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JTM30 Problem -- Please Help!

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3/12/2000 7:42 PM

JTM30 Problem -- Please Help!

My JTM 312 started fading out, going from full volume to a faint, scratchy level after it had been on for about 5 minutes. Sometimes if I really attacked the strings it would pop back to full volume, but fade again. I'm not much of a tech. Most of my electronics experience is at the level of rewiring guitars, building foot switches, or fixing guitar cables.

I first checked the tubes -- all OK. I pulled and checked all the caps (only with a multitester, not for capacitance) -- all OK. I put all the caps back in and now I get nothing, no sound at all, not even hiss.

My more-experienced brother-in-law and I went through the board and traced the signal. I'm getting signal at the emulator-out jack and all the power circuits are working, but the signal's not making it to the power section.

So, with that background, I have two questions about the problem. Is there anything obvious that could have caused the first problem, ie the fading out? Could whatever caused the fade-out problem failed entirely and caused the break between the preamp and the power amp?

I also have one more question about resources. I found a schematic, but I'd also like to find a service manual. Anyone know where to look online?

3/12/2000 10:55 PM
Andy	Hi Maybe too simple, but have you tried bridging the FX-loop with a cord? This way you can eliminate the jacks themselves as being the problem (this happens a lot of times!) Andy

3/13/2000 1:12 AM
rebel420	I've had this happen befrore... 99% of the time it's tubes.. try new tubes, aslo try the Caig ProGold GXL (available from lord vavalve at NBS)... the stuff is AMAZING on crackly sockets... aslo make sure you bias the amp properly as well... if the tube s are running too hot or cold you can SOMETIMES get weird results

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