One of the many ways to start with this trouble shooting: a protection circuit always kicks in with a reason!
For example:
a) short circuit (some clues: electrolytic caps? does anything get hot?)
b) faulty protection circuit
Let me clarify what I was trying to tell Ian in my previous comment: I would check all the major components one by one. Ian said the voltage rails are fine. So far so good! The next I would go for is the opamp. Since there is power, just apply an external low DC voltage on all inputs and check their corresponding opamp outputs, instead of the end stage's output. Go through all channels like this to narrow down the possibilities.
If I was you, I'd make a list of all major components and each time I tested something I would strike it through and focus only on the remaining ones, otherwise it's just like finding a needle in a haystack.If all good, I would carry on with the electrolytic caps. They are among the primary causes of failure because they have a relatively short lifetime as the electrolyte tends to evaporate. This changes their internal resistance, sometimes so badly that a cap can short out. Check the aluminium top on them visually to see if there is any "bump" (when internal gas builds up, it is usually a sign of a bad cap). If all good, I'd go through all of them with an ESR meter and match the read outs with their factory specs. If you don't have an ESR meter perhaps a multimeter could give you a clue to identify possibly shorted out caps.
Sorry, that photo doesn't help much.
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Ian, use as low voltage as possible. For a conventional audio preamp input I wouldn't go above 200-300mV test voltage and 3v would be HUGE. But I don't know that particular amplifier and normally what supplies it's input signal.
From our correspondence I am assuming that you aren't very equipped for the task so you might want to leave the signal test to the very end and just jump on the cap tests I described earlier. Let's hope that will solve the issue.
Make sure that you have discharged all caps before start testing them! (don't kill yourself)
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