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Old 19th January 2014, 04:54 PM   #11
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I've already checked all the transistors (out of circuit), and they all work.

Now I've done the following:

Disconnect everything that takes power from the supply, minus the B+ and B- going to the power amp (that means the phono amp, pre amp and tuner are not getting any power).

Move input signal from volume control directly into power amp.

Now the B+ voltage still goes down to 0V a couple of minutes after power down, whereas the B- stays at about full voltage.

DC offset is now 16VDC. A quick five second test reveals: Some hiss through the speaker (DC offset of course) but music plays nice and clear. Robust speaker, that. Started smelling again.
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Old 19th January 2014, 05:29 PM   #12
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How embarrassing.

I somehow managed to miss one small TO-92 transistor (Q403 if anyone cares), and of course - it was the culprit. It was open in all the directions.

Preliminary testing: having replaced that garners about 1mV offset (with new diff pair transistors), and music plays fine through headphones. I will stitch everything back to normal now (it's a mess) and see if it still works after.

Thanks for your thoughts, everyone. Greatly appreciated.
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Old 20th January 2014, 12:07 PM   #13
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Old 20th January 2014, 05:32 PM   #14
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Just a note from my meager experience, when you see + dc voltage on the output, it usually indicates that something on the - side of the circuit is not functioning. Either a busted transistor of an open resistor. That also accounts for the + rail draining more quickly than the - rail on shutdown.
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Old 20th January 2014, 06:32 PM   #15
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Quote:
Originally Posted by still4given View Post
Just a note from my meager experience, when you see + dc voltage on the output, it usually indicates that something on the - side of the circuit is not functioning. Either a busted transistor of an open resistor. That also accounts for the + rail draining more quickly than the - rail on shutdown.

+++10000 very correct Often people that see + Dc offset in the output run for NPN transistors in the output stage or drivers ...while the real fault is an open resistor on the other side

Well said still4given !
Kind regards
Sakis
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