Adcom GFA 565 DC offset problem...
I have a pair of these amps and one is showing a drifty -20Vdc offset at the speaker. I found that the 220UuF 25V caps that filter the servo op amp rails were physically leaking, so I replaced them after thoroughly cleaning the PCB. I replaced a 10 ohm resistor that established a ground ref for the servo op amp because it was reading high. Still no cigar. Servo op amp is railed out positive. I tried blowing a hot air gun on the PCB and it did not seem to affect it much.
Earl😕
I have a pair of these amps and one is showing a drifty -20Vdc offset at the speaker. I found that the 220UuF 25V caps that filter the servo op amp rails were physically leaking, so I replaced them after thoroughly cleaning the PCB. I replaced a 10 ohm resistor that established a ground ref for the servo op amp because it was reading high. Still no cigar. Servo op amp is railed out positive. I tried blowing a hot air gun on the PCB and it did not seem to affect it much.
Earl😕
Earl,
If you read the entire link I posted earlier in this thread and the other threads about this, and this whole thread too, you would find that your experience is a very common one.
The solution is to clean it more thoroughly.
You will probably also have to remove the opamp (and socket if there is one) and clean it and its pins and the holes in the PCB, especially well.
Anyway, read the link(s), too.
Cheers,
Tom
Saga of the ailing GFA-565...
Earl
Decided to take the PCB out of the GFA-565 after I did a revealing little experiment on the amp. As I mentioned before, using hot air on the PCB and watching the offset showed that the heat had little effect, but after it cooled down, I blew on it using my breath in an attempt to make it humid. The offset shot up and down like crazy when I did that! So there was probably conductive contamination on the board. I removed the three 220uF 25V 'lytics and found that was capacitor juice on the board around all three of them, and there was a lot of it around C109, mounted in a sensitive area on the PCB. I probed some of this juice on the board's top surface with an ohmmeter and found a few 100K of leakage. It had spread all over the board. Horrible! I ordered some switch mode type 105C replacement caps for the 220's as well as replacements for the 100uF 100V rail caps C114 & C115. Those caps did not leak, but I figured I might as well replace them too. I washed the board in isopropyl alcohol and then hot water with a detergent. I found that resistors R153 & R154 (1.1K) were kind of brown looking and replaced them with a 1 watt size of resistor, stood on end. The board now looks pristine. I socketed the servo op amp. When I get all the caps, I'll install them and give 'er a whirl.
Earl
All most informative. I have been considering picking up a 535 just as a comparison of this designers work to the other amps I have in my quest to isolate the design parameters that effect my wife's super hearing. I would not have guessed the servo to be that sensitive.
There was a power outage & my computer restarted during my reply; I do not know if it got out or not. At any rate, the impedances around the servo are high and the leakage paths caused by the capacitor juice leakage are low, and the 85V rails on the '565's board make this even more troublesome. Doesn't take much to upset an apple cart like that. I'll post again when I reassemble the amp after I get replacement caps.
Earl
Earl
Saga of the ailing GFA-565...
Finally got the caps for my GFA-565 and replaced them. I found that the boards needed to be washed in isopropyl alcohol (IPA); using just deionized water helped, but did not cure it to my satisfaction. I soaked-washed-scrubbed the boards in IPA, and then soaked-rinsed twice in DI water, and now I have stable offsets in both of them of 0Vdc +/-5mV or so. The drifting effect is a property of the servo circuits, and <10mVdc won't bother me at all. By the way, I replaced the yellow tubular 4.7uF mylar input caps with nice polypropylene ones. All is well with them now, and unless placebo effect is at work here, I think they sound better, too.
Earl
Finally got the caps for my GFA-565 and replaced them. I found that the boards needed to be washed in isopropyl alcohol (IPA); using just deionized water helped, but did not cure it to my satisfaction. I soaked-washed-scrubbed the boards in IPA, and then soaked-rinsed twice in DI water, and now I have stable offsets in both of them of 0Vdc +/-5mV or so. The drifting effect is a property of the servo circuits, and <10mVdc won't bother me at all. By the way, I replaced the yellow tubular 4.7uF mylar input caps with nice polypropylene ones. All is well with them now, and unless placebo effect is at work here, I think they sound better, too.
Earl
That "Yellow" cap you replaced with a PP was in fact a very high quality Polycarbonate cap from Electronic Concepts. Not only was it an expensive cap it was no doupt in good condition and good for years to come. The very few times I have had to replace that cap I order driectly from EC for the same cap, 5MC series.
Yellow caps
Thanks for the correction on the caps. I measured my two and they showed a little higher D.F. than the polypropylene replacements I used, so since I had the PPNs lying around, I decided to use them. Another reason I replaced the yellow caps was that the two caps were different models; one was longer and thinner than the other one was, although both were spec'ed at 5uF 50V 10%. Both caps were from Electronic Concepts. I held onto them and will likely use them in some non audio project. They appear to be in good condition and were not contaminated with the leakage from the electrolytic caps.
🙂
Thanks for the correction on the caps. I measured my two and they showed a little higher D.F. than the polypropylene replacements I used, so since I had the PPNs lying around, I decided to use them. Another reason I replaced the yellow caps was that the two caps were different models; one was longer and thinner than the other one was, although both were spec'ed at 5uF 50V 10%. Both caps were from Electronic Concepts. I held onto them and will likely use them in some non audio project. They appear to be in good condition and were not contaminated with the leakage from the electrolytic caps.
🙂
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