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Old 1st May 2011, 07:54 PM   #1
zvir is offline zvir  Croatia
diyAudio Member
Join Date: Feb 2009
Default Fender M80 bass amp repair

Hi everyone.
I recently got an old Fender M80, that had been sitting in the basement a long time after it died. When I started checking it for problems, I found that several things there are blown. So I just went from the beginning. I fed it a sine wave and started following the signal path with an oscilloscope. I found the first problem in the EQ section of the preamp. One of the op-amps (U3B) is oscillating at RF, and I can't figure out why. Can anyone have a look? I can post o'scope traces if necessary, just tell me where to stick the probe
Thank you.

Schematic: http://www.pmfst.hr/~zvodom/m80.pdf
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Old 2nd May 2011, 08:38 PM   #2
mjf is offline mjf  Austria
diyAudio Member
Join Date: Jan 2009
Location: Austria
have you tried out to solder a cap (47nF........100nF) across the power supply pins of the tl072 ? perhaps this can help.
if it does not,i would simply change the opamp,the tl072 are cheap and easily available.
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Old 3rd May 2011, 10:21 AM   #3
zvir is offline zvir  Croatia
diyAudio Member
Join Date: Feb 2009
I appreciate your input, but unfortunately, it doesn't help... Soldering a cap does nothing, and I already replaced all of the op-amps in the circuit. Also, I noticed that, when I ground the non-inverting input of the op-amp, the oscillations stop. That would mean that they are coming from somewhere before this point, right?
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