Adjusting Bias on a DC coupled Amplifier. DENON PMA-630

I have this DENON PMA-630 DC coupled amplifier from 1978 when bought new. Recently I had an intermittent problem with no output on both channels. Thinking it was a transistor in the power supply that I changed in 1985(!), I went back inside and found all the voltages from the PS are correct and the unit is in fact functioning. I did find that the speaker offset voltage is about 35-45 mv on both speakers which seems a bit high. The bias adjustment in the service manual is explicit about setting the speaker offset to 0 +/- 10 mv, but goes on to have you set idling current (at a test point} when warmed up without giving a final value for the speaker offset voltage. The procedure has you turn the gain all the way down when making the adjustments and gives no guidance of where the offset voltage should track when the volume is ramped up. Measuring the speaker voltage per the manual is done with a high impedance voltmeter, and in doing so, I see the voltage go to over

2.5 volts without a speaker load. Both channels are similar and I have had no history of speaker popping or any anomaly.

There is no spec or procedure calling for checking the bias at other than zero volume. Is this voltage rise normal?

Since this is probably a good time to go through the bias adjustments, I would like to do so. However I am concerned that the pots for the bias adjustments might be dirty and scratchy from old age.

I don't want to spray them with cleaner and find them way out of whack and blow a transistor. Can anyone suggest a good method for maintaining the nominal bias while cleaning those pots?

Finally, what is a good pot and switch cleaner that will not damage the parts?

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©
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RFI-EMI-GUY
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The two controls should be nearly independant.

Depends on the circuit.

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I would like to see a schematic and photographs of the adjustment pots. Some pot constructions do not lend themselves to using cleaners, and not all cleaners are the same. .

Reply to
JosephKK

Is there a cleaner that is generally considered "safe"? The pots are open frame with clear plastic bodies. Not sure of the maker.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©
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RFI-EMI-GUY

"RFI-EMI-GUY"

** Your dumb questions have ALL l been thoroughly answered already.

Stop being a tedious PITA.

..... Phil

Reply to
Phil Allison

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A couple are, they are generally pressurized R22 and isoporpyl alcohol with a surfactant them help them mix.

Clear plastics are a whole new game. Photos are required before i even guess.

Damn, i had not guessed that the marketeers had managed to mess Denon up that much that early.

I live in north central California, not very far from Jeorg. .

Reply to
JosephKK

Caig makes a great selection of cleaners. Generally I use DeOxit, but you might want to read up on their product line.

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BTW, WD40 is about the stupidest thing you can spray on electronics. The residue attracts dust.

Reply to
miso

Caig makes a great selection of cleaners. Generally I use DeOxit,

** Caig ( aka DeOxit) is a massive scam.

** Never spray WD40 onto electronics - it does no good whatever to electronic components to have any kind of lubricant sprayed onto them.

But hardware items like pots, switches and connectors are another matter - they get dirty and corrode so the contact areas need cleaning from time to time.

WD40 is just about most effective and safest cleaner and protective lubricant you can use.

** That is just so utterly asinine it is hard to fathom out why folk say it.

...... Phil

Reply to
Phil Allison

WD40 is not a lubricant at all. Its purpose is to protect against water ("WD == Water Displacement) and is a also a solvent. It is a pretty good solvent, it also will clean oils and grease. It also tends to move grease around so may help "lubricate" by moving grease back into moving parts.

s/attracts/collects/ and he's pretty much right with any "grease", no matter how it got there.

Reply to
krw

No, any change in the DC output voltage with volume control adjustment should be temporary (it should go back to its original value in a few seconds) and small, if it happens at all. I suspect leaky electrolytic capacitors feeding the high side of the volume control potentiometers. There may also be leaky electrolytics in series with the wiper, and I could see that also having this effect. Electrolytics often go bad over long periods of time. Replace every electrolytic in the signal path, or even every electrolytic in the unit (leaky caps within the power ampilifier section can cause changing offsets over time) with a new one rated at

105C or greater, then readjust the bias and output offset with the volume set at zero as the manual says. Again, changing the volume should have little or no effect on the offset voltage.
Reply to
Ben Bradley

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