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Post by rbstereo on Jan 23, 2024 18:28:29 GMT -5
I have a unit that has a bit of hum at high volume no input. I recapped main PS pcb. It has more PS on main board.
Wondering on bias setpoints. Have to get the right 2 pin molex (my scrap heap?) To get to the test points.
But the thing gets pretty warm and has 2 quiet fans. Id like to kick on at least one of the fans without triggering protect. Pulling posistor plug starts fan then it protects.
Can i trick or wire around for fans? (No 12v source?) i believe they switch 15v through a circuit to generate the 12v output (pull down?) needed?
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Post by rbstereo on Jan 23, 2024 18:39:33 GMT -5
Update. I see there is a 78m12 regulator so fan should be easy to turn on. If it doesnt trigger the protect, i'll feel lucky.
Posistors seem to read wide open?
They are listed as 100r0 16v so i think they should read near 100 ohms?
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Post by rbstereo on Jan 24, 2024 10:02:20 GMT -5
So pulling down the fan and floating the circuit didnt force protect. But i still have the high volume hum.
Can a bias offset cause this?
Or do i need to keep chasing for a leaky cap or a bad diode?
On a scope its not showing up as straight 60 hz. Its messier than that but sounds to be focused in the 95hz range. What part of a receiver like this runs in that frequency range? Display driver? Really annoying at high volume and between tracks.
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Post by rbstereo on Jan 24, 2024 17:58:51 GMT -5
I am answering my own question for the benefit of others. This may save someone time. I'll attempt my guess as explanation.
Bias settings can create noise on this and likely many amplifiers. I adjusted to spec and noise (intermodulation distortion?) nearly disappeared even at max volume. I am guessing by getting the bias drift out of a narrow band (measured values varied off setpoint of -23.2mv where worst drifted down to -69 mv) gets against thevrail of one power supply or the other generating clipping effects.
Does this make sense?
Likely to cause excess heating and possibly clipping that could damage amps or speakers.
I am guessing modern amps could use a digital feedback bias circuit?
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Post by mastertech on Jan 24, 2024 19:40:49 GMT -5
Good to see that you got this fixed. Shows the importance of setting the idle current.
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Post by rbstereo on Jan 24, 2024 21:58:07 GMT -5
The partial recap won't hurt, though not the problem. On to the Denon AVR 5800.
Any pointers on the schematics and service manual would help.
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