ADCOM GFA-565 DC Offset Fluctuation - diyAudio
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Old 4th March 2011, 11:39 AM   #1
retrex is offline retrex  United States
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Smile ADCOM GFA-565 DC Offset Fluctuation

Sorry to rehash this but I have read the threads I could find on the subject and never really got enough info to quell my concerns. I have just purchansed a pair of GAF 565s, one going back to seller to rectify a hum and a .5V DC offset issue, the other with a DC Offset that will not settle down to a stable value. It is fluctuating between -4mV and +7mV, constantantly cycling and never resting on any one value for any period of time. I tested with pre hooked up, no signal, speaker attached, after about a 20 min warm up of playing and 10min rest w/no signal. I have a GFA 585 that does not exhibit this behavior so I am a bit worried. Can someone in the know please let me know if I have reason to be concerned? All replys are welcome and appreciated. Oh and first post to forum after many hours of enjoyable reading. Thanks.Click the image to open in full size.
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Old 7th March 2011, 05:20 AM   #2
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The problem:

There could be few bad leaking electrolytic capacitors on each input board. The electrolyte from bad capacitors spills over the board.
The electrolyte is a conductive liquid which causes servo DC and differential circuits to malfunction. At this point the amplifier can not properly control DC voltage at the speaker output terminals, which has to be about 0 volts without input signal applied. As a result you notice that the cold amplifier make loud turn on/off pop or has distorted sound, but works fine after warming.
On the other hand, the electrolyte is corrosive liquid and can cause damage of some parts on the board.
As a consequence, the amplifier gets dangerous voltage at the speaker terminals, up to 70 volts, and can destroy the speakers.
This process is very slow and can take several months before the amp blows the speaker, so you have a chance to notice the problem and save your lovely speakers.
Diagnosing:
Simply check DC voltage across the speaker terminals without speakers hooked up and without input signal by using a multimeter.
When you turn on the amp, the DC can jump up to 0.2 volts, then it slowly goes down. In 1 minute it must be stabilized at 0-30 mV (less than 0.03 volts).
If your amp has more than 30mV DC, it must be repaired ASAP
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Old 9th March 2011, 11:34 AM   #3
retrex is offline retrex  United States
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Default Adcom DC Offset Fluctuating

Thanks for the repy amadork9. Yes I was aware of the leaky cap issue and solutions with regard to rectifying it. I was trying to get a sense of whether the fluctuation is a symptom of the the same problem and the degree of damage, if any, the variance poses to operation of the amp. And if is not a problem now when will it become an issue, if at all? Can I monitor the DC Offset periodically and move to correct above a certain level? I believe I still have the option to return the second unit for repair as well but I am trying to get a sense of the seriousness of the problem before I go down this path. I don't have a problem working on electronics, EE background, but I still don't own it fully in the sense that I view it as a good transaction and accept the deal as is. I have only listened to the amp breifly due to the other one being sent back for repair soon after I received them, but it did sound ok. I am just going to the experts for some additional insight and support.
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