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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,
Gary

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
 
:redhot:


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,


Thanks,
Gary

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.


Gary


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