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Old 27th June 2010, 02:27 AM   #1
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Default Adcom GFA 585 faults again

Hi Folks,

It seems my Adcom 585 is unwell again. Some 2 or 3 years ago I took on repair of this amp, after it had succumbed to the well documented leaky electrolytic capacitors. This had seemed to be entirely successful, and I suceeded in seemingly perfect operation with very low DC offset, in the 1 mV range on each channel.

I turned the system on this evening, and suddenly noticed the right channel sounded "fuzzy", it quickly degraded, and eventually lost output altogether, except for some very distorted traces of signal. I turned everything off. Later a quick check showed around 3V offet on the right speaker channel, and 50mV on the left speaker. Fortunately there's no damage to the speakers.

I'm disappointed to encounter problems again, and will document my troubleshooting and repair attempts in this thread. First steps will be to dig out the service manual and take a peek at the innards for obvious problems. The amp has seen fairly light useage since the first repairs and has not been pushed hard. I hope for a simple explanation, but I know this is a complicated circuit, constructed from parts that are becoming harder to locate

Cheers,

Ed
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Old 2nd July 2010, 10:58 PM   #2
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Brief update,

A very preliminary investigation, poking about with a multimeter after the amp had time to discharge (90V rails and giant reservoir caps...) seems to show no glaring problems such as shorted devices in the output stage, drivers or voltage amplifier transistors.

Next I plan to pick through the input stage with a fine toothed comb. I have a service manual, and have been reading some of the older threads here on the 585 and 565 amp repairs. It seems there are some voltage references that can develop problems, and thanks to DIY audio, I know which components are appropriate substitutes for those with cryptic Adcom part numbers.

This amp has already been fixed following a typical case of the notorious leaking electrolytic capacitor disease. This culminated in its devouring the previous owners' speakers with full DC rail output before the output stage's power supply fuses finally put it out of it's misery.... He was told it was beyond economic repair and passed the remains on to me. I managed to clean and restore both input boards with new capacitors, restoring the amp to apparent good health. Perhaps though, there was another fault initiated by the leaked electrolyte, but not fully developed at the time of my repair?

This time around, all I noticed was distortion & signal fading to nothing in the affected channel, a couple of minutes or so after switch on. Mild but ultimately harmless DC was subsequently measured on the output. Luckily my beloved (and recently completely re-voice coiled) Magnepans were spared. However, I am prepared with a set of beefy dummy load resistors in preparation for testing... when I get there.

Cheers,

Ed

Last edited by Ed Holland; 2nd July 2010 at 11:08 PM.
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Old 30th January 2012, 01:21 AM   #3
gootee is offline gootee  United States
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Ed,

Did you ever find and repair the problem?

Regards,

Tom Gootee
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Old 20th June 2012, 10:12 PM   #4
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OK, wow, was it really so long ago?

My workspace and available spare time to fix this problem eluded me, and the poor amp sat unloved for a long time. I was also wary of causing major damage in the pursuit of the fault... it's not a trivial circuit, or of a construction that is easy to probe. However, in an chance conversation with a work colleague, I was directed to a repair shop with a seemingly excellent reputation, and decided to take the plunge with them. They were immediately confident that it could be returned to factory spec., and I decided to take the plunge. Not very DIY, I know, but this equipment is worth it, in my opinion. I did copy the service manual for the engineer

Anyway, I just collected it this past weekend (June 2012). Parts replaced were a couple of resistors, and the bias servo opamp. I was heading in this vague direction with my own diagnostic thoughts. Open resistors have been mentioned before on this forum, and the opamp was drenched in electrolytic cap sludge before I made the original repairs to get the unit running after it was given to me. At that point, this channel was out of servo control, and putting out -90V (full rail voltage essentially). Although I corrected that issue (new caps & vigorous cleaning of the PCB) and proved the amp was still capable, it seems these other parts had been stressed, or succumbed to chemical exposure and gave out later.

Anyway, my beloved Adcom is back home and well again. Even given the repair bill, it's a bargain.

Cheers

Ed
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Old 21st June 2012, 03:21 AM   #5
gootee is offline gootee  United States
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Ed,

Great! I am very glad to hear that!

Given the relatively low prices of components, and the nasty characteristics of the electrolyte that leaks, and the time it takes to partially disassemble the thing, it would probably be a good idea, for anyone repairing one of these, to replace any components that the electrolyte might have touched, while they're at it.

Cheers,

Tom
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Old 22nd June 2012, 09:16 PM   #6
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^ you're probably right there Tom, good idea indeed... cap electrolyte is ugly stuff.

The discrete components are trivial enough to replace, and I took care of all the electrolytics on both input boards during my original resurrrection of the amp. Where one can run into trouble is if semiconductors are suspect - they all have Adcom part numbers, and finding info about device equivalents is tricky at best. Many items are also close matched in pairs or even (I think) quads, according to the service manual. I admire the attention to detail that went into matching, but the disguised part numbers are a headache.

For instance the bias opamp (Labelled Adcom 2A) was replaced in this recent shop repair. Somewhere on this forum there is a thread detailing suitable replacements (micropower spec, predictable non latching behaviour etc.) amid warnings against simply using any pin compatible substitute.

The repair took a while to complete, because (so I believe from what I was told) that the tech was attempting to get an original part - something I have not opened the case to check. I'd certainly use this shop again . They were friendly, and willing to tell me a little about the nature of the fault, and what was needed to put it straight.

Now, I'm off to listen again,

Ed
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Old 22nd June 2012, 09:36 PM   #7
a.wayne is offline a.wayne  United States
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The 555 mk2 seems to be the best of these older adcoms, the bigger amplifiers 565, 585, sonics and reliability are not as good IMO ...
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Old 24th June 2012, 09:07 PM   #8
gootee is offline gootee  United States
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Quote:
Originally Posted by a.wayne View Post
The 555 mk2 seems to be the best of these older adcoms, the bigger amplifiers 565, 585, sonics and reliability are not as good IMO ...
Interesting. I have heard quite a few people opining that the 535 and 535II sound better than any of the 545 or 555 models.

I have a 535 II, 2x 545 II, and a 585. My son and I both agree that the 585 is somewhat better-sounding than the other two models I have, with both my Magnepan MG-12 and my Vandersteen 2Ce speakers. It is not a large difference but it is noticeable.

The harmonic and intermod distortion specs are certainly MUCH better, for the 585, than any of the 535/545/555 models, according to the tech manuals.

Personally, I don't think there is a big difference between any of the models I have. I like all of the ones I have, quite a lot.

I originally bought the 585 mainly just for its power ratings, because I wanted to be able to try getting near the MG-12s' maximum power-handling capabilities without clipping, mainly just to find out how loud they could go before the bass started audibly modulating/distorting the mids and treble due to excessive panel motion. The 545 II could never reach that point, with 150W/ch into 4 Ohms capability, before its warning indicators lit up. The 585 did it easily, being capable of 400 W/ch into 4 Ohms. (It is at a point that is loud-enough that I would never desire to listen anywhere close to that loud, with those speakers at least, which is good to know.)

And with the 585, I learned that the 2Ce speakers have red lights behind their socks, which flash when speaker detruction is imminent!

Also with the 585, I learned that if the remote-controlled motorized volume knob on my Adcom GTP-500 II preamp gets stuck in clockwise mode and it maxes-out the volume control, both tweeter fuses in the Magnepan MG-12 speakers can blow, instantly.

And with the 585, if I ever wanted to, I could run 4x pairs of 4 Ohm speakers in parallel, since it's capable of driving 1 Ohm per channel with 1000 Watts of continuous average power. I would probably never want to do that. But the fact that the tech manual gives that rating probably means that the power supply is relatively robust.

But now you've got me curious about the 555 II, which I've never heard. Maybe I should acquire one, just to round out my "collection".

Last edited by gootee; 24th June 2012 at 09:11 PM.
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Old 25th June 2012, 04:34 PM   #9
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Funny this comparison should come up. When I acquired the 585, a 535 came along for the ride (it was part of a surround system that my friend abandoned when the 585 detonated his expensive B&W speakers).

The 535 has been driving our Magnepan SMGa's in place of the 585 during the big amp's extended absence. It does a fine job in this role, but I never push the system terribly hard. I think I like the 585 better, but can't come up with an objective reason for this at the moment.

Ed
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Old 26th June 2012, 01:09 AM   #10
gootee is offline gootee  United States
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Yeah, I can't claim to be especially objective, certainly. And I've never done any type of blind or double-blind testing with them. For all I know, maybe the 585 sounded better to my son and me because it's bigger, or because it costs more. As I said, all of the ones that I have sound great; superb actually. They all seem to be nearly transparent, to me. And since my goal is always the most-accurate reproduction of the input, whether I "like the sound" or not, they are truly-great amplifiers, to me. It's almost difficult to believe that they are as cheap as they are, on the used market.
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