Adcoms with lots of DC offset. Lots! - diyAudio
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Old 9th March 2008, 08:24 AM   #1
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Exclamation Adcoms with lots of DC offset. Lots!

Well I have done my GFA-585 with great success. Thanks Chris!
Well I found a couple deals I could not pass on. I ended up with 4 Adcoms for less than $300.
2 565's
545
and a 555
545 done. Just needed jacks. Sleeves ripped off
555 full dcv to one channel's speaker output 78vdc
This is with the input board disconnected.
The 565's have 40+ vdc in one, and 10vdc in the other.
the 555 may have a blown output transistor?
One of the 565's may have the same issue? (its roughly half the PS voltage).
The other 565 is 10vdc. This one has me really puzzeled.
The caps have been replaced by a lame shop. I am going to clean them with my ultra sonic cleaner, and simple green. Been through this on my 585.

Thanks guys
Ben
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Old 9th March 2008, 10:17 AM   #2
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OK this is kinda a surprise. After leaving the amps (565's)on for an hour the dc offset drops from 40+ to less than 1vdc. Sometimes it drops to within spec The other one that was 10vdc is down to 7vdc, and slowly dropping.
This can't be the effects of electrolyte on the boards can it? 40vdc offset at the speaker terminals drops to spec, and bounces around under 1vdc. The electrolytic have been replaced, but I don't know about them cleaning these.
Could it just need a good cleaning? 1vdc is what I was struggling with on my 585.
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Old 9th March 2008, 10:39 AM   #3
h_a is offline h_a  Europe
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Quote:
This can't be the effects of electrolyte on the boards can it?
You mean aged caps? Then no. Or do you mean some caps are blown?

Quote:
Could it just need a good cleaning?
No.

Anything above some hundreds of Millivolts is a defect. 10 V DC heat the driver coils already with more than 10 W continously. For sure not a good idea. You need to get your units fixed.

From your questions I would strongly advice you not to kill yourself by trying to fix them.

All the best, Hannes
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Old 9th March 2008, 04:24 PM   #4
john65b is offline john65b  United States
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There should be a DC Offset pot in there somewhere...check out schematic to find it and trim it down while shorting the inputs, or plugging in your powered pre with no input..your bias setting may stray on trimming the DC offset

I had a fiend with a couple Hafler 555's and they too had higher DC offset on outputs. Upwards of 400 - 600mv. Don't know if this is common with Haflers...
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Old 9th March 2008, 05:55 PM   #5
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Quote:
Originally posted by h_a


You mean aged caps? Then no. Or do you mean some caps are blown?



No.

Anything above some hundreds of Millivolts is a defect. 10 V DC heat the driver coils already with more than 10 W continously. For sure not a good idea. You need to get your units fixed.

From your questions I would strongly advice you not to kill yourself by trying to fix them.

All the best, Hannes
The elna caps that came in the 585's, and 565's aged, and leaked all over these amps. Elna had a bad run of these caps which plagues what I find is an exceptional pair of Adcom amps. The caps were replaced by a service shop, and are not blown.

Thanks for your concern, but I have done a bunch of work to my 585 including... finding, and replacing bad transistors, diodes, resistors, caps, and output transistors. I replaced the op amp with an opa97 also just as insurance. I have been bitten by 80vdc once, and thats all it took to learn some patience for that caps to bleed off. As for the 10 W continuously I would assume that the amp would be running hot at idle, but it is barely warmer than room temp. I started it again today, and this time it went from 30vdc to .7vdc in less than 5 minutes

Well I know that I am going to remove the boards, and do a thorough cleaning in simply green with the ultrasonic cleaner, and test some of the know diodes that have given me problems before, and do some resistor testing. I may just completely strip one of the boards, and scan the copper side to make an image to create a positive for making my own circuit boards. I should have done this with my 585.

Ben
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Old 10th March 2008, 12:43 AM   #6
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Well I think Leonard Skynerd said it best. "Ooooh Ooooh that smell" I just cleaned my input board with my ultra sonic cleaner, but before I removed all the Electrolytics, and the VR. What a stink that electrolytic gives off!
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Old 10th March 2008, 03:18 AM   #7
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Another update. Boards are cleaned, and repopulated with new caps. Hangs at .5vdc when warmed up. Shut it down, and it climbs to almost 5vdc. Bias is at 24mv. Haven't bothered setting it properly. I know I still have other issues. Whether is electrolyte still left, or failed devices I don't know yet. With that much dc offset in the beginning I would not be surprised if something was blown.
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Old 10th March 2008, 05:42 AM   #8
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OK more info.
With an 8 ohm resitor the DC offset does not climb during shut down. Just drop flat quick.
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Old 14th March 2008, 08:08 AM   #9
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Well I hooked up my amp that goes from .09 vdc offset, and it plays just fine. Beautiful really. But when I turn off with no resistance across the speaker terminals it climb to near 5vdc still.
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Old 14th March 2008, 01:43 PM   #10
john65b is offline john65b  United States
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I have my friends Adcom GFA-555 II - plays fine, but the offset is 5.5 vdc always - both channels. I thought he said 500mv, and was quite surprised to see it at 5500mv when I checked it. He was using mono-ing the amp on some very expensive electrostatic speakers (Soundlabs) and hopes he did not do any damage to them...

I was discussing the issue with an Adcom specialist that said just check and replace the T0-126 transistors on the input board. There are 6 of them.

At .28 each, I will be replacing all four of the NEC A1142, and leaving the two compliments alone as they look fine (the 1142s look a bit burnt)

The Adcom specialist said the DC offset is not controlled by a pot as I originally thought (I guess I can stop looking for it now), but by a servo. The servo should be fine, but he said 90% of the issues around high DC offset on these GFA-555 II amps are these six T0-126 transistors.

I am having a bear of a time getting the input board out of the unit as the RCA input connectors are glued to the chassis. I really don't want to cut and splice the input wires to get the input board out...

More later
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