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SA-8800 protection mode help

Cool - I ordered those, so it was an easy replacement!

First checked the DC output voltages to make sure we weren't just triggering Q3 or Q4, but both appeared to be pretty close to 0V (mV level fluctuations on the DMM - probably the actual signal?)

Replaced Q2 with the 2690AYSTU, and initially, it stayed in protection mode when I turned it back on. I started probing the Q2 voltages to see what was going on, and the instant I hit the collector pin (which looks like it goes straight to the relay...), it clicked on. Appears to be working fine now - I tried cycling the power a couple more times, and the relay kicked on on it's own. Any guesses on why it didn't kick on without the probe the first time?

Q2c has been +50VDC each time I've checked it... in and out of protection mode. Amp worked for about 15 minutes after the relay initially kicked on, when went back into protection mode.
 
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Q2C should be less than half a volt when the relay is pulled in, and not in protection. When you power on the unit starts 'in protection' then, as everything settles down and becomes normal C24 charges up to a voltage higher than the Zener D15 voltage and the BE junction of Q2 (about 15.6vdc) and Q2 saturates, pulling in the relay.
The only thing that comes to mind about the probe kicking on Q2 is a cold solder joint being moved and making a connection :dunno:. Reflow the solder on Q2 and the relay coil.
 
Ok, I went ahead and took a bunch of voltages in the circuit while the amp was in protection mode... Had to relabel a bunch of points on the schematic for clarity (for myself as much as anyone else!) - marked up schematic is attached.Capture.PNG Capture.PNG Capture.PNG

Voltages (all with respect to case gnd):
A: 0.44VDC
B: 50.44VDC
C: 50.20VDC
D: 0.48VDC
E: -0.02VDC
F: 50.65VDC
G: -1.46VDC

My first thought is that there is either a problem with:

C24 - it's replaced with the film cap @merlynski recommended... my reading of the datasheet was that it wasn't polar - is that correct? Could I have installed it backwards?

D15 - bad zener diode?

Happy to accept other suggested as well! I tried to get subs for everything in the last mouser order, so I at least hopefully have the parts to fix - just want to find out what's gone bad first!
 
For the unit to come out of protection Node A must rise above +15.7vdc to cause Q2 to saturate and pull in relay RL1.
Node G is -1.46vdc so Q1 should be in cutoff.
If the output of either power amp channel is greater than about +/- 1vdc, either Q3 or Q4 will turn on, pulling Node A towards 0vdc, keeping unit in protection.
Measure VDC on AWR-182 (power supply) pins 28 and 29.
 
Measurements:

Q4b: -2.1mVDC
Pin 28: -2.4mVDC
Pin 29: -11.1mVDC

All pretty low, so I don't think we're turning Q3 or Q4 on...
 
D15 looks ok. Acts like a diode with positive voltage, negative ref voltage is -14.8V - pretty close to the spec.

C24 was a little out of spec - 121uF... it was also a 100u/16V instead of 10V, so I replaced it with a 100u/35V electrolytic I had some extras of.
 
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