Basic troubleshooting help Lab Series L3

[Johnnykrome]

Hey,

New here looking for some help/suggestions. I have a Lab Series L3 with no sound coming with a guitar plugged into either high or low inputs. No hiss, pop, nothing. Amp powers on, and I have found that I can get sound from plugging my guitar ( active pickups) HALFWAY into the jack at the end labeled "external processing".
All the way in = no sound. But halfway produces good clean sound which is adjustable by the master volume.
The other volume, equalization, and reverb have no effect on the sound when plugged in this way.
By the way I bought the amp in its current condition.
Any repairs I could do would have to be layman level because I am not experienced repairing electronics. Would appreciate any thoughts. John

I got these schematics from another thread.... I really dont know if they are relevant......

Lab_Series_L3-1.pdfLab_Series_L3-2.pdfLab_Series_L3-3.pdf

[The Dude]

The fact that you can get sound when using the "external processing" jack tells you the power amp section of the amp is working. It could be the jack itself that is the problem. It is a stereo switching jack where sleeve= GND, tip= send, ring= return. That is why it works plugging in to the first click. The jack should short tip to ring with nothing plugged into it allowing the preamp signal to pass on to the power amp. To verify this is the problem, plug a stereo plug into the jack and short tip and ring together with an alligator clip and try the amp. Or, you could grab an insert cable and plug it into the amp and then use a dual 1/4" female adapter to tie the two mono ends together. If the amp then works, you must clean up the contacts on the jack or replace it. That's where I'd start.

If not that, you're probably looking at a bad FET, bad op amp, bad +-15V supplies, etc., which may be beyond "layman level" depending on what exactly you meant by that.
GOOD LUCK!

[Johnnykrome]

Dude, thanks. I am going to try these tests although I need to educate myself about cables, I think. " Layman level " means all this working on the amp makes me want to go lay down.