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My Marshall JCM 900 50 watt head (2500) has low out put

I bought this amp on ebay, it didn't work the first few weeks (no output) we found the first problem the impedance select switch was old so i fixed that, it worked for a few months and then the problem cam back, this time, we though it was the tubes cause i put in and old set and it worked so i got some new one and it worked for a week and then it happened again with he low/no output. this time i though it was the volume pot so cleaned it and greased it up and no such luck still has REALLY low output i have looked though this amp with he schematic and my dad and i have decided that there is possibly a lose connection some were in the amp we can't see. i want to see what all of you think before we send it in ANY ALL SUGGESTIONS ARE APPRECIATED!! thanks

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Did you replace all tubes preamp and power amp? Just a clarification I assumed so. Some common problems with amps in general are after time the effects loop relay can be going bad. This happened on my 5150. I eventually replaced the relay and all was fine but until I did I used a patch cable to go from send to return. Unfortunately this amp employs the use of opamps in its signal path for the EQ. You may also what to check these. First try the effects loop patch to see if that works for you. Since the problem is intermittent it sounds like that is the problem. If you need any other help let me know, I have pretty good experience troubleshooting/building tube amps.

Posted on Feb 08, 2010

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