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SA-8100 protection circuit issue

intermittent is always hard to figure out . with cars etc i say let it develop .. i dont think for one second that applies here though .
 
More than likely the protection board itself. The scope will help point in the right direction. Just set the sweep to something around 200Hz to 1kHz for starters, AC scale, connecting the probe ground to the chassis nearby and probe the legs of the transistors. We should see some clean lines on the scope. If the lines move up and down erratically or you see noise or a waveform, there's quite possibly a transistor breaking down. I'm not saying its the best way to do it, but it helps me to visualise the circuit and what is going on and a scope responds faster than a DMM so you can see these "noises" and weird behaviour.

Sorry, I was writing my post looking at the previous page - all of a sudden P14.
 
Was able to get readings on pins 16 and 14. Pin 16 did not change when the unit went into protection after 16 minutes. Voltage was a constant +34.4V.

Pin 14 went from +8V to > +20 V when the unit went into protection.

I'll have to try to get pin 17 readings tomorrow and will redo pin 16.
 
Well, turned the amp on this morning and waited over 20 minutes, but it has not dropped into protection. I'll do some more testing over the next couple of days, but I'm wondering if this is something random that I am just going to have to live with? As I mentioned previously, it did get better over time. After replacing Q3 & 4 with the 123As, it was dropping in an out of protection a lot. After a day it was only once or twice during play. Then became once during play. I tried last night after the amp had been off a couple of hours, and then this morning, and neither time has it dropped into protection. Of course, this is with nothing connected to the amp and pre out/power in separated. We'll see...
 
Was able to do a test this evening. After 15 minutes, the amp tripped into protection. Pin 17 was at +7.4VAC and did not change in protection. Pin 14 was at +8V and jumped to +30V in protection.
 
and if that makes no difference you could try lifting r9 and r10 ..then the protect circuit is isolated from the amp .
 
OK, with D5 pulled, after about 10 minutes the amp went in and out protection for about a second.
 
So far with R9 and 10 out it has not dropped into protection for about 20 minutes. Will need to repeat this when the amp is cold to confirm.

If this proves to be the case after another try, does this indicate the power amp board is giving a bad signal that is tripping the circuit? What would the next step be?
 
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