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gdnichols31 20th September 2019 02:45 AM

Adcom GFA-555 II Amp Oscillating
Bought a defective GFA-555 II off EBAY. Right Channel was good, Left channel was dead and the instan. Distortion LED was on.

I replaced Q107 & Q108 and the left channel came back to life.

I replaced all the elec caps.

I biased both channels and they came in right at 1mV.

Output DC offset is less than 1 mV on each channel.

When I tested the amp in bridged mode, 1 kHz signal into an 8 ohm load, when the output was at about 150 watts, it went into oscillation and blew a fuse.

It's done that twice now.

When I replaced the fuse and test again in Stereo everything is fine.

Any ideas what I should check/look at? I'm guessing a cap somewhere but not sure.

Thanks for the help,

PB2 20th September 2019 03:58 PM

I'd suggest that you put 4 ohm loads on it in stereo and bring up the power to
see if it does the same thing. Output transistor parameters change most dramatically
with current/voltage changes and if they were replaced then it might not be stable.

gdnichols31 21st September 2019 03:47 AM

My face is RED

I figured it out not long after I posted the question.

I remembered I still had my scope connected to the output, shorting one side to ground!

I'm lucky the fuse is all that blew.

Once I un-grounded the output it worked great!

Imagine that,


Phloodpants 21st September 2019 04:18 AM

Were you testing the amp with a signal generator with a grounded plug? If so, you might have sent a current through the patch cable from the signal generator. The GFA-555 II has a 100R buffer resistor between each channel's signal ground and amp ground. Check resistance between your amp's RCA shields and speaker ground. You might have popped those resistors. It's a metal-oxide type if you need to replace it. (Metal oxide for its high frequency performance.)

gdnichols31 21st September 2019 06:24 PM

Pretty sure my signal generator is isolated but I'll check it and the resistors.

Thanks for the tip.


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