Adcoms with lots of DC offset. Lots!

[MDchanic]

Ben and John,

I haven't needed one myself, but this looks like it would do it:

Fuse Holder FH052

If you can't find an insert that fits your holder, then the entire assembly can be easily replaced with a slightly different one.


Mariachi,

Adcom will e-mail you a scan of the service manual (with poor quality wiring diagram) if you enquire through this link: Contact Adcom
If you don't get a response, I've got the guy's direct e-mail.


Ben and Chris,

Last night as I was "depopulating" the input board in preparation for US cleaning, I found an open resistor (R155 or R156, I think - not one of the usual culprits) leading into the V+ (or V-) of the op amp. I think this might be the cause of my DC offset (no, I am not so curious that I would reassemble everything and test it with a new resistor BEFORE cleaning it...). What do you think?

- Eric

 

[ben62670]

I don't think that would knock the offset that far out of whack, but it may also feed some of the smaller trannies that do control offset. The opamp controls down to mv where the 970's, and 2240's control the higher voltage offsets.

 

[el_mariachi]

Mariachi,

Adcom will e-mail you a scan of the service manual (with poor quality wiring diagram) if you enquire through this link: Contact Adcom
If you don't get a response, I've got the guy's direct e-mail.




- Eric

Whow, great service from the guys at Adcom, i sendt the reguest for about 16 hours ago and i already got the documents in my mailbox.

Way to go Adcom!

And thanks to Eric for the url.

 

[MDchanic]

De nada.

 

[MDchanic]

Success!

I don't know whether it was the 3 cycles of ultrasonic cleaning with Simple Green, followed by rinses (each 380 seconds), on top of the 3 earlier sessions of toothbrush-scrubbing with strong detergent, or whether it was replacing open resistors R156 and R114, but my errant GFA-565 has now gone from 27VDC offset to +/-3mV.

One question I have is whether there is a normal amount of "wandering" or "hunting" of the offset. Mine now slowly tracks up to +3mV, then down to -3mV, over a period of about 5 minutes or so, without any obvious pattern (I'm using the OP97 op amp). This seems fine to me, but, then again, I wasn't even worried about the 27VDC until I happened to measure it "just for kicks."

If anyone has any questions about what I did and learned, as a decidedly amateur electronic tinkerer, I'd be happy to answer them.

Thanks to all of you, and especially to Chris and Ben, for all of your help, most of which you gave me indirectly over the past year in your posts to others (before I even knew I had a problem) - I couldn't have done it without you!

- Eric

ps:
I'll let you know how it actually SOUNDS when I get all connected in its new home, which as of now is still waiting to be wired for electric service...

 

[ben62670]

Re: Success!

I don't know whether it was the 3 cycles of ultrasonic cleaning with Simple Green, followed by rinses (each 380 seconds), on top of the 3 earlier sessions of toothbrush-scrubbing with strong detergent, or whether it was replacing open resistors R156 and R114, but my errant GFA-565 has now gone from 27VDC offset to +/-3mV.

One question I have is whether there is a normal amount of "wandering" or "hunting" of the offset. Mine now slowly tracks up to +3mV, then down to -3mV, over a period of about 5 minutes or so, without any obvious pattern (I'm using the OP97 op amp). This seems fine to me, but, then again, I wasn't even worried about the 27VDC until I happened to measure it "just for kicks."

If anyone has any questions about what I did and learned, as a decidedly amateur electronic tinkerer, I'd be happy to answer them.

Thanks to all of you, and especially to Chris and Ben, for all of your help, most of which you gave me indirectly over the past year in your posts to others (before I even knew I had a problem) - I couldn't have done it without you!

- Eric

ps:
I'll let you know how it actually SOUNDS when I get all connected in its new home, which as of now is still waiting to be wired for electric service...

Congrats🙂
It sure does feel good to know you did something other people doubt you can do. Your offset is better than normal.
Enjoy
Ben
BTW
Chris gets all the credit here. He taught me most of what I know.

 

[BTG]

OK folks, it's been a while but here is some info to help you all.

First, the electrolytic caps could be bad after so many years. I would replace them all whether they are bad or not if you already have your fingers in the amp.

There are 10 ohm fusible resistor in the ground path, they do have a habit of opening and could cause some of the issues being mentioned here.

The main cause of DC offset in all the version II amps and GFA-565 which is the same basic design is the Adcom 3A servo amplifier.

Input transistors will cause DC offset if they are not matched properly but with a servo amp it's a non issue. Even a poorly matched pair wouldn't be more than 120mV and that would have failed Adcomm's QC. If the transistor went bad over the years, you should be able to measure that plus they usually work or they don't.

Good luck. BTW, if anyone can tell me what IC the Adcom 3A is I would appreciate it. It's a Linear Technology LT10xx something, just don't remember what.

 

[ben62670]

I have Ben using the OP97's with great success. Walt Jung has recommended these as replacements, and I have found these to work great. For the price I will from now on replace these instead of messing around with cleaning, and removing these. The same with the caps, and the 970, and 2240's that are on the input boards.
Ben

 

[EchoWars]

The Analog Devices AD820 is also a fine replacement.

 

[K-amps]

ok
good luck. Btw, if anyone can tell me what ic the adcom 3a is i would appreciate it. It's a linear technology lt10xx something, just don't remember what.

lt1022

 

[K-amps]

lt1022

I stand corrected, it is not LT1022 but LT1012.

The LT1022 might demand more current than the precision zeners can provide.

Both LT1012 or OP97 should work. They have lower DC offset than the other recommendations and are drop ins for the servos in the 565.

 

[anatech]

Hi Arif,
The current draw of the replacement op amp is important. It it's too much, the "zeners" will become current starved and drop out. There really isn't a good reason to replace the op amp unless it has failed. They rarely go bad.

Hi BTG,
Those resistors will only open if an external ground fault occurs.

The cause of the DC offsets is not the Adcom 3A. The main cause is electrolyte from defective capacitors.

Input transistors will cause DC offset if they are not matched properly but with a servo amp it's a non issue. Even a poorly matched pair wouldn't be more than 120mV and that would have failed Adcomm's QC.

I have to disagree with you on this. It is critical to have a properly matched input pair. The DC servo circuit does not respond quickly, it's response time is set by the integrating capacitor. This can take over 10 sec to null out an offset. In the mean time, you may have a large offset that drops slowly over this time. If you have a DC servo supplied by a bipolar supply, the match of the input pair determines the DC offset until the DC servo has corrected the imbalance. If the op amp runs from a single supply, there may be other reasons for the initial DC offset. These can start around 10 ~ 12 VDC (as reported here earlier) until they are corrected. This is not abnormal behavior, but it's not what you want to see. No danger to tweeters unless you are running an active crossover where the tweeter is driven directly.

If the transistor went bad over the years, you should be able to measure that plus they usually work or they don't.

I wish that were true. Most often they may have a beta change, so the input pair becomes unbalanced. They may become noisy or even leaky, but they will often continue to function rather than go open or short. Of course, it is possible they can fail open or short. Never discount the ways parts can fail, I've seen too many odd failures in my time.

Hi Ben, Eric,
I'm really happy you have your amplifiers running again. That's great, and better still that you are helping others. Thank you for that.

-Chris 🙂

 

[gootee]

Hi John,
There's your problem. 100 ohms to speaker ground from the RCA outer shell. The resistors will appear just fine, but they are open.

Get those caps out of there or don't use the amp until they are replaced. The electrolyte is not normally visible to the naked eye.


Yes.

-Chris

It sure was nice to find this old thread, today. I just received an Adcom GFA-555II from an eBay seller, and found that each channel had about 4.5 Volts of DC offset, on its output. After reading the post above, I checked the resistance from input ground to chassis ground and found between 1 and 2 Megohms, for each channel, instead of 100 Ohms. After I replaced R104 and R154, the two channels' DC offsets were both only about 0.3 millivolts!

I didn't have the replacements needed for all of the small caps, on hand, so I have ordered those.

In the meantime, I took a chance for a few hours, so I could see what she could do, through some Vandersteen 2Ce and some Magnepan 3.6 speakers. The sound is exquisite!

"Thank you" to everyone who contributed to this old thread, and to diyAudio for keeping it around. You saved me some frustration and ill will, today.

- Tom

 

[void97]

I've 5V+ DC offset on both channels, and it go away after I replace the R104 and R154 resistor. Thanks for the great tips.


Louis

 

[cogeniac]

Hi Void;

If you have not seen it, I'd suggest you check out the other long Adcom 565 thread. We wrung those amp offset problems out pretty thoroughly there.

Yet Another Adcom GFA-565 Thread

Scott