LAB SERIES L3 - troubleshooting mysteries

[Baguette]

Hello gang,

I've just acquired an old Gibson / Moog / Lab Series L3 amp from about 1980. It's the smaller one of the bunch (1x12"). It's a fine amp, buuuuut....

It has 2 problems.

1. The input jack labeled "LOW" does not work. It's not crackly or anything, it just does not work at all. I've sprayed it clean and still, no sound. There's a faint, muted sound when the guitar jack is plugged half way in. Other than that, no sound. The other input jack ("HIGH") works fine.

2. There's a loud hum (low pitched hum) + hiss (white noise type of swooooooosh). This buzzing sound increases when the Master control is turned up, but does not increase significantly when the Volume,TMB and Reverb controls are turned up. Also, I do have a sound when I plug the guitar halfway into the "External Processing" jack (aka FX loop). So I guess the power amp part is fine.


What I have done so far is spray the jacks and chopstick the heck out of the circuit board. But I did not find a bad joint or something with this technique. I'm stumped and do not want to randomly change parts.

Do you have an idea what the problem(s) may be? and could my 2 problems (jack + noise) actually be related?

Your help is greatly appreciated, thank y'all in advance!

[Latole]

Did you find the issue ?

Easy to fix if jack HI is working.

Service manual ; https://www.dropbox.com/scl/fi/93g3vcaru3tlidowr6yjp/Lab-Series-L3-Service-Manual.pdf?dl=0&e=1&rlkey=mznd3t8uwwiosxj8cwe64zy8w