Adcom GFA 585 faults again

[Ed Holland]

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

[Ed Holland]

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

[gootee]

Ed,

Did you ever find and repair the problem?

Regards,

Tom Gootee