Help me troubleshoot my LAB SERIES L3

[rileyismycopilot]

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!

[Loudthud]

Probably a bad connection on the preamp circuit board.

Do you have the schematic correct for you amp? Post it or a link.

[g1]

Service manual attached.
Sounds like 'Lo' input is the higher gain circuit (meant for lower output pickups) which is opposite to the usual arrangement.

[rileyismycopilot]

Hey!

Thanks for your answers and sharing the schemo. Great stuff.

Could someone please help me pinpoint an area of the circuit / component, according to the symptoms I have described?

Could my two problems be related?

Thanks!

[Loudthud]

Problem 2 would seem to be Q9, part of IC2. I think you can still get these parts at Jameco in the USA. Check the -15V supply for ripple, it comes from A1A.

[rileyismycopilot]

Problem 2 would seem to be Q9, part of IC2. I think you can still get these parts at Jameco in the USA. Check the -15V supply for ripple, it comes from A1A.

Hey Loudthud, thanks for the great answer.
1. IC2 is marked "14007" on the schemo. Could I swap it with a "MC14007UB" or "CD4007"? (those 2 seem to be readily available).
2. How do I check the A1A ripple?

Thanks!