Fender M80 bass amp repair - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument amps.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
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
  Reply With Quote
Old 2nd May 2011, 08:38 PM   #2
mjf is offline mjf  Austria
diyAudio Member
 
Join Date: Jan 2009
Location: Austria
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
  Reply With Quote
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?
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Help for repair or rebuild power supply from bass amp. Fredenando Power Supplies 1 14th December 2012 01:30 PM
zapco m80 outputs damage Car Audio 2 15th February 2011 10:14 PM
Fender Jazz Bass Mexico made jazzman06 Instruments and Amps 1 10th July 2009 09:55 PM
Want to build bass cabinet for Fender BXR 300 timzog Multi-Way 14 11th June 2004 11:50 AM


New To Site? Need Help?

All times are GMT. The time now is 05:37 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki