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SA-8100 protection circuit issue

Pulled one end of each of those: R13: 15k, R14:14.8k, R18: 148.9k

So, all are in spec. I was thinking R13 might be a problem also, but I guess not.
 
sort of leaves r21 .. if i remember right all has been verified that should matter apart from it .
voltage divider for q5 emitter .
 
R21: 14.8k

I'm wondering if I can just leave D7 out? Voltages without it are near spec on q5.
 
I wouldn't recommend it. Without D7, you won't have protection against a large DC offset.
 
It appears to me that the differential amplifier formed by Q3 and Q4 is out of balance since the collector voltages should be identical under normal conditions (ie. no significant DC offset present). With the base voltages on Q3 & Q4 both being 0V, the differential amplifier should be balanced and not in protection. Since it is not, one would first suspect Q3, Q4, R13 or R14. But those were either replaced (Q3,Q4) or tested good (R13, R14). So, someting else must be causing the collector current imbalance.

Here's one more check to make. First, return all resistors and diodes to normal. Then with the power off, watch the pin 16 voltage until it's been completely bled away. Then measure the resistance from pin 16 to the collector of Q3 and Q4. They should be very close in value confirming that there are no continuity issues.
 
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All resistors and diodes reconnected. Pin 16 voltage at 0. Resistance between pin 16 and Q3 C = 15k, pin 16 and Q4 C = 14.8k.
 
This one's a real head scratcher. I think the only thing we haven't looked at yet in the differential amplifier are the emitter resistors R15 & R16 which should be 82 ohms.
 
Could you maybe also post a couple of photos of component & trace sides of the protection board - maybe there's something to spot there ?
 
Yes, I agree we are at that point. Here are pics of original board, and after recap (top, side, foil side):

Original protection board 8100.jpeg Recapped Prot board 8100.jpeg Recapped Prot Board 2.jpeg Prot foil side 8100.jpeg
 
Has Q5 been changed ? to what ?
Also, what is it that looks to be 'liquid' near Q5, and to a lesser extent Q3 ? If it's leakage from the old caps, it needs to be cleaned up.
Will keep looking...
 
Q5 is the only transistor that was not replaced. The residue on the board is glue that was used when the caps were originally installed by the manufacturer. I was told that they glued down all caps so they wouldn't move.

Thanks for looking!
 
As an alternative, could I just lift the base of q5 and see if the collector readings at q3 and q4 equalize? Should they if q5 base is out?
 
Pulled Q5 base. Turned on and amp came out of protection. Voltages:

E C B
Q3: -0.6 18.6 0
Q4: 0.3 1.7 0
Q5: 16.4 -1.2
Q6: 0 9 -1.2
Q7: 8 8.2 8.7
 
need to get q4 collector voltage back up . lift d5 to rule out q1 q2 . see if q4 collector alters . leave q5 base out for now and d5 . if still same lift d6 .
 
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