Also, R53:
40ohm
200ohm
C26 to ground:
0.0
62.8
Q8:
0,0,0
61.1, 60.8, 61.8
Q10:
0,0,0
62.5, 63.4, 62
Apparently the bad channel doesn't turn off everything left off Q17.
Also, R55 was disconnected a long time ago.
40ohm
200ohm
C26 to ground:
0.0
62.8
Q8:
0,0,0
61.1, 60.8, 61.8
Q10:
0,0,0
62.5, 63.4, 62
Apparently the bad channel doesn't turn off everything left off Q17.
Also, R55 was disconnected a long time ago.
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Hi Peter,
I'm sorry I'm so slow to respond. Life intervenes...
I annotated the partial schematics with your measured voltages and will post them for comparison, tomorrow. (Late here.) An interesting phenomenon present in both working and failed units is the behavior of the emitter voltages at Q11 and Q9. In all four cases, the base-emitter junctions are reversed biased. The only explanation I can guess is that bias currents are so small the meter load currents to ground are sufficient to generate reverse biasing.
I suggest connecting the reference lead of the meter to the positive rail and probing the base and emitter voltages of Q11 to see if Q11's b-e junction then shows expected forward bias. If Mute were released in the good channel, I think this phenomenon would disappear.
The other glaring phenomenon in the problem channel is the roughly 60V on the collector of Q11. This implies 0.3W dissipation in both R73 and R48. Assuming the center ground path is intact, these resistors should be getting HOT! Are they? And it they are hot, where is the ~5mA+5mA current coming from? And how does this square with the small currents seemingly implied in the previous paragraph?
I think the problem lurks in this area.
TTYL
I'm sorry I'm so slow to respond. Life intervenes...
My apologies, as I intended to type left of Q37 rather than Q17, but didn't notice the error. In Mute, the balanced differential input stages are unbiased by virtue of having their current sources disabled. Later stages pass minimal current when the amp is Muted.
I annotated the partial schematics with your measured voltages and will post them for comparison, tomorrow. (Late here.) An interesting phenomenon present in both working and failed units is the behavior of the emitter voltages at Q11 and Q9. In all four cases, the base-emitter junctions are reversed biased. The only explanation I can guess is that bias currents are so small the meter load currents to ground are sufficient to generate reverse biasing.
I suggest connecting the reference lead of the meter to the positive rail and probing the base and emitter voltages of Q11 to see if Q11's b-e junction then shows expected forward bias. If Mute were released in the good channel, I think this phenomenon would disappear.
The other glaring phenomenon in the problem channel is the roughly 60V on the collector of Q11. This implies 0.3W dissipation in both R73 and R48. Assuming the center ground path is intact, these resistors should be getting HOT! Are they? And it they are hot, where is the ~5mA+5mA current coming from? And how does this square with the small currents seemingly implied in the previous paragraph?
I think the problem lurks in this area.
TTYL
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