Good news!
I switched to the other channel, set RV101 to max and got almost nothing on 4-6 (as it was).
Adjusting RV101 I could set it to 5mV. As I understand it should be between 5 and 10. It was not linear though; only about halfway of the pot did I start to see a change.
Happy to be making progress! Once again thank you so much for the amazing support 🙂
Now onto the other channel, will try to inspect and compare values. Maybe start with the resistors that where suggested in the beginning of the thread.
I switched to the other channel, set RV101 to max and got almost nothing on 4-6 (as it was).
Adjusting RV101 I could set it to 5mV. As I understand it should be between 5 and 10. It was not linear though; only about halfway of the pot did I start to see a change.
Happy to be making progress! Once again thank you so much for the amazing support 🙂
Now onto the other channel, will try to inspect and compare values. Maybe start with the resistors that where suggested in the beginning of the thread.
Excellent. So that channel you have powered up now seems basically OK.
The way to approach the other channel (well what I would do anyway 😉) is to power it up again (with the bulb of course) and look at voltage readings to see where and why the current is flowing. Of course give it all a visual look for burnt resistors and so on but remember that checking values in circuit doesn't usually work because of interaction with other parts.
The idea with faultfinding is to disturb and unsolder as little as possible. Try and gather the evidence by measurement.
Having said that the very low values may be possible to test in circuit. The two 10 ohms left uncircled should be fine. R129 would read 'short' in any case as its across a coil. Those two will be OK.
See what you find. Next step would be voltage checks to see where the current is flowing.
If they are easy to unsolder you could remove pins 3 and 4 and pins 6 and 7 which would isolate the output transistors. You could then see if the supply voltage comes up.
Ultimately I suspect you will be looking at more than a failed resistor and more likely a problem with one or more transistors but we need to isolate the problem first.
The way to approach the other channel (well what I would do anyway 😉) is to power it up again (with the bulb of course) and look at voltage readings to see where and why the current is flowing. Of course give it all a visual look for burnt resistors and so on but remember that checking values in circuit doesn't usually work because of interaction with other parts.
The idea with faultfinding is to disturb and unsolder as little as possible. Try and gather the evidence by measurement.
Having said that the very low values may be possible to test in circuit. The two 10 ohms left uncircled should be fine. R129 would read 'short' in any case as its across a coil. Those two will be OK.
See what you find. Next step would be voltage checks to see where the current is flowing.
If they are easy to unsolder you could remove pins 3 and 4 and pins 6 and 7 which would isolate the output transistors. You could then see if the supply voltage comes up.
Ultimately I suspect you will be looking at more than a failed resistor and more likely a problem with one or more transistors but we need to isolate the problem first.
One other thing you can try.
Add a link across this cap so it shorts out the bias transistor. This would force zero bias current normally. I think its worth doing just to see if the output stage responds to that. If it did the current would fall to zero and show there is a problem with the biasing.
I suspect the fault will still be present (high current flowing ) but its such a quick check that its worth doing.
I'll look in tomorrow 🙂
Add a link across this cap so it shorts out the bias transistor. This would force zero bias current normally. I think its worth doing just to see if the output stage responds to that. If it did the current would fall to zero and show there is a problem with the biasing.
I suspect the fault will still be present (high current flowing ) but its such a quick check that its worth doing.
I'll look in tomorrow 🙂
Another good news! From visual inspectio R122 is toast (10ohm). I already removed it, need to find a replacement. Could that be the cause?
I have 10R's but I believe they are for low current stuff. Tempting though😆
Edit: The resistors are blue so 2w right? Will swap.and see what happens
I have 10R's but I believe they are for low current stuff. Tempting though😆
Edit: The resistors are blue so 2w right? Will swap.and see what happens
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Quick update: Swapped R122 and now the bulb is lighting up real strong. Desoldered pin1 on the good channel to work only on the bad one.
WIll read your post carefully and check those values 😉
WIll read your post carefully and check those values 😉
Tomorrow you should test all the power transistors before you do any such thing. Don’t apply power until you’re sure all the silicon is OK.
Do basic checks on the 6 transistors in the output stage.
You can isolate pins 3 and 4 to isolate the upper output transistor and pins 6 and 7 to isolate the lower one. Do that to test them.
See if the others read as faulty in circuit. Compare with the good channel if you are unsure. You typically see a low reading if one is faulty.
Lets see what all that shows.
You can isolate pins 3 and 4 to isolate the upper output transistor and pins 6 and 7 to isolate the lower one. Do that to test them.
See if the others read as faulty in circuit. Compare with the good channel if you are unsure. You typically see a low reading if one is faulty.
Lets see what all that shows.
I count 8 transistors in the output stage and I believe 5 in the power stage right?
Going with my multimeter in diode mode I checked all 8 of them. All but one gives me a delta reading of around 10, ex: 740 and 750. (mV?)
Tr105 gives me the same value in both legs(710) and is reading in both directions.
If I understood correctly I can desolder 3 and 4 to isolate that section and see if it powers up without too much current.
Going with my multimeter in diode mode I checked all 8 of them. All but one gives me a delta reading of around 10, ex: 740 and 750. (mV?)
Tr105 gives me the same value in both legs(710) and is reading in both directions.
If I understood correctly I can desolder 3 and 4 to isolate that section and see if it powers up without too much current.
From right to left on the circuit you have the two main output transistors, then the drivers and then the 'pre' drivers'. Divide those into an upper and lower half and you have what are called a pair of 'output triples' which essentially form a high gain complementary output stage.
The transistor with the bias preset is similar to one of these:
https://en.wikipedia.org/wiki/Rubber_diode
TR102 forms part of the main voltage amplifier stage.
Remove this one and test it out of circuit. With the red lead on the emitter and the black on the collector it should read open circuit. With the black lead on the base it should read as a diode between base and emitter and base and collector.
Uncharted territory but I think this would work...
If you remove TR105 and also TR106 and also isolate 3 and 4 and also 6 and 7 then the amp should power up with correct voltages.
The transistor with the bias preset is similar to one of these:
https://en.wikipedia.org/wiki/Rubber_diode
TR102 forms part of the main voltage amplifier stage.
Remove this one and test it out of circuit. With the red lead on the emitter and the black on the collector it should read open circuit. With the black lead on the base it should read as a diode between base and emitter and base and collector.
Uncharted territory but I think this would work...
If you remove TR105 and also TR106 and also isolate 3 and 4 and also 6 and 7 then the amp should power up with correct voltages.
I don’t know why you are so obsessed with powering up an amplifier with a fault in it. Lose this obsession. You can identify the fault statically. Then you fix it, and then you can try powering it up again. Not before.
Tr105 is reading as diode between B-E and B-C. Giving a voltage drop of 770 for both.
Unfortunately it is giving continuity between E-C so toast right?
It looks that way. TR105 is a PNP and so should not read from C to E with the red lead on the emitter.
Does the transistor (which looks like a T05 style) pop out of that heatsink?
Does the transistor (which looks like a T05 style) pop out of that heatsink?
No, even applying a great deal of force it didn't budge.
There seems to be a ring inbetween the transistor and the heatsink (isolation?), maybe with a x-acto knife I can get in there. Or just ask my father, the king of angle grinders 😆
From my search the replacement should be BC461 or 2N5322
There seems to be a ring inbetween the transistor and the heatsink (isolation?), maybe with a x-acto knife I can get in there. Or just ask my father, the king of angle grinders 😆
From my search the replacement should be BC461 or 2N5322
I was wondering about replacements as well. All these are very old devices now. Using something in a different package outline might be a possibility as well such as BD139 and BD140. Its all uncharted territory with regard to stability in the output triple configuration. Power dissipation in the drivers is low when the amp is idling but will increase at very high output levels, and so small heatsinks are needed really.
You may also find other issues (perhaps more transistor failures) so don't just order the BC461.
You may also find other issues (perhaps more transistor failures) so don't just order the BC461.
Maybe I can score some vintage transistor from ebay. Some old listings had 34896's
I will do a prayer to the audio gods 😆
You will never find them and you don’t want them. Use a BC461, 2N5322, or any other equivalent, and a modern heatsink.