HELP!!!!!!! JCM 900 4100

[alexzanderepair]

My Jcm900 is in the dumps.Im getting no sound. Its getting power to the speakers so I assumed it was the preamp. I opened the sucker up and found that all four led lights were not lighting up. Something is wrong but what should i be looking for?

http://www.drtube.com/schematics/marshall/jmp52-02.gif

[Seljer]

The LEDs there in the top left of the schematic wouldn't be lighting up normally, I think they're some sort of actual clipping diodes, so they'd probably only light up when you played something, if at all

If you think its the preamp try hooking up the FX send to something else to see if you're getting any sound from it. Or try hooking up something else to the FX return to see if the poweramp works.

[petemoore]

  First thing to look out for..is Potentially LETHAL high voltages.
  The next would probably be an amp forum, such as Ampage or other.
 

[alexzanderepair]

Yea im def getting sound from the effects return. (oh and yes I am being very careful regarding voltage. I like my sweet fingers )
So its def the preamp. Here are the symtoms. the only way I get sound is if i turn the volume up all the way. Im searching but not to sure what to be looking for

[R.G.]

1. take your voltmeter, clip the black lead to ground
2. put on rubber/foam soled shoes; put your left hand in your pocket
3. power on the amp
4. touch the red probe to places in the signal path at different places; you should hear a click when you do this.
5. work back from the effects return (where you know you get audio) toward the input, listening for clicks
6. when you don't hear clicks, you've backed up before the problem

[alexzanderepair]

Im gonna try the tracking back thing. But let me just put this out there because this issue has been driving me nuts and I refuse to give up.

THE FIRST PERSON THAT HELPS ME GET THIS DARN THING GOING WILL RECEIVE 50 BUCKS VIA PAYPAL

you guys have been at this alot longer than I have(schematics etc.)

please help me

Ive been looking over the schematic and Im gonna try the back tracking thing

[MetalGod]

a common problem with send and return jacks is that the contacts don't 'make' and therefore you get no sound.

try a patchlead between the send and return jacks - if that solves it then you've got a dodgy send or return jack.

[lovekraft0]

JCM900s also commonly have power supply problems, especially filament supply - are all your filaments lit?