the right socket is to follow the lead to the rectifier and that would be this lead right here now this could have been replaced you know it's a little bit crusty here and this could have been replaced from this point all the way back to the socket but you can see this runs directly over here to the rectifier right into this which is a filter capacitor all right so this tells us right now this is where the whole chain starts and we have filtering going on over here so this is going to be correct see that right here rectifiers there so we can leave that alone that's looking pretty good so now let's just remove this tube see what's going on i'm gently rocking this back and forth while pulling up you don't want to be careful that you don't you know very you know give it a lot of rocking you want to just rock it like this as you pull it up reason being is you don't want to break this little locking area this is an index on the bottom here and you see that little you see kind of an extrusion on there this is known as an octal tube base and they you can see the little index there well they're pretty loose on this area here and that allows you to rock them and pull them out but you don't want to excessively you know i guess put excessive force on them because i've seen many people break these index pins or this little index area off this is very common in guitar amplifiers they always seem to be broken off in those so anyways this is a 12 sq7 so this is the detector and first audio so this is going to be right next to the audio output tube and i can guarantee you that the audio tube will not be here because this is the if transformer so this is going to be the iaf okay so this should be a 12 sk7 it is okay so 12 sk7 here so we're going to want to put that in this socket right here because this is right next to the if chain going here and then it's going to run from this tube into the detector in first audio which is most likely going to be this tube right here we'll just verify this in a moment i'll take a look at the bottom of the chassis plug that in there and then the audio output tube should be the furthest one down the chain so it's almost almost like every tube except for this one here was in the wrong socket this is why it's so important to take note of everything that's done so let's take a look here so the audio output tube is going to have a large value resistor from the cathode which should be number eight this is pin eight right here and this is running to ground and then we're gonna have our signal coming in on five this is the audio output tube right here so this is the coupling capacitor this is the coupling capacitor right here that's running from the plate of the 12sq7 running over here to the grid which is number five pin number five of the 50 l6 that's this one right here right so we go eight seven six five okay so that's the signal in cathode resistor right here that's it so what do you think let's try it out so i am going to plug this into my current limited isolation and variac supply reason being is because the chassis on these radio receivers can be hot or not depending on which way you plug it in as you can see there isn't an indexed or polarized plug here so you can plug it into the wall this way or you can plug it in this way which gives you a 50 50 chance of making the chassis of the radio connected directly to the ac line and that's why the knobs are bake light and there is no connection to the outside and the antenna connection which will most likely be isolated by some form of a capacitor underneath the chassis so that this here doesn't cause any issues that's one of the biggest problems that cause issues with antenna tuning coils and things like that is the cap on the antenna that isolates the antenna connection you know basically over time because they're wax capacitors they end up well coming close to a short you connect this to an outdoor antenna or something like that and maybe it's close to ground and if this is plugged in the wrong way bad things happen you get big sparks and you wreck the antenna connection sometimes you actually blow the connection right off over here so whenever a radio like this is worked on it's always advisable to use an isolation transformer mine is current limited and i also have it on a variac as well so i can bring it up slowly which we are going to do right now so plug this in there it is turn my variac down and let's see here i'll put this on like so i'll turn this up to about variac up to about 50 volts turn this on that's normal you'll see the light light up just a little bit and as the radio starts to pull current so basically as the rectifier tube heats up and the rest of the radio starts to draw current through the rectifier this here will get brighter and since i'm at 50 volts on the line that is going to take some time see if i can get some of the light out of here so we can see this just a little bit better so it's looking good okay and it's not drawing excessive current okay so what i'm going to do is turn this up to about 100 volts i think you hear a bit of hum now i do [Music] that it's coming to life it's coming to life and it's not humming so he did a nice job with the caps and getting everything all put together okay i'll turn this down and give it full line voltage all right everything should get nice and warm now [Music] so there's an oscillation happening here this is good for halloween make it uh make it into a halloween radio or something okay that's enough of that at any rate so what's happening is there's an oscillation happening here so what i'm going to do is i'm going to go grab a little tool and we'll just see if that oscillation is happening anywhere in the chain here and we'll most likely be able to address that here in just a moment so i'll just go get that right back let's try and find the oscillation using the carlson super probe here so this is something i put together a number of years ago there's a video right here on youtube about how to build one of these things if you're interested in it and if you're interested in the printed circuit board diagrams and alignment procedure and all that kind of stuff everything pre-sized and all that that's all up on patreon as well so there's lots of attachments up there for this at any rate let's take a look and see if we can actually find this weird oscillation so this is a very very sensitive tool turn this on here turn that up so this listens to all sorts of things you'll see here in a moment so i'll just connect this to the chassis here to keep it somewhat quiet and i'll just move the probe around you'll see this little green led light up when we get a substantial signal coming into the probe so you can hear this is fluttering right now we would see this led start to flutter with the oscillation here if we get close to it so i'll move around here [Applause] to hear that this is picking up the rectification going on inside this rectifier right now [Applause] so that's changing ac to dc and we're hearing it that's what that sounds like anyways i'll move this along here i don't see anything by the actual oscillator itself we can see the oscillator is working see that that indicates that the oscillator is working it's very strong and then again this is the rectifier doing its thing so let's move over here to the iaf stage so we can see that the oscillation is in the if section by me moving the probe closer and further away from this glass tube is actually affecting that oscillation [Music] and we might even see yeah we can hear a little bit of filament buzz in there and a bunch of other things going on i tuned this into a station we'd actually hear the station in here most likely so what does this tell us shut this off and turn this down so what does this tell us well i'm about to forecast what's going on here every one of these radios tells a story all right and as you work on this kind of stuff more often you get to realize what's going on and you get to see a chain of events what's happened is the tubes have been put into the wrong spots what's the next thing that happens well nothing happens right because all the tubes are in the wrong spot chances are now i don't know this yet because we haven't you know stuck screwdrivers in any slots here and tried to tune into the if transformers or anything like that but usually what happens is since nothing comes out of the radio at this point the next best thing to do is start turning everything in sight so you screw driver the radio right out of alignment and you put it into oscillation and i can almost almost guarantee that so tubes put into the wrong spot turn the radio on nothing happens so next thing you want to do is try and tune everything to see if you can get a radio signal so i can almost guarantee that these if transformers are right out of tune so what i'm going to do is i'm going to go grab a tuning tool and i'll be right back we'll work on this thing and see if we can just peek this up by ear and see if we can get this thing performing somewhat normal without going through a complete routine alignment right now okay let's address this and see if we can peek this thing up and get rid of that oscillation so i'll start with this one right here using an insulated tuning tool wow listen to how that's coming up eh was here right about there it's even receiving a station just like [Music] that i just want to peak for maximum static right now okay we'll go over to this one all right then i'll go back over to this one here okay so we want to go to the upper end of the dial and give this a bit of a tune we'll see what happens here this is the antenna tuning section turn that down and how we can tell that we're going to the upper end of the dial without actually looking at the dial face how can we do that you might want to take a guess on that right now well when the capacitor is fully meshed so all the plates are inside of each other it's at the lowest frequency when the plates are completely out of each other it's at the highest frequency so again this is just a a quick preliminary tuning as you can see i'm moving this open okay so this would be the upper end of the dial right here if the plates were to go this way and be fully meshed that's going down in frequency and as they open it goes up in frequency okay so now what i'm going to do is i'm just going to peek the antenna section here and see if we can get any more sensitivity out of that so turn the volume up so we can hear it okay look at that everything's been turned in this okay so watch now this one here and this one here will adjust the dial tracking to make this track correctly and these here are adjustment points so you adjust this and these are these correspond to frequencies on the dial so you don't have to have the dial glass there we won't worry about that right now all we're going to do is just see if we can get this thing working and at that maybe we'll dedicate an actual full alignment video to this thing if you're interested in that you can leave that down below how about that okay let's see how far and uh you know how well this gets us into the received portion of this radio see if it's working well all your favorites it's doing extremely well for being in the lab here because the lab is there's basically no reception in here whatsoever so i'm going to do is i'm going to get an external antenna attached to the back here and then we'll sweep through the band and see how well it does with an actual external antenna again the lab is um i guess you could say very well shielded i'll be back all right let's attach this to an external antenna and see how well this thing receives so this is still attached to my current limited isolation transformer and variax supply so i feel quite safe about attaching this common attach that to the tuning capacitor there and i'll attach the antenna signal lead right over here there's here radio stations already [Music] [Music] or they don't wind do a hard down there are radio stations everywhere this thing is really trying to receive now normally there's a religious station right at the bottom of the dial so in order to get a quick idea if the oscillator is somewhat in alignment i should be able to hear a religious station right now and what does that tell you well just like everything else and this thing was screwdrivered i imagine the oscillator is screwdrivered as well right [Music] and there's no religious station down there whatsoever it's it's out of alignment here so it's really trying to receive and it seems to be working just fine so if you're interested in actually seeing a full alignment of this radio receiver so online the iaf the antenna section and the oscillator section using some test equipment let me know in the comments below and we'll address it at that point we'll go through and give this thing the uh the supreme tune up and then see how well this thing actually performs so all in all really the only thing that's wrong with this thing was the tubes are in the wrong socket and it was basically screwdrivered into outer space so the fellow that worked on this before did a great job on the under half you got everything right because it is working just something as simple as the tubes are put into the wrong socket and then of course after that nothing was received so everything got twisted and made everything all that much worse if you're enjoying my videos you can let me know by giving me a big thumbs up and hang around there'll be many more videos like this coming in the near future we'll be taking a look at vacuum tube and solid state electronic devices alike so a lot of very interesting things coming in the near future if you haven't subscribed now would be a good time to do that as well if you're interested in taking your electronics knowledge to the next level and learning electronics in a very different and effective way and gaining access to many of my personal electronic inventions and designs you're definitely going to want to check out my ongoing electronics course on patreon i'll put the link just below the video's description and i'll also pin the link at the top of the comment section so if you click on the link it'll take you right there all right until next time take care bye for now you

Channel: Mr Carlson's Lab

Views: 98,102

Keywords: Fix a radio, radio repair, radio restoration, electronic repair, fix electronics, electronic maker, radio alignment, electronic alignment

Id: MZ0iXQ-VUXI

Length: 30min 53sec (1853 seconds)

Published: Mon Oct 26 2020