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SA-8100 protection circuit issue

LesE said:
I believe that D5 was lifted to isolate Q1 & Q2 which didn't indicate a problem. Having D6 & D7 lifted shouldn't have a significant effect on Q3 & Q4 collector voltages. Both collector voltages should be balanced at around 18 to 19V.
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Yes, just wanted to make sure we were all on the same page. Q3 had bad signals yesterday, but that was a different set of conditions than today. But, I understand that q3 and q4 should essentially be identical, and right now q4 is not in spec. D6 is disconnected, which has an effect on the voltage at q4 collector, correct? Whereas d7 is connected, which has an effect on q3 collector.
 
When testing the Q3 & Q4 collector voltages, It's best to have both D6 & D7 either installed or lifted.
 
LesE said:
When testing the Q3 & Q4 collector voltages, It's best to have both D6 & D7 either installed or lifted.
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Yes, thanks for pointing that out. That was the condition (all diodes in, but base of q5 out) for the voltages I measured in post #159. Under those conditions q3 looks good, but not q4 E and C.

Should I reconnect D5 & 6, or wait until after I replace R14 and measure voltages?
 
You could probably leave all three diodes out for now but I thought you measured the resistance of R14 twice yesterday and it was fine.

I take it that Q3 & Q4 bases are still at or near 0V ?
 
LesE said:
You could probably leave all three diodes out for now but I thought you measured the resistance of R14 twice yesterday and it was fine.

I take it that Q3 & Q4 bases are still at or near 0V ?
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Yes, that is true. Since D6 is disconnected, if R14 is not bad, that would leave q4?

I suppose I could swap either R13/14 or Q3/4 to see if the problem follows one of them.

Yes, the base of q3 and q4 are both at zero.
 
Possibly but that's a new C1845. Swaping Q3 and Q4 would provide an indication if one or the other is bad.

Out of curiosity, did you test the two C945's that were replaced with C1845's after they were removed?
 
BTW, I don't recall having checked R11 & R12 (2.2K ohms). These resistors provide a small amount of current from ground to the bases on Q3 & Q4.
 
LesE said:
Possibly but that's a new C1845. Swaping Q3 and Q4 would provide an indication if one or the other is bad.

Out of curiosity, did you test the two C945's that were replaced with C1845's after they were removed?
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Just tested them now and they both appear to be functioning, but only checked them with the diode test function on my dmm.
 
KeithD said:
That was the condition (all diodes in, but base of q5 out) for the voltages I measured in post #159
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If I look at the voltages in post #159: The Vbe for Q3 (0.6V) shows Q3 is ON, with Q3 Ic = Ie = ca. 1.1mA, all more or less as expected. The Vbe for Q4 shows Q4 is OFF, passing no current - why ? if R15 & R16 are both tested good, how is Q3 emitter at +0.3V ? I'd look at R16, including the soldering & traces around it - if Q4 is off, there's no current path through R14 or D6, or am I missing something else ?
 
Checked R16 again: 82 ohms, checked continuity between solder pads on foil side and connecting lead on other side. Both ends good.

Out of curiosity, I swapped R13 and 14. Put d5 and 6 back in. Base of Q5 still out. Readings:

E C B
Q3 +0.8 +34 +5mV
Q4 +0.6 +1.7 +1mV

Swapped R13 and 14 back. Readings:

Q3 +0.8 +34 +4mV
Q4 +0.6 +1.4 +1mV

So, I don't think R14 is the problem. When the resistors were out I tested them again with dmm. R13 was 14.9k R14 was 14.8k.

The only other thing I can think of is to swap Q3 & 4 to see if the problem follows the transistor? However, with the diodes in, I'm still getting bad readings for E and C on both.
 
I thought that was the conditions you were using when you measured those voltages in post #159 ? The values are very different.

Yes - in post #159 Q3(E) was -0.6V, but Q4(E) +0.3V
 
Goldie99 said:
I thought that was the conditions you were using when you measured those voltages in post #159 ? The values are very different.

Yes - in post #159 Q3(E) was -0.6V, but Q4(E) +0.3V
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Agreed, so something has changed or is variable?

Now, the only way I can get negative voltage on the emitters of q3 and 4 is to disconnect the respective diodes d7 and d6. With D5 in, D6, D7, and Q5 base out here are my readings:

E C B
Q3: -0.2 +31 6mV
Q4: -0.35 +0.6 1mV
 
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