Another one, so soon!
Mine's a bit different than the other posted a few days ago, though.
This amp was given to me to repair, and initially it didn't look too bad - one failed output transistor which I think may have been caused by age rather than abuse.
So initial work involved just replacing the blown transistor, removing and testing the other 3, and removing, testing and replacing the driver transistors (all tested Ok but I destroyed them to get them off rather than destroying the board. I also replaced all small electrolytics and tantalums as a precaution.
It powers up OK but the problem is that I cannot get consistent bias current on the right channel (the one which had the failed output transistor). The left channel is absolutely fine, but on the right channel it comes and goes - one minute it will be there, and then a few seconds later, zero. It does not wander up and down - it's either there or it's not. When it's there I can adjust it fine with the bias adjust pot.
After a frenzy of component replacement (including the bias adjust pot itself), plus a very careful examination of the board for faulty solder joints and cracked tracks (none found), it's still like it.
Attached is the right channel schematic followed by the same diagram marked up with components I've replaced with new (red) and components I've tested carefully in situ and I'm 99% sure are Ok (green).
The question: which of the remaining component areas should I focus on to address the bias issue? Almost everything left to change now is SMD so I want to focus on likely things, rather than everything. Thanks!
Mine's a bit different than the other posted a few days ago, though.
This amp was given to me to repair, and initially it didn't look too bad - one failed output transistor which I think may have been caused by age rather than abuse.
So initial work involved just replacing the blown transistor, removing and testing the other 3, and removing, testing and replacing the driver transistors (all tested Ok but I destroyed them to get them off rather than destroying the board. I also replaced all small electrolytics and tantalums as a precaution.
It powers up OK but the problem is that I cannot get consistent bias current on the right channel (the one which had the failed output transistor). The left channel is absolutely fine, but on the right channel it comes and goes - one minute it will be there, and then a few seconds later, zero. It does not wander up and down - it's either there or it's not. When it's there I can adjust it fine with the bias adjust pot.
After a frenzy of component replacement (including the bias adjust pot itself), plus a very careful examination of the board for faulty solder joints and cracked tracks (none found), it's still like it.
Attached is the right channel schematic followed by the same diagram marked up with components I've replaced with new (red) and components I've tested carefully in situ and I'm 99% sure are Ok (green).
The question: which of the remaining component areas should I focus on to address the bias issue? Almost everything left to change now is SMD so I want to focus on likely things, rather than everything. Thanks!
Do the output transistors cool down when this happens? If so it is real.
When the bias reading goes away, measure the DC voltage wrt ground at the emitters
of the output transistors, and at the leads of the emitter resistors. A reading much different
from zero would imply an open circuit.
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A purely bias related fault will not affect the audio output.
Does the amp still play when the bias goes down ?
The bias generator seems to be partly controlled via the opto isolator. When the opto is on the bias will increase. I can't see the logic in that given that only one bias current will be correct for the topology.
T538 (bottom left) has the ability to shut of the current sink to the input stage. What controls that ? and is it kicking in ?
Does the amp still play when the bias goes down ?
The bias generator seems to be partly controlled via the opto isolator. When the opto is on the bias will increase. I can't see the logic in that given that only one bias current will be correct for the topology.
T538 (bottom left) has the ability to shut of the current sink to the input stage. What controls that ? and is it kicking in ?
That transistor is tied into the protection Thyristor across the output emitter resistors.
If I understand the protection correctly it appears that ~6Apk through both emitter resistors would trigger the Thyristor, or ~12Apk through one emitter resistor flowing into a load or into a short should also trigger the Thyristor.
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