Fisher 800B Part 6b - Tuner MPX Module & Final Test

here's the bottom of the mpx 95 multiplex module and remember how this is a module that plugs into this chassis in this module there is one electrolytic capacitor and it looks like this right here this is the original one that comes in it see this little orange capacitor it's rated at one microfarad at 350 volts okay it is almost always bad replace it with just replace it and that's the one where i put this one in here i didn't have an axial lead one i only had a radial lead so i put that in there but it's fine replace it with either 350 or 400 volt one microfarad electrolytic or if you can fit a film cap in there you have one you could put that in as well and it would actually be probably be a little better even but anyway we are now at a point where we can we're going to have to adjust the multiplex because if you recall that was the only thing in this entire tuner that had problems so we're going to go through and we're going to align that now okay we are now on this multiplex decoder unit and the first thing we want to do is if you remember from the last video or from uh earlier in this this video i'm sorry we saw that number one there was some strange looking uh waveforms you know on the in the output and that we were not getting good stereo separation if any at all now the first rule of circuitry is thou shalt always measure thine power supplies so that's what we're going to do first and i'll let you know where i am with this because that's what i did and i'll open this up a little and the first thing i'm going to do is check the plate supplies you have two supplies coming into this module you have 125 volt and a plus 160 volt and of course your heater pins and these are going to be a little bit high because the line voltage what i found is this amplifier really likes about 115 volts for the incoming line and at 119 to 120 everything runs high as you can imagine now what i did do was if you look down here in the corner i added a cl 60 thermistor i usually frown on using these because they produce a lot of heat and they also can be a kind of a current limiting device for dynamic power so i normally would never use this in a solid state amplifier but in some of these vacuum tube amps they actually are kind of advantageous this gives you just about two to three volt drop on the in input which is still going to give us a little bit high power supply a little bit higher than the schematic but it'll at least be safe so anyway with that in mind these are going to be a little bit high but they should be roughly this and you can see this first plate supply should be about 95 volts and then this one should be about 40 volts and then over here they're higher voltage these ones are 160 volts and then this last audio output which i think has nothing to do with the problem are 94. but we just want to see where our voltages are so right now i have this connected to the 40 volt supply and spoiler alert this 75 volt one i've already tested and it's a little bit high it's probably around 80 something and again that doesn't surprise me so let's bring this meter in here and let's turn this on and we'll see what it does now see how high that is it's supposed to be 40 volts isn't it but wait until everything warms up you have to wait for the tube to warm up and see it drops down really well and it actually gets down towards that 40 volts it's around 55 but then look all of a sudden it goes right back up and it's high and it'll keep creeping up a little bit and it'll go all over the place and after leaving it settle for a while we're about 60 volts and by the way i have the fm multiplex turned off in other words this thing's powered but i don't want to have the influence of the tuner or anything in there right now okay so let's take this off and let's turn the power off i'm going to let this bleed down a little bit and we're going to do some static checks okay we're now looking as i bang the camera around we're now looking at pin seven which is the control grid of this tube element it should be roughly zero volts you shouldn't see any kind of voltage and you can look it's sitting at about negative point seven and i don't think that's right i don't think that's what we should have there but then again maybe we should now looking at this schematic again we have this diode it's you have this coil which is going to block any current from flowing through here because of the high frequency this is this is your 19 kilohertz right here and this should not that basically you have a one mega ohm resistor in parallel or i mean in series with a 10 mega ohm resistor up to this plate which should kind of or to this grid which should keep that down especially with this one nano farad capacitor here so i don't think we should be seeing that so the first thing i want to do is i want to measure all of this i'm just going to measure some resistances and i'm going to unplug the amplifier it's been turned off and everything we're going to move to resistance testing and if i check from here to ground you can see i'm getting 10 megs and i should be getting roughly 11 mega ohms see right here so let's move back and go on this branch right here and you can see i'm getting 8k and what i'm looking at right now is this with respect to ground and it's reading 8k and it should read one mega ohm now if this were leaking okay this coil acts pretty much as a short to ground when there's no high frequency on it so we may be reading something through here so what i want to do is i want to lift this diode to take it out and see if we have our one meg in there so let me get to get this heated up and put this in all right we got that out and look what happens we're now at one meg so now let's go back to our voltage and go back on that grid there let's plug this in and turn it on and see what's happening now let's see if we get our 0.78 volts there okay it's different but it's still going up there let's see what our plate voltage is like this is hard around the camera and look at that now it's at 45 where it's supposed to be all right let me get a jumper here and let me connect this diode back up so here's one end okay here we go boom look at that so that diode's bad it's leaking and the reason i went through this i pretty much had a suspicion this was the problem but i wanted to go through with you and kind of show you the symptoms of what these do this is notorious uh for doing this these these diodes leak these are germanium point contact diodes and there are ten of them in this circuit so looking here you have one two three four five six seven eight nine ten they're all the same diode type they're these point contact germaniums and it wouldn't surprise me if every one of them has that issue now let me turn the power back off and i just want to kind of show you something that we have a luxury here today that we can that we can do okay as it so happens i actually have another one of these mpx65 multiplex modules now this is a newer version the older version used 12at7 tubes this one uses 12a x7s the later models of these had that and the reason they did that was remember 12ax7s weren't collectible expensive you know tubes back then they were very popular and common and by using these this amp already uses 12ax7s a 12a t7 is not used anywhere else in these in these amps so that was just another tube that they had to stock so by going to this of course these aren't ideal for that circuit but it was reworked to do that so this is a drop in replacement i could literally just drop this in there and it would work but we don't need to we're going to fix the one that's in there i'm going to save this for another project but if you notice they still have the germanium diodes but they used a different type in this one and if we go to resistance even with the diode connected remember this was 7k with that diode in circuit and one meg with it out because you're reading across this one mega ohm resistor right here so let me connect this up and you can see it's one meg or 990 k and that's what it should be and you can see with this one still disconnected it's also one meg and if i just take a jumper here and connect it again you'll see that's going to go down to the 7k if i can connect this around the camera there and see what happens 7.8 k so this this diode's bad and the funny thing is uh if i just read the diode itself which is what we were doing you see that 7 k if i go to these other diodes in the circuit in the same manner like this one over here this just another the other one you can see it still reads funny i'm not getting a really good uh connection hold on but these are all all 10 of them there you go 5.4 k and if we go to to this one uh let's see where's that diode in that one it's hiding in here see here's those the eight they actually neatened it up on this but that other one where is it they moved it give me a second and of course in this one the reason i couldn't find it is that diode has been replaced by a one meg resistor so they took the diode out this again they reworked this circuit there are changes that they made to it there's a second coil here that's not on this one they moved it into a actually they moved it into a different location so some things have been moved around and changed and improved and the the two bridges that are used for uh for the for the 38 kilohertz doubler circuit they're really neatly mounted on this these two little bridge boards now whereas on this one if you look closely and i'll move the camera up you can see they're just kind of hanging out there in midair see they're just floating around but they're all reading not good you can see and that's the capacitance in the in the the diodes in that direction why that makes your meter jump like that you see 7.3 k they're all doing that the funny thing is if you read if you read this in diode mode it'll read a pretty normal forward drop 0.24 volts and if you read the other way around it doesn't it reads blocking so it seems like it's okay okay the phone is ringing today i feel like the wonder pets [Music] there's another easter egg for you all to figure out anyway um i'm going to replace these diodes before i do anything else and what i'm going to use is and these may be the wrong thing for this job but i'm going to use these russian point contact germanium diodes now i have several of these on order and they take a long time to come in but these are the ones i have in stock and we you could probably also use a 5711 schottky diode unfortunately i'm fresh out of those the only shotkeys i have really don't have the forward voltage drop and everything that i want so i think these are going to be a better fit and they are not a real high voltage uh maximum forward voltage which is or reverse voltage i'm sorry i think these ones are 30 volts or something like that however you know the these are similar to a 1n34a which i don't i think there may be 60 volts i would have to look it up but i think in the positions that they're being used i don't think there's going to be any problem with voltage so i think it'll be safe to use them if they don't work we can always swap them out with a different one when i get some other ones in so let's get that done and then we're going to go through and test this once again and see if everything looks good now that looks better already the diodes are replaced and now let's connect our meter and see if that corrects all of our problems with our voltages so the first one i'm going to go to is that one that was problematic and remember this is going to read a little bit high because our mains voltage is a little high so let's move this over here let's turn this on and let me back this out a little bit as you can see it and just what we want should go down to about 45 to 47 volts i would say and it is okay and according to this pin one should be 95 volts so it's probably closer to a hundred or so so not pin one that's the same as this lead right here and it's 100 volts excellent okay now if we go to this lower tube this next one in line here we need 160 volts on both pins one and pin six the two anodes and that's going to be our frequency doubling circuit right there so here's one pin and look at that 167 so it's 7 volts high they're all we little bit high and how about this one same thing so that side's good and really the audio output i'm not too worried about because we were getting audio out of that anyway so i'm thinking that one's probably going to be okay and we can always come back and revisit that i'm more concerned about everything prior to this bridge here very good now after doing a brief test on the stereo separation we're still having some problems adjusting that and what it turns out to be is this resistor or this potentiometer is bad and you can see it is the stereo separation pod so you can see right there it's a 50k pot and i noticed that the wiper on it is very loose to begin with which really doesn't surprise me but i don't think it is making any contact or good contact uh with the wiper you know the the shaft versus the wiper so let's move this over where you can see it a little bit and i'll kind of show you how this works so going across the two terminals of the pot you can see we're right about halfway it's a 50k plot and it's right about at 25k and if i rotate it i can't get my fingers in there as it is but if i rotate it you can see turning it makes absolutely no difference so we can't adjust the separation see it just never moves so we're going to need to replace that and i have a few ideas of what i want to do i don't really like this design so i think we're going to try to use a different type of pot in there wow where did i leave off so it's been a while since i was on this it's been a few days at least and things have been really busy at work so i'm going to try to pick up where i think i left off and fill you in so if i repeat some things that were in a previous clip that's why this is where we are we replaced that bad potentiometer and let me see if i can find it in here i don't even know where it went the the old one was defective it needed replaced and i don't know there it is so if you look this is right here and the little carbon track and it is just broken so it essentially was doing nothing in the circuit and where it's located is the uh let's see here in this part of the circuit this is your separation control and that's that pot that sits on top and if you look at the extra module i have you can see it up there and i just replaced it with a normal potentiometer and i soldered it on there and i think i showed a picture of it if not we'll look at it later but i kept these original leads as they are and i soldered the pot right directly to the chassis just here and here kind of similar to how they did on this one and it worked okay so we have separation adjustment now and if i feed a stereo signal into it this is what we're getting out right now we're going to have to explain a few things so you can see right and left and you can see how this is a very distorted signal this one's distorted as well and right now i am at about 75 percent modulation of that of the audio and if watch what happens as i turn the modulation down things start to clean up but as you notice the amplitudes are very different if i turn like i'm at 30 there's 35 percent modulation you're getting a sine wave now but you can see that the there's a lot of difference in the amplitude of right and left channel and you can see as i turn this up the signal starts to distort i mean right there we're only at 65 percent modulation and if we take it up to like 90 percent that's 90. it starts flat topping pretty badly there now if i go to right channel only you can see we are getting some separation now with that pot and if i adjust the pot you can see i can kind of i can really flatten that but if i move to the right channel now this one's got some bleed over and if i flatten that one out just like that now i go to this channel and it's off so you can't balance it even though the balance pot works now you can never get the two channels balanced all right and if we just go to just straight mono or if we just go to mono on the stereo unit so in other words i shut the multiplex unit off here and i go just straight fm there's fm perfect signal even even with the 90 modulation uh all of the noise is gone i have a little bit of that ringing in there you could see but again the the ant the receiver is open right now i have a bunch of things on the bench we're picking up some noise so ignore that but you can see the amplitude is the same you know the fm section is working perfectly it's only when we switch that module in you have to understand how this receiver works it's not like other receivers there's actually a loop when you select here we go when you select fm stereo since this is a standalone multiplex decoder module it actually kind of breaks the circuit path and it inserts that module in line with that and then the audio loops through this so this this unit this becomes part of your audio path and you can see right now when you when you switch to normal fm it's like this module is not even in there it just totally bypasses it so this is like a mono fm tuner right now but if i take this knob and i switch it to stereo mode like that it's now looping the the if through this okay using the the multiplex and everything and you can see even in stair in mono that's mono that's i'm putting a mono fm 400 hertz modulated signal in there you can see what it's doing so this multiplex module still has issues all right now why is it doing this so what we're going to be interested in is this section right here back here is where you're going to detect your 19 kilohertz pilot tone and that 19 kilohertz pilot tone is going to be filtered out or extracted let's say and it's going to be used number one to create our 38 kilohertz to regenerate that and it's going to also control the phase of everything because that's really important with all this so essentially you're going to have three adjustments you have your 19 kilohertz you have your 38 kilohertz where this is your frequency doubling circuit and then you have this 67 kilohertz uh sca trap that that you adjust this isn't really important right now for what we're looking at once your 38 kilohertz is regenerated it's carrying all of the audio information for your left and right channels and through these full bridge rectifiers we're going to split that off and we're going to generate our right and left channels here and these are those germanium diodes that we replaced because if you recall in the in one of the previous segments several if not all of these were pretty bad i mean all germanium silica or germanium semiconductors have some degree of leakage in them these ones were exceptionally bad and were causing problems we also replaced this pot here but now what we're going to do is we're going to actually look at that information if you notice there are three 33k resistors here and they're all kind of giving kind of isolating this from here okay or current limiting or whatever you want to call it changing the impedance and what we're going to do is we're going to read what kind of signal we have at the input of these this these series of resistors or this set of resistors and then we're going to look at the output of each resistor and what we should see since we're using a mono signal right now we should see basically the same information on each point here just out of phase with one another all right so we're going to look at the information here and then we're going to look at it basically on either side of each resistor i don't know if that makes sense so zooming in it's kind of hard to see it but you have this this this one down here and this one down here those are your four resistors and down here at the very bottom you see these two little tiny capacitors those are these capacitors here and here okay and what we're going to do is we're going to look at the junction of these coupling capacitors and these four resistors so we're going to look here and here first okay let's get you up on the scope and if i look at the one side with recent these are all with respect to chassis okay and you can see we have a nice sine wave ignore this noise this will be filtered out all that noise that you're seeing is going to be filtered when it goes through this little network here before it goes out to the audio we're actually looking at it before all of that and that filter may have problems too we're gonna have to check that later but for right now that's one side there's the other okay one and the other they're pretty pretty symmetrical i would say all right now we're going to look at the other end of each of those resistors so now we're going to look at we're going to look here here here and here again they should all be the same just with different phase okay so here's the first one not very good is it here's the other one and then here's the other channel those two resistors there's one there's the other so you can see they all are asymmetrical they're not correct so if i clip on this channel i actually have to increase this in amplitude on the scope just to even get it to lock see that doesn't look anything like it should and if i look at this one all kinds of amplitude but it's still very distorted then i look at this channel that's not too bad and then this channel and it clips and and again you should see the clipping because you're looking at each half of the waveform so what i'm saying is one side is not bad okay this one is actually working but there is a little bit of amplitude difference but the other channel i don't know we'll call it right channel and left channel is not it's really bad that one's halfway decent this one's really bad okay now there could be other components theoretically that could affect this a little bit but it should affect it the same way on each channel all right so if i go resistor number one remember these are all 33k that one's a little low it's 27.4 k if i go to the other one it's 31.2 k so that's a little closer to 33k but you can see they're quite a bit different now if i go to the other side and i clip on here and i check this resistor it's 34.5 so it's a little bit high and if i look at the other side it's 24.6 so almost you know 10k difference so these resistors are bad they're probably really out of tolerance i mean they should read 33k and they're not so either the resistors themselves are bad or some associated uh component that's in the circuit is bad okay bad lighting but hey we're gonna live with it so if i'm reading with my own meter from here to here there's really not a whole lot that you're reading through without these you know these diodes i mean everything is very high resistance compared to this 33k so essentially these should be pretty close to 33k and as you saw they are not they're very different so at the very least right now these also will need to be replaced whenever i don't understand something you know me i redraw it so what i did was i just went into paint and i just cropped out the part of the circuit we're interested in and i put the original one we have in the schematic versus what we have in reality when we look at the circuit that we have and it's pretty interesting because in this circuit you have these two resistors these one mega one meg resistors on each bridge that are kind of shunting across the output like this and you don't have it in our multiplex unit the other thing is this charge capacitor here this one microfarad it has a one mega ohm resistor referencing it to ground but in hours it's not there you just have these two 100k resistors with the cap going into it and nothing referencing it to ground and i can see this is factory nobody has modified this and if i go in really close i'll try to get as tight as i can with the camera without it kind of getting wonky on us but pretty close so there's our one microfarad capacitor and here is one of the 100k resistors that goes to it that's this back terminal is where this connects to and then underneath the little board here is the other 100k so here's where here's the the output of the one bridge for the one channel here's the output of the other bridge for the other channel and underneath here my pointer is touching it right now is the other 100k resistor you can't see it but it's it's right down here the tip of the pointer is touching it now so those are the two 100ks and they both converge right here on this back terminal and there is no one meg resistor going from that terminal to ground and just to verify that if i read from that point and i'll back you way out now if i read from ground which right here this is on chassis ground and i read to that point back there i'm reading 98k now i don't know if we reverse the leads if we're reading something across a diode or something again these germanium diodes are kind of nope it's it's 100k in either direction so with respect to ground we are reading 100k from here to ground and really we shouldn't when you think about it because you have a capacitor here and you have two resistors here and there is no going to the big picture there is no reference to ground here okay you got a cap here you got a cap here so there should be no reading of anything with respect to ground from here it should be almost like an open circuit because unless you're reading back through these diodes which you shouldn't technically be able to read all the way through all these diodes to ground and by the way i did check the diodes all of them are good there are no i mean they have normal uh leakage that you would see on a germanium diode but not 100k or whatever so there's something weird going on maybe my capacitor is bad we're gonna have to check that okay now this is where we are and there's been quite a lot since the last clip i decided to pull the module to make it easier to get to everything and to check everything i replaced the four resistors and you can see them in there and you can see the addition of some other components first of all i removed that electrolytic because it just didn't fit in there right and i found this film capacitor that should be just as good if not better and it was an axial lead which was a lot easier to fit in there so i replaced that those five extra resistors if you recall so in the original module as we found it this this all of these these two resistors these two here and this one here were all not in there so it kind of looked like this so no one meg to ground and no bypass right here what those are those are mods that were made later on probably in a later production run that were put in there in order to reduce distortion and i think that's part of why we're getting some problems with um like especially when you're over modulating and so forth it should help with that a little bit so i added those one meg resistors and you can see they go across these uh these four diodes here and then that other one you can't really see it well maybe you can't is right there and that goes from this lead to to ground now strangely enough when we look at our newer module the updated one that i have mine's a lot newer first thing you notice is you see how neat the two bridges are and you can see there are four one meg resistors so here's your two one megs and they go to here and then this goes over to there so that's that one and then these two go here it goes to here so you can see how much neater it is instead of them kind of flying leads you know the way that the old module is and then that one meg resistor to ground it's actually down inside here it's very hard to see but it's located right down there see it and it's going to that ground lug right there and it comes from over here so this is what the newer module looks like with the modifications much neater you can tell this was like an early production and it had some issues with the layout this one is much neater how it's laid out and again i could plug this one right in there and use it but i chose not to the schematic for instance shows these two capacitors as 3 900 picofarads and you can see in this one they have 5100 i'm going to leave those for now and you can see a couple places where they used ceramic in the original and they went to a mylar or a film capacitor in the later ones here and here so there have been changes that have been updated we could continue doing all this but i'm going to just do this much that we did the other thing is you have to remember they changed some of those other values because the later ones like this one they used all 12ax7s and this one uses two 12a t7s and a 12ax7 so that's also it's going to change a few of the values of the resistors and so forth for the biasing and that's not i don't have a schematic of that so that's going to be another difference and of course we're not going to that's why we're not going to modify it beyond those resistors there's one meg resistors that i added so let's get this module back in and see if we fixed it oh most important thing of all another problem i found was this diode right here you see it this one back here it was in backwards i had to reverse it so when i checked my work the first time i missed that and i think that was causing the strange clipping problem on the one channel so it'll be pretty exciting to see now if they match and with these mods if it'll be a little bit cleaner and won't clip as much okay everything's back in and i'm going to first probe here and here the two outputs of the 38 kilohertz circuit so essentially we're going to look here and here in the circuit just before the diodes let's get you up to the camera driving to the oscilloscope okay now here's the first one side one and look at that all the noise is gone and it looks really clean and let's go to side two same thing all right now let's look at our bridge rectifiers our our bridge circuits so now we're going to look at here and here and we're going to look here and here and what we should see is we should see half of the waveform okay so here's the first side of the first channel that looks good there's the other side of the first channel and you can see it's the opposite then we have the other channel first side and the other cha and the other side okay i think we got it fixed so now let's switch over um and look at the output of our record out jack so this would be like what the amplifier and what the speakers would be would be hearing okay this actually looks really good so here's your stereo right and left channels as the record output jacks look and if i go to left plus right that's your mono left minus right it should invert and it does right only and you can see right there we have pretty good separation and left only and i got to change the trigger right there and we can probably even flatten that out a little bit more by adjusting the separation control so let's see what we can do right there is about as good as we can get it there and not too bad there now all this noise here i think we're still having problems with our filter we're going to have to fix that but we have stereo sound now and the stereo beacon is working as well okay the radio is sitting on its side so this isn't the greatest view but i have this set to mono right now now watch what happens when i turn it on to stereo see what happens so this when you're in stereo mode they repurpose this uh tuning indicator to be a stereo beacon so when when the eye is closed that means it's a stereo signal and when it's open it's a mono signal and you can kind of see on the scope how it changes a little bit yeah let me turn the light back on so if we go from stereo to mono you can see what it's doing right there and again this is the this is the module doing this if we switch into straight fm it bypasses that module and you see how nice it looks and it doesn't really care there's your stereo signal again it's picking up that pilot so there's some there's still some problems in the tuner that we're going to have to get to filter out all the frequencies above the 15 kilohertz that's the bandwidth of fm but we're very close to having this totally fixed now all right let's take a quick look and see why this is why it is up here is your ratio detector and the output of that is right here this is where your detected audio comes out so in in normal fm mode all you're going to see is your fm signal passing through here this little filter here passes right through this multiplex output jack through this little coupling capacitor .05 microfarads and then this goes down to your switching network to be switched into your amplifier and your tone control section and everything so this is your actual audio right here and if you notice there's a little capacitor here that doesn't look like it's very important but it actually is this is the is a .0018 or 1.8 nanofarad capacitor and what this 1.8 nanofarad capacitor does is it takes it's a high pass filter it's a low pass filter i'm sorry low pass filter and what it does is it's supposed to attenuate that 19 kilohertz pilot tone from the multiplex but of course this is about the simplest filter you could have so there's a trade-off this this will only roll off at something like 6 db per octave or maybe even worse than that and what that's going to cause is that it's not just going to clamp off the 19 kilohertz it's going to start clamping it off before 19 kilohertz a little bit and at 19 it'll be rolled off a certain amount and then as you go higher than 19 kilohertz it'll just it'll attenuate the signal more and more the problem is if we increase this to the point that it totally squelches out that 19 kilohertz pilot tone it will also attenuate the high frequencies of your audio so they had to use a capacitance that was a trade-off between those two remember this is designed to to perform up to 15 kilohertz that's what fm audio is and so of course it's not the full fidelity that you would hear 20 to 20 kilohertz now normally in more modern circuits and more complicated circuits they would have a an actual either a notch filter in here to notch out that 19 kilohertz or they would have a really sharp filter that would very sharply at 19 kilohertz cut everything above that out but that requires more circuitry and they just weren't going to do that in this it's not really important because your ears can't is it's so far rolled off with this capacitor that your ear isn't going to hear that so if we go back up to our oscilloscope waveform and we look and i'm going to try to make this so you can see it a little easier so you're looking at a five kilohertz stereo signal okay so we have the stereo module switched out of the circuit right now so i have a stereo signal that we're tuned into but we're not using the multiplex module so you can see just the tuner by itself is not getting rid of that 19 kilohertz pilot when we switch in the multiplex circuit which is right here you can see that a lot more noise comes in through that circuit and of course some of this has to do with everything being open and everything but you can see there's noise there and if we switch through left plus right left minus right right only and left only you can see the left the right channel has a lot of noise in it but you are getting the stereo separation but here's where it gets kind of strange if i go to right only so you can see the left channel is pretty much out okay there's pretty good separation but if i go to a one kilohertz tone it's not too bad yet if i go to a five kilohertz tone you can see it it starts to bleed over even more and this is just this is the old technology here and if i go to the left only same thing and again i'm not syncing very well on anything and i'm using chop mode instead of we'll have to go over what chop and alt means one of these days but anyway you can see that that's some of the limitations of this really really basic looking circuit and of course so there you go that's just what you're going to have that is old stereo for you now we modified a few things to make it better but it's still not perfect but you can but you would get the effect of stereo if you were to listen to this and tune in a station we'll do that but um this should give you an idea now i still have to figure out what this ringing is on this channel i'm not sure what it is okay we finally got it and i'll show you what all it was there was a couple things number one right in between the filament pin and pin three there was a lead that was just kind of sticking out of the end it was when they when they made it in the factory they didn't crop it off far enough down and it was almost touching that filament pin and that was causing that oscillation in the one channel additionally we don't have quite enough drive signal the original capacitors that were in there were these ceramic .02 microfarad if you can see them right there and those were right here in this position and if you notice in the schematic this is an updated schematic they're 0.047 in addition later models change this from a 22k to a 27k so this all has to do with the filtering network and everything now this makes it very touchy to adjust but once you get it you get really good stereo separation and let's get you up here on the monitor or on the oscilloscope i'm sorry and if you look that's a stereo signal right now here's mono and you can see there's nothing in there stereo and you see your little pilot tone and that and that's as i said before that's just the way this thing's going to work you're not going to get rid of that tiny little bit but you can't hear that here's left plus right left minus right and you can see how sometimes it gets a little bit flaky and then there's right and left and you can see we're going to have to touch this up a little bit yet and i'm still trying to figure out this see it's just right on the edge if you go too much and the problem is this control is very touchy and it you can't really turn it precision enough so right there see that's not right it's it needs to be it's still a little bit off peak but you can see there's your right and your left and you can actually get it really flat if you want with the adjustment so we're still going to need to work on it a little bit but i'll have to fine tune it with uh just see if you just touch it with the tool but i can't move it if i move it it jumps because that slug in there is kind of sticky right see right there it jumped i can't move it close enough but let me fiddle with it and we'll show you the finished product okay another mod that had to be done so we have this 5100 picofarad silver mica capacitor and you can see we replaced it with a 3900 and if you look at the schematic that's what it has in this later version and what that does is that makes this coil a little less sensitive to adjust it doesn't get it gives it less range is what i should say to adjust it but it also gives you more precise adjustment so i think that's why they changed that because it made it easier to tweak in now that we have it let's take a look up here there's left plus right left minus right right and left and you can see right there's left and we can adjust the kind of get the get it flattened and you really have to mess with it to get it and it's still not quite perfect it still needs a little bit of tweaking but it just this is what this stuff is is totally analog there's no phase lock loop anything like that so any component that changes value if a capacitor or a resistor gets heated up or something and changes its value it's going to throw this whole thing off so this is what you deal with when you're when you're working with this old technology like this but it is working okay one last check there's right left left minus right left plus right and stereo i think it's time to listen to it [Music] well anyway you can't really hear anything over a mono lapel microphone but i will tell you it's working and it sounds pretty good there is some static if we um of course my my setup here if i stop the music and just play it with a carrier you can hear some static in the background part of that is is you know the old circuitry in the tuner and so forth some of it is my setup on my bench but i am getting pretty good stereo separation um it's not perfect and it never will be because this is a really early generation multiplex unit but i think it's as good as it's going to get right now and it sounds nice and it's pretty surprising how good it does sound one last thing is i replace these burned out bulbs in the face plate with these i was able to order online and if you notice these are not normal you know six six by thirty two or whatever they are fused type light bulbs they're actually five millimeter i think five or five point five by uh 60 or 40 or something 52 i forget well we could measure them they are yeah they're 40 42 millimeter by let's see four millimeters they're four by 42 millimeter so they're in odd shape but they're i found these on ebay a company that was making them so i gave it a shot and they work they look really good they look very natural they don't flicker they almost look as good as the incandescents that were in there originally so well let's give it one last listen with you know on the auxiliary input the tuner is working now and just so you know we're tuned into the carrier right now with no signal and if i shut the signal generator off we'll kind of move you up here and show you and if i shut this off so pretty quiet not bad and if i turn it back on let it fire up pretty nice and that's with a stereo signal tuned in so not bad all right let's try one last song and then we're going to wrap this up okay let's try mirage by the youtube library i like this one it doesn't have a whole lot of uh dynamic range but it does have it's a nice clean sounding song and it also it's very good with the high frequencies and so forth so let's get the recorder going and then let's start it up and then i'll get out of your way [Music] [Music] so [Music] [Applause] [Music] [Applause] foreign [Music] so [Music] not bad not bad at all well that ends this series everything works we did the phono stage we did the amplifier the preamp the tone section we repaired the multiplex module and cleaned it up a little bit replaced all the capacitors and did the proper alignment on it or at least verified the alignment and that's it so i'm going to wrap this one up i have another project i want to do before i start any more repairs or restorations and we'll see if i get to share that or not online or not i don't know if i'm just going to get the job done or if i'll record it but we'll see but anyway i wish you all peace joy happiness and good health in your lives and i look forward to the next project to get posted and i look forward to all your comments and there goes the computer and uh well we'll see you again real soon this is going to be an epic video i can tell you right now this last part because there's so much that i did so we'll see how it turns out how about it all right until then take care bye

Channel: xraytonyb

Views: 1,766

Rating: 4.9754601 out of 5

Keywords: electronics, vintage electronics, stereo, fisher, fisher 800, fisher 800b, tube amplifier, tube amp, tube receiver, amplifier, tuner, repair, restore, restoration

Id: 4wgmQsEHCUM

Length: 68min 53sec (4133 seconds)

Published: Sun Oct 03 2021