When your failure detection system… doesn’t actually fail.

Couple of years ago, I repaired a NAD T751 which turned on and then immediately off again. These NADs have a protection circuit that shuts the amp down if something is wrong, and in this case, the protection circuit was at fault.

So when my brother spotted a T760 with exactly the same problem, he bought it, figuring that this was a solved case.

This turned out to not be the case. While the T760 is almost identical to the T751, this one lacks the bodge that caused the problem last time. And while the prime culprits (C173, C174, C176, C199 on the AC3 board) were all out of spec (measuring 70pF to 10nF), replacing them did not solve the problem.

(If your PCB is discoloured around the voltage regulators, the amp ran hot for long, and that dries out the caps).

Anyway. so I went where I should have started, and measured the offsets on P501, P502, P503, JP801 and JP802 (representing the amplifier quescient current).

And that explains it (even if it’s a b*tch to get at).

Attended to a couple other suspicious-looking solder joints, buttoned the whole thing up again. Offset on the centre amp (the one which had the bad joints) doesn’t want to come down below 2.7mV so it’s going to run a bit hot, others all at 2.0mV (over 0.22 ohm, so 9mA idle current).