Adcom GFA-5800 DC offset off - diyAudio
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Old 18th January 2013, 07:43 PM   #1
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Default Adcom GFA-5800 DC offset off

I'm using an Adcom GFA-5800 (the one with fans), and today I thought it would be a good idea to do some simple service on it.

I adjusted the bias alignment and everything looked fine, but when adjusting the DC offset on the right channel the best I can do is to adjust to -92mV. The left channel adjusts fine to 0mV as specified in the service manual.

Does anyone have a pointer on what could be causing this?
And if anyone have a simple explanation on how this affects the output.
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Old 31st January 2013, 05:14 PM   #2
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Does it have the original electrolytic caps on the input boards? If so, they could be leaking, which can cause this. This is a very common problem on this amp and there are quite a few threads here that discuss it.
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Old 31st January 2013, 05:26 PM   #3
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Thanks for the reply.

This is a 2nd hand unit, and the caps have been replaced.
I can see signs on the circut board from what seems to be cap leakage.

Do you think that its possible that there might be leakage I can't see under some of the components? Specifically the IRFD210, I have trouble visually inspecting it without lifting it from the board.

And sorry for starting a new thread if there are others discussing it already.
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Old 11th February 2013, 06:18 PM   #4
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Yep, it sounds like the board needs to be cleaned. There are threads on this site that discuss the process, but in a nutshell you use an ultrasonic cleaner.
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Old 12th February 2013, 06:53 AM   #5
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Thanks.
I'll do some research on how to do so, and see if I can get some equipment.
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Old 12th February 2013, 06:08 PM   #6
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Check those variable resistors to adjust DC offset, sometimes they can wear-out or have dirty contacts, also check for bad caps (as mentioned) they can look good but under an E.S.R meter will show something different!. I would recommend Panasonic FC caps 105c, they are very good. Also clean the boards very well. The easy and cheap way is to use an old toothbrush and alcohol (70% alcohol or higher from any pharmacy) also acetone works great. We used to clean electronic boards at the shop with these and We never had any problems and got excellent results.
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Old 12th February 2013, 09:10 PM   #7
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I have done in circuit measurement on resistors, including the variable ones. And all of them seem to be ok. I have also cleaned what I can with isopropanol and q-tips, and no luck so far.

I guess it doesn't hurt to change the caps, but they seem new so I want to explore the other posibillities first.

Also, I have seen someone having a similar issue with the GFA-5802, and solved it by changing the input board. But as far as I can see there are very few components on the input board on the GFA-5800.

Hopefully I will get some time the next few days to have a look at the amplifier again. I always get impressed by how massive and nice its built when I open it.
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Old 25th February 2013, 06:07 PM   #8
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I just went through the adjustments on my Adcom 5800 and finally had enough of trying to adjust the very touchy original pots. I replaced the four original pots with 10 turn 5K pots and now the adjustments are much easier to make and the settings won't be disturbed by any shock to the 5800 cabinet. I noticed that I was unable to use my multimeter for the DC offset as the scope showed a bit of a noise level, but it was very low. I ended up doing the adjustment using the scope. Anyone else noticed that situation, or maybe I have an unusual level of noise with no input? No noticeable noise level through the speakers with no signal.
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Old 26th February 2013, 03:59 AM   #9
Apogee is offline Apogee  United States
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Default Check your resistors also!

I have a pair of the 565 monoblocks that had the cap leakage issue. Besides replacing the caps, I ended up needing to replace several resistors because the electrolytic substance had wicked its way up some of the resistor legs and caused the resistors to corrode under the exterior coating. There was no outward difference in appearance. I found it because as I was troubleshooting the board I lifted the leg of one resistor and it fell out of the shell... That put me on the trail and found several others that were also bad.

I hope this helps!

Steve
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Old 26th February 2013, 05:18 AM   #10
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Apogee:
Thanks, I guess that I will have to start lifting components eventually. Its hard to know whats hiding underneath where I can't see. And the resistors were the only components I could measure in circuit.

JustDoIt:
As far as I could see there were no noise on my output channels, only the offset. I didn't check with a scope, but I'm pretty sure my meter would have gone crazy if there were any noice.
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