Adcoms with lots of DC offset. Lots!

ben62670

Member
2006-12-27 7:58 am
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:hot:
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The caps have been replaced by a lame shop.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
 

ben62670

Member
2006-12-27 7:58 am
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:confused: 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.
 
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?

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
 

john65b

Member
Paid Member
2005-01-09 2:32 am
Chicago
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...
 

ben62670

Member
2006-12-27 7:58 am
h_a said:


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:confused:

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
 

ben62670

Member
2006-12-27 7:58 am
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.
 

john65b

Member
Paid Member
2005-01-09 2:32 am
Chicago
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
 
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.

A servo??

That thing adjusts DC within fractions of milliseconds! No chance that this is normal behaviour.

Another thing: your intention to change resistors and diodes is nice, but in nearly 99% of all cases not the cause of any failure. If resistors are damaged, they're usually burnt and that, again, has different reasons, with killed transistors the most probable.

All the best, Hannes
 

ben62670

Member
2006-12-27 7:58 am
Thanks for sharing guys. I am pretty sure the 555 is a stuck output transistor. With the input boards connected it still gave full voltage out minus the currant sharing resistors. Then I disconnected the input board with the same results.
As for diode, and resistors on my 585 this was a large part of the issue, and I was able to fix it with DIY's help (a big thanks to Chris). I am definitely going to check those TO-126 trannies.

Thanks much guys, and good luck John.
Email me if you need original Toshiba output trannies.
Ben
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Ben,
I've been away from the computer, sorry.

Your issues are pretty well traceable to both conductive residue on the boards and possibly the components themselves. Remove the op amp and clean it on it's own. Make sure the op amp power supply is working properly. I have seen those regulators short when the electrolyte issue occurs.

Those 100 ohm ground resistors should always be tested - every single time you get in there. They can look fine and still be open.

If you replace transistors, be sure to match the ones that should be matched. Make sure your compliments are in the same ballpark for beta. The input pair should be closely matched if you expect good sound quality. Remember, this is the place where distortion is corrected. That can't happen if those transistors are not matched.

Hi Hannes,
The DC servo does not operate quickly at all! Otherwise it's operation would affect the sound directly. The time constant is long, therefore expect the offset to correct over a few seconds time.

Hi John,
Most of the above comments apply to your situation. I'd be very hesitant to replace those transistors unless you know for sure the replacements are not copies. Replacement brands are complete junk. If the transistors are overheated, try to find new ones, otherwise clean them off and retest them. Chances are they are fine.
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.
John, I can't see this fellow being an expert on Adcom. Sorry. The DC offset faults are almost always caused by capacitor electrolyte or open ground resistors. Polk SDA's will do this every time unless special measures are taken to connect the two speaker commons together between the channels.

All,
Cleaning the electrolyte off these boards requires effort and attention to detail. I do this reliably with "simple green" and an ultrasonic cleaner. It takes a few cleaning / rinse cycles to do this. Some focused scrubbing may also be required. You can easily tell by hitting the area with a soldering iron. Ben knows that particular odour well now! :D Therefore, you can't clean them once and declare the method does not work. You must clean under components and the components themselves. I depopulate the area to make things easier. Everything gets washed. Do not forget to get that stuff out of the holes in the PCB!

-Chris
 

ben62670

Member
2006-12-27 7:58 am
Thanks Chris. I was missing ya.
Yes glowing pretty shinny boards from the other one sure did look nice. I went to solder on the components, and there was that sharp stink.
Wash, and scrub repeatedly.
This can't be emphasized enough! If you go to solder these puppies, and you get that smell stop, and clean again when you are less frustrated. I am down to 1vdc now. I think the boards are holding trace amounts of moisture. This happened on my other one. Bias is good. I am pleased even though I know I have more work ahead. No major parts, but finding some little misbehaving little ones.
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Ben,
Try drying the boards out in an oven (remove all caps!). If there is any hint of that odour when soldering, then the board is not clean enough.

Also, consider that what is on the PCB is also on the outside of the components. That's why I dismount the parts and clean them by hand. The op amp and regulators would be very sensitive and harder to clean. Resistors could be replaced easily enough, or cleaned as well.

That is nasty stuff in those caps, isn't it Ben?

-Chris
 

ben62670

Member
2006-12-27 7:58 am
anatech said:

That is nasty stuff in those caps, isn't it Ben?

-Chris
We certainly know I know. I did dry the boards with a heat gun using caution. I have a theory that the ultra sonic waves (for lack of better terms) drive liquid into the boards. I have my amp Idling, and it is down to .5vdc. I shall let it cook for a while long, and tomorrow I will do a cold start,
Ben
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Ben,
Water will tend to creep into everything. Ultrasonic agitation tends to allow water to get in faster.

What we should be looking for is a way to dry the PCBs out without over heating them. A low humidity environment should do the trick. Therefore, after your heat gun treatment to evaporate the greater volumes of liquid, freezing should be safe to do. A few days in that situation might draw whatever moisture is left out and away.

I have to say that I've not run into a water retention problem with these boards yet. I do tend to get the washing / rinsing over with and allow the boards to sit for a couple days after some heat drying. Possibly this is why, no idea.

I have had problems getting all the electrolyte off the op amp body. I do eventually succeed.

-Chris