Jcm 900

[ALEXZANDEREPAIR]

Ok so I recently fixed my JCM it had a bad solder joint.Quick fix.
I then started on my long awaited combo amp for my JCM
After i was done with the cab then mounting the speakers.
I got NO SOUND!!!!!!!!!!!!! see before when my preamp had a bad solder joint I was getting power to the speaker and when I plugged into the effects return I was getting sound but now Im getting nothing to the speakers
The cabinet took me about a day and I put the marshall in and the speakers
but when I turned it on I wasnt getting any signal to the speakers but now when I turn it all the way up the transformer makes a slight buzzing sound
Im confused because it was working fine and wondering if When I was trying to get the marshall in the cab that maybe in the process I banged it and loosen something
I pulled all the tubes and placed them back in to see if they maybe got loose
then pulled the preamp board and did a visual check for loose solder joints did the same for the amp board
Im not to sure why something could have gone wrong by me just maybe knocking it around a bit.
I hope you can lead me in the right direction again.

[Amp Kat]

It's kind of a routine thing when I get no sound on Marshalls but it works. Of course making sure the power is off I measure the Output transformer primaries and make sure I'm getting that 16 1/2 ohms from center tap to winding and around 32 ohms across each side. Then I check the speaker jacks and make sure it's around 3 ohms from tip to sleeve and make sure the sleeve is grounded. If that's all cool I fire it up and start making voltage readings. Of course first visually inspect for heater glow then Check pin 3 and make sure your getting your 450 to 500 volts dc on the plates and slightly less on pin 4, then check pin 5 and make sure you have your -50 to - ? there. If that's all good then I insert a cord from send to return to make sure that connection isn't causing loss of signal. If it still isn't there it's time to get the signal generator and scope out. You can try your send out to another power amp in or return of another amp to verify and isolate the preamp from power amp. If the send out works you've cut your time in half and can focus on the power section. Sounds like a connection problem to me but maybe some of the other fine folks here will chime in on some other issues.

[tim]

What amp kat said...also, a few weeks back a guy called me in a panic from a gig...He had just set up his Marshall amp and was getting no sound out at all....he said that during his practice just an hour before that all was fine....after going through the normal checklist....speaker plugged into correct jack, no bad fx loop, etc, it was found that his speaker impedance switch was between clicks so no nothing was going to the load.....clicked it to a detent and wham-o...lots of sound. All this to say, sometimes it's the simplest things and we just overlook them.

[Enzo]

And some of those have a mute button. If you never use the mute button and it gets pushed in somehow, it can be "invisible."