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Old 26th May 2017, 11:56 AM   #1
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Default Adcom 535 troubleshooting help

Hi - I'm new to the site, looks like a massive font of useful info.

I've got an Adcom 535 that has a bad buzz in the right channel. I've replaced all the electrolytics on that channel PCB, as well as the output transistors. Since this amp has essentially a dual power supply, I swapped the left channel supply to the right amp and the buzz remained. I assumed (perhaps wrongly) that confirmed the PS was not the issue.

Here is where I'm confused: When I switch off the power, the right channel will start to lose the buzz and I will begin to hear some input signal, as it is running off the PS caps as they discharge. This would seem to point me back at the PS.

Any debug ideas would be welcomed ... thanks!
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Old 26th May 2017, 12:06 PM   #2
Mooly is offline Mooly  United Kingdom
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Welcome to diyAudio

I'm not familiar with the amp, however can you clarify:

Quote:
the right channel will start to lose the buzz and I will begin to hear some input signal
If you had said the right channel instantly losses the buzz................. and I instantly begin to hear...

it would make a little more sense.

Do you really mean the buzz fades away when you kill the power ?
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Old 26th May 2017, 12:45 PM   #3
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Just tried it again to see. The signal does come up almost instantly on power off, then tapers as the caps discharge. I've checked the grounds as best I can. The PS voltages to the B+ and B- rails look OK ...
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Old 26th May 2017, 01:32 PM   #4
Mooly is offline Mooly  United Kingdom
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Signal coming up as rails collapse suggests the DC conditions of the amp are changing in response to the falling rail.

Does the bad buzz stop instantly you kill the power ?
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Old 26th May 2017, 02:11 PM   #5
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Yes - the buzz stops instantly when I kill power. I do hear a tiny bit of signal in the background with the power on, but the buzz overwhelms it. Kill power, buzz stops and it seems that the signal comes up in strength (but this might be auditory illusion) for about 5 seconds before fading away.
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Old 26th May 2017, 05:01 PM   #6
Mooly is offline Mooly  United Kingdom
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I've no quick fixes I'm afraid. The correct way to fault find on this is going to be to use an oscilloscope and look not just at the rails but also use it to look at the grounds and the wiring around the input to see if anything is amiss.

Has this amp some unknown history... an acquisition or ebay purchase etc, or have you owned it for some time and its suddenly developed an issue ?

If the former then I would be looking for physical damage such as cracked print or damaged wiring.
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Old 26th May 2017, 05:19 PM   #7
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Much appreciated, Mooly - thanks for the suggestions. No scope at the moment, but I might have to fix that.
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Old 26th May 2017, 05:27 PM   #8
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OK
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