MA600 dc offset at power-down

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Hello, wonder if I can pick your brains

I have an MA600 here which powers-up fine from cold and works just fine.

But after a few minutes, if you power off, we get a small residual DC of about 4V on Ch1 output terminals. This means that if you switch it on again, it goes into IOC protect. If you leave it a few minutes, the capacitors on Ch1 discharge, the offset of course disappears then the IOC light goes off and the power relay clicks in.

I have exchanged the op amps on the main pcb between channels to prove it isn't that. It has had new electrolytics on the LV psu as routine.

Once the amp has powered up successfully, there is no DC offset at all.

Any ideas folks?

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FYI this was traced after some time to being diode D305 (1N4004) which had a 100K reverse leakage when warm.

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