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,
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.
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!
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.)
Pretty sure my signal generator is isolated but I'll check it and the resistors.
Thanks for the tip.
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