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Magnavox astrosonic loudness pot issue

JURB said:
You see that the front balance is that funky type that boosts one side as it cuts the other.

The balance pot to which you refer I think is the FM balance pot. I am pretty sure he has the same problem on other sources. I'll relook at the print to make sure, but I think that is only for FM stereo.
Click to expand...

The pot i referenced is "R3" balance pot. The stereo balance (R6) is on the MPX board, facing the dial. There is NO Balance knob on the Front panel.
 
Yeah, now I remember that. R3 is not on the front panel, it is on the amp board or something.

Still, looking at page two he sent, that control can't knock a channel completely out like the usual type that mainly shorts the audio to ground.
 
"Q11; e=2.4, b=2.5, c=7.2. Q12; e=7.0, b=7.1, c=9.8"

Looks alot like Q11 has an open collector. You can check that using the diode function just like any other transistor but the forward readings you get will be lower. Also, it may check good and be bad. I have seen it. It checks good, not open or shorted but the damnthing just does not transist. If you are good with a soldering iron you can just switch them with the other channel, Q11 and Q16 that is. Careful of those old foils.

Germanium transistors can degrade over time and it might transist but have really low gain. But in this state, Q12 only has 2.8 volts to work on. It might have such low gain at that operating point that even if some signal is getting to it, it is so low you can't hear it.

Know what is funny ? I might actually have NOS components for that. A long time ago I bought a load of parts for Maggie audio gear. Surprisingly the load had a couple of Audio Technica AT13Ea carts in it ! I do not believe they ever used those in anything they ever built, but I was glad to get the deal on them. And there were dome tweeters, I've never seen any dome tweeters in their stuff. So apparently the company that went down sold parts for other things as well. But just for fun now I have to go look and see if I got some 36P3s. I also have some OEM parts for 1980s Sony TVs LOL.

But not to worry, a modification can make a silicon transistor work in there just fine, in fact probably better.

But next thing is, can you switch those transistors ? If you do and both channels work then that is the problem and you healed it with heat. This also happens to high value resistors like 100K. You desolder one pin to check it and the heat apparently heals the internal connection. That is why I go by live readings when possible, like nothing is shorted.

You could also try shorting C83 and 90 to each other. Short the side toward the selector switch first, it MIGHT work but doubt it with those voltage readings. Then short AFTER them, where they go to the resistors and then to C11 and 16. Do not short the bases of the transistors directly, short the junction of C83 and R54 to the junction of C90 and R76. If the dead channel speaks on the second test it is likely that C83 is bad and Q12 works just fine on 2.8 volts. If that's the case great, it is a matter of caps. If not then switching Q11 and 16 would be the best test. If the problem doesn't follow it to the other channel then we are in the "weird" territory. But all is not lost.

Never is.
 
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Magnavox did use AT magnetic cartridges in a few models in the 70's, the first I remember was a four channel quad compact. They also started using dome tweeters in their consoles around 1975 or so.
 
JURB said:
"Q11; e=2.4, b=2.5, c=7.2. Q12; e=7.0, b=7.1, c=9.8"

Looks alot like Q11 has an open collector. You can check that using the diode function just like any other transistor but the forward readings you get will be lower. Also, it may check good and be bad. I have seen it. It checks good, not open or shorted but the damnthing just does not transist. If you are good with a soldering iron you can just switch them with the other channel, Q11 and Q16 that is. Careful of those old foils.

Germanium transistors can degrade over time and it might transist but have really low gain. But in this state, Q12 only has 2.8 volts to work on. It might have such low gain at that operating point that even if some signal is getting to it, it is so low you can't hear it.

Know what is funny ? I might actually have NOS components for that. A long time ago I bought a load of parts for Maggie audio gear. Surprisingly the load had a couple of Audio Technica AT13Ea carts in it ! I do not believe they ever used those in anything they ever built, but I was glad to get the deal on them. And there were dome tweeters, I've never seen any dome tweeters in their stuff. So apparently the company that went down sold parts for other things as well. But just for fun now I have to go look and see if I got some 36P3s. I also have some OEM parts for 1980s Sony TVs LOL.

But not to worry, a modification can make a silicon transistor work in there just fine, in fact probably better.

But next thing is, can you switch those transistors ? If you do and both channels work then that is the problem and you healed it with heat. This also happens to high value resistors like 100K. You desolder one pin to check it and the heat apparently heals the internal connection. That is why I go by live readings when possible, like nothing is shorted.

You could also try shorting C83 and 90 to each other. Short the side toward the selector switch first, it MIGHT work but doubt it with those voltage readings. Then short AFTER them, where they go to the resistors and then to C11 and 16. Do not short the bases of the transistors directly, short the junction of C83 and R54 to the junction of C90 and R76. If the dead channel speaks on the second test it is likely that C83 is bad and Q12 works just fine on 2.8 volts. If that's the case great, it is a matter of caps. If not then switching Q11 and 16 would be the best test. If the problem doesn't follow it to the other channel then we are in the "weird" territory. But all is not lost.

Never is.
Click to expand...
Okay, when I shorted c83 and 90, on the side of the resistor, my sound just got fuller on the one working channel. The other side, my sound on the working channel started throbbing. Neither way gave me any noise or even a crackle out of the bad side. I switched q11 and q16, nothing changed. The same channel still doesn't work. The good one still sounds fine.
 
Ronnie baisden said:
Okay, when I shorted c83 and 90, on the side of the resistor, my sound just got fuller on the one working channel. The other side, my sound on the working channel started throbbing. Neither way gave me any noise or even a crackle out of the bad side. I switched q11 and q16, nothing changed. The same channel still doesn't work. The good one still sounds fine.
Click to expand...
When I said the other side, I meant shorting the other side of the capacitors together.
 
I guess the next thing is to switch Q12 and 17. There is also a slight possibility that R56 has gone high in value. At least it is a low enough value to check in circuit, so check that. In fact that is worth a quick ohmmeter check before swapping the transistors, much easier. Another thing is that C15 might be leaky. You could see if taking it out changes the DC voltages on the transistors but there is a chance the circuit will oscillate. If so, just put about any cap in there to quiet it down and see if the DC readings change.

In fact if it oscillates with that cap out of the circuit it is a very good chance it is leaky and causing the problem. It cannot oscillate without being able to amplify.

So yeah, check the resistor, disconnect the capacitor and see what you get. Then switch the other transistors. If all this fails there is something wrong with that big orangish yellow thing. But luckily, all that is is a bunch of resistors. It can be dealt with.
 
JURB said:
I guess the next thing is to switch Q12 and 17. There is also a slight possibility that R56 has gone high in value. At least it is a low enough value to check in circuit, so check that. In fact that is worth a quick ohmmeter check before swapping the transistors, much easier. Another thing is that C15 might be leaky. You could see if taking it out changes the DC voltages on the transistors but there is a chance the circuit will oscillate. If so, just put about any cap in there to quiet it down and see if the DC readings change.

In fact if it oscillates with that cap out of the circuit it is a very good chance it is leaky and causing the problem. It cannot oscillate without being able to amplify.

So yeah, check the resistor, disconnect the capacitor and see what you get. Then switch the other transistors. If all this fails there is something wrong with that big orangish yellow thing. But luckily, all that is is a bunch of resistors. It can be dealt with.
Click to expand...
Okay JURB, tested r56 out of circuit, got 5.43k ohms on it. Schematic shows 4.7k. Took that capacitor out (it was one I had replaced when I recapped) no oscillation (which I'm guessing is the pulsing of sound out of my good channel.) it tested out of circuit as 4.75. I switched 12 and 17 and no change. Same good channel, same bad channel. And because I hadn't even heard a crackle out of my bad channel in a while, I shorted the loudness pot again just to see and same as before it came right in. So should I go ahead and order a replacement resistor? Would that reading completely kill my channel?
 
JURB said:
I guess the next thing is to switch Q12 and 17. There is also a slight possibility that R56 has gone high in value. At least it is a low enough value to check in circuit, so check that. In fact that is worth a quick ohmmeter check before swapping the transistors, much easier. Another thing is that C15 might be leaky. You could see if taking it out changes the DC voltages on the transistors but there is a chance the circuit will oscillate. If so, just put about any cap in there to quiet it down and see if the DC readings change.

In fact if it oscillates with that cap out of the circuit it is a very good chance it is leaky and causing the problem. It cannot oscillate without being able to amplify.

So yeah, check the resistor, disconnect the capacitor and see what you get. Then switch the other transistors. If all this fails there is something wrong with that big orangish yellow thing. But luckily, all that is is a bunch of resistors. It can be dealt with.
Click to expand...
Whoops, I forgot to take the DC readings. If it didn't oscillate and switching them did nothing, do I still need to get those?
 
No, that resistor being off that much will not kill the channel. Could possibly make it distort but no way it could kill the channel.

I have to so some thinking now. I am pretty sure the culprit is PC5. You can't just get one. Mainly, PC5 provides for the biasing of those two transistors and their own idea of negative feedback. It is just a bunch of resistors. One of them is open.

As you now see, troubleshooting isn't really about finding out what the problem is, it is as much about finding out what it is not. What I am having trouble with now is that it is not making sound at all. Most failures I think, resulting in those DC voltages should make some sound, but probably distorted.

With it off, measure the resistance from the emitter of Q11 to ground. It should read like 11K. Also, it should read 1K to the negative side of C13.

If all else fails, we just eliminate all this **** and find a spider board and an OP AMP. Half of the tone circuit in these things needs to be scrapped anyway for good sound. If we go that route, it is not that expensive, but can be a bit tricky, and in no way can be considered a restore, nor will it be restoreable. It will sound better than new, but it will not sound like new.

Actually it could be done, but really if you get it fixed "right", which means tacking on resistors to the PC5, I had such a list of modifications anyway that the end result would be the same. I just like to avoid modifying if possible.

They had no reason to use that PC5 dohickey. What, to keep their design secret ? All it is is a low impedance preamp. And right there is their circuit right in front of your eyes. Just a bunch of resistors, and Howard W. Sams company reverse engineered it quite handily. Or did they ? That is another thing, they could be wrong. Wouldn't be the first time.

Anyway, too much more of this and we throw up our hands and say "ENOUGH" and find, literally, a 4558 generic IC and be done with that ridiculous circuit. They sell a little board you can mount the IC to, even a surface mount, and hook up wires. Problem is most OP AMPs are split supply but are adaptable to single supply. Now the ones that are intended for single supply are intended for a positive source and this thing has negative sources. So we have to adapt a split supply unit for this, It's just a matter of a few resistors.

So, I hope you find too high resistance from Q11 emitter to ground. If so, the fix will entail one resistor. If not, well you know...
 
Okay, this might be interesting to you. I didn't trust my resistance readings on q11 so I tested them all, each leg. Here's what I got:
Q11; E=0.L, C=16.1, B=0.L.
Q16; E=8.0, C=14.3, B=59.8
Q12; E=2.6, C=3.8, B=16.1.
Q17; E=2.5, C=3.3, B=14.3.
Does that look like 11 and 16 are both bad?
 
JURB said:
No, that resistor being off that much will not kill the channel. Could possibly make it distort but no way it could kill the channel.

I have to so some thinking now. I am pretty sure the culprit is PC5. You can't just get one. Mainly, PC5 provides for the biasing of those two transistors and their own idea of negative feedback. It is just a bunch of resistors. One of them is open.

As you now see, troubleshooting isn't really about finding out what the problem is, it is as much about finding out what it is not. What I am having trouble with now is that it is not making sound at all. Most failures I think, resulting in those DC voltages should make some sound, but probably distorted.

With it off, measure the resistance from the emitter of Q11 to ground. It should read like 11K. Also, it should read 1K to the negative side of C13.

If all else fails, we just eliminate all this **** and find a spider board and an OP AMP. Half of the tone circuit in these things needs to be scrapped anyway for good sound. If we go that route, it is not that expensive, but can be a bit tricky, and in no way can be considered a restore, nor will it be restoreable. It will sound better than new, but it will not sound like new.

Actually it could be done, but really if you get it fixed "right", which means tacking on resistors to the PC5, I had such a list of modifications anyway that the end result would be the same. I just like to avoid modifying if possible.

They had no reason to use that PC5 dohickey. What, to keep their design secret ? All it is is a low impedance preamp. And right there is their circuit right in front of your eyes. Just a bunch of resistors, and Howard W. Sams company reverse engineered it quite handily. Or did they ? That is another thing, they could be wrong. Wouldn't be the first time.

Anyway, too much more of this and we throw up our hands and say "ENOUGH" and find, literally, a 4558 generic IC and be done with that ridiculous circuit. They sell a little board you can mount the IC to, even a surface mount, and hook up wires. Problem is most OP AMPs are split supply but are adaptable to single supply. Now the ones that are intended for single supply are intended for a positive source and this thing has negative sources. So we have to adapt a split supply unit for this, It's just a matter of a few resistors.

So, I hope you find too high resistance from Q11 emitter to ground. If so, the fix will entail one resistor. If not, well you know...
Click to expand...
Wait a minute, I think I did that wrong
 
JURB said:
No, that resistor being off that much will not kill the channel. Could possibly make it distort but no way it could kill the channel.

I have to so some thinking now. I am pretty sure the culprit is PC5. You can't just get one. Mainly, PC5 provides for the biasing of those two transistors and their own idea of negative feedback. It is just a bunch of resistors. One of them is open.

As you now see, troubleshooting isn't really about finding out what the problem is, it is as much about finding out what it is not. What I am having trouble with now is that it is not making sound at all. Most failures I think, resulting in those DC voltages should make some sound, but probably distorted.

With it off, measure the resistance from the emitter of Q11 to ground. It should read like 11K. Also, it should read 1K to the negative side of C13.

If all else fails, we just eliminate all this **** and find a spider board and an OP AMP. Half of the tone circuit in these things needs to be scrapped anyway for good sound. If we go that route, it is not that expensive, but can be a bit tricky, and in no way can be considered a restore, nor will it be restoreable. It will sound better than new, but it will not sound like new.

Actually it could be done, but really if you get it fixed "right", which means tacking on resistors to the PC5, I had such a list of modifications anyway that the end result would be the same. I just like to avoid modifying if possible.

They had no reason to use that PC5 dohickey. What, to keep their design secret ? All it is is a low impedance preamp. And right there is their circuit right in front of your eyes. Just a bunch of resistors, and Howard W. Sams company reverse engineered it quite handily. Or did they ? That is another thing, they could be wrong. Wouldn't be the first time.

Anyway, too much more of this and we throw up our hands and say "ENOUGH" and find, literally, a 4558 generic IC and be done with that ridiculous circuit. They sell a little board you can mount the IC to, even a surface mount, and hook up wires. Problem is most OP AMPs are split supply but are adaptable to single supply. Now the ones that are intended for single supply are intended for a positive source and this thing has negative sources. So we have to adapt a split supply unit for this, It's just a matter of a few resistors.

So, I hope you find too high resistance from Q11 emitter to ground. If so, the fix will entail one resistor. If not, well you know...
Click to expand...
Okay, round two of testing and I got new readings for Q11: E=8.2, C=16.1 still, and B=58.6. No
JURB said:
No, that resistor being off that much will not kill the channel. Could possibly make it distort but no way it could kill the channel.

I have to so some thinking now. I am pretty sure the culprit is PC5. You can't just get one. Mainly, PC5 provides for the biasing of those two transistors and their own idea of negative feedback. It is just a bunch of resistors. One of them is open.

As you now see, troubleshooting isn't really about finding out what the problem is, it is as much about finding out what it is not. What I am having trouble with now is that it is not making sound at all. Most failures I think, resulting in those DC voltages should make some sound, but probably distorted.

With it off, measure the resistance from the emitter of Q11 to ground. It should read like 11K. Also, it should read 1K to the negative side of C13.

If all else fails, we just eliminate all this **** and find a spider board and an OP AMP. Half of the tone circuit in these things needs to be scrapped anyway for good sound. If we go that route, it is not that expensive, but can be a bit tricky, and in no way can be considered a restore, nor will it be restoreable. It will sound better than new, but it will not sound like new.

Actually it could be done, but really if you get it fixed "right", which means tacking on resistors to the PC5, I had such a list of modifications anyway that the end result would be the same. I just like to avoid modifying if possible.

They had no reason to use that PC5 dohickey. What, to keep their design secret ? All it is is a low impedance preamp. And right there is their circuit right in front of your eyes. Just a bunch of resistors, and Howard W. Sams company reverse engineered it quite handily. Or did they ? That is another thing, they could be wrong. Wouldn't be the first time.

Anyway, too much more of this and we throw up our hands and say "ENOUGH" and find, literally, a 4558 generic IC and be done with that ridiculous circuit. They sell a little board you can mount the IC to, even a surface mount, and hook up wires. Problem is most OP AMPs are split supply but are adaptable to single supply. Now the ones that are intended for single supply are intended for a positive source and this thing has negative sources. So we have to adapt a split supply unit for this, It's just a matter of a few resistors.

So, I hope you find too high resistance from Q11 emitter to ground. If so, the fix will entail one resistor. If not, well you know...
Click to expand...
Okay, new results: Q11: E=8.2, C=16.1 still, and B=58.8. Not sure why I got open on E at first. It was steady too. Hmm.
 
Ronnie baisden said:
Okay, round two of testing and I got new readings for Q11: E=8.2, C=16.1 still, and B=58.6. No

Okay, new results: Q11: E=8.2, C=16.1 still, and B=58.8. Not sure why I got open on E at first. It was steady too. Hmm.
Click to expand...
(My phone died mid typing which is why I repeated myself there. Didn't see that it had saved it.)
 
An open on E would cause this.

This is getting weird. Need some time to think. However if you do happen to find a bad connection to E of that transistor it would explain it.
 
So if I just short that loudness pot like youshowed me, do I lose stereo? Just get mono? I'm thinking about doing that just to get sound from both sides. Shorting various wires going into and out of that PC5, but at that junction is the only place I get the same volume to both sides of speakers. Should I alternatively just build what the SAMs thinks is the circuit there and see how it goes?
 
Yup, if you just short it out you get one channel in both speakers.

Didn't mean to neglect this, my head is into a couple of things at work. Bad new parts etc. I still don't actually know what to try next, that you can do there. I would be about to the point of going through my stash of resistors and trying some tricks with biasing those transistors, but you have no stash like that.

With the weekend upon us maybe I can come up with something to try next.
 
JURB said:
Yup, if you just short it out you get one channel in both speakers.

Didn't mean to neglect this, my head is into a couple of things at work. Bad new parts etc. I still don't actually know what to try next, that you can do there. I would be about to the point of going through my stash of resistors and trying some tricks with biasing those transistors, but you have no stash like that.

With the weekend upon us maybe I can come up with something to try next.
Click to expand...
Well no hurry, I appreciate the help. I could just start testing every part to see if it is in tolerance, but I'm not sure of the area to do that in. I don't want to screw with what works. So maybe between the loudness pot and transistors on the schematic.?? No hurry though, take your time. I'm up for anything.
 
So maybe this is helpful, when I connect a piece of wire between the collector on q17 and the negative leg of c217, the other speaker starts working. Not sure if I'm just replicating the other channel again, and it sounds bad. Scratchy and a lot louder than the other speaker, but at least it was making noise. Does that sound like its actually the other channel? Or just the good one splitting?
 
Ronnie baisden said:
So maybe this is helpful, when I connect a piece of wire between the collector on q17 and the negative leg of c217, the other speaker starts working. Not sure if I'm just replicating the other channel again, and it sounds bad. Scratchy and a lot louder than the other speaker, but at least it was making noise. Does that sound like its actually the other channel? Or just the good one splitting?
Click to expand...
Make that the negative leg of c26 according to the schematic.
 
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