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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

JTM30 Troubleshooting help

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I'm fairly new to amps and have so far only repaired simple fender champ type things. Unfortunately my main amp has developed a fault so i decided to take a look.


The fault started after transporting the amp to a practice, only to find I couldnt get anything from the clean channel. The dirty channel was ok.

One side of V2 handles the clean so i thought the valve was loose or the solder on the socket came away.

Unfortunately re-seating the valve did't work.
Swapping valves about and a new valve in v2 didn't work.

I had to take the main PCB out to get at the sockets etc but they look fine and ive reflowed em just to be sure.

I was thrown for a bit as I had no plate voltage, then i realised the main ground for the main board is actually the risers used to fix the board in to the chassis. So i ran a temp ground wire. and presto.. voltage.

But now i can get no audio from either channel, I get audible noise touching the grid of any pre amp valve but nothing from the input jack.

Touching a screwdriver to where conn 101 comes in seemed to get a noise.

All plates have sensible DC voltage in the order of 150v
The first two valves have about 1.5v on both cathodes,
The third valve has 75V at the cathode.. this sounded wild to me but i dont really understand what its doing.

If you plonked the amp in front of me as it is right now is say problem with the input jack, but previously only one channel was effected.

I tried clipping a guitar direct to con 101 thinking that might come through (i need to buy a signal generator).. That didnt work.

I'm a bit lost on this one... Can anyone assist with troubleshooting?
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When I have to deal with something like this, I'll send a 100Hz signal in with my generator, then poke around with my scope until the signal doesn't make it through a certain section.

I think that would be the kind of approach I'd go with. but i dont own a tone generator or a scope..

Would it be ludicrous to take an old MP3 player and a mono 3.5 mil cable, cut one end off, and connect the ground to ground on my chassis then lift my coupling caps one at a time and touch the hot of the MP3 output to where the caps were connected?
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