Fender M80 bass amp repair

zvir · 1st May 2011, 07:54 PM

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

mjf · 2nd May 2011, 08:38 PM

hello.
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.
greetings

zvir · 3rd May 2011, 10:21 AM

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?

1st May 2011, 07:54 PM	#1
zvir diyAudio Member Join Date: Feb 2009	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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2nd May 2011, 08:38 PM	#2
mjf diyAudio Member Join Date: Jan 2009 Location: upper austria/near linz	hello. 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. greetings

3rd May 2011, 10:21 AM	#3
zvir 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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