Help with a JCM 900 mkiii 2100 100w

[gkelm]

Just picked this up locally for $700. Has some cosmetic wear, but not too bad. Has a couple functional issues…

1. The channels won’t switch. I’ve tried two know-to-be-working basic footswitches—the light changes but won’t activate volume A.

2. The sound fades and cuts out occasionally. I’ve heard this may be dirty jacks, etc. or perhaps some wiring issues?

3. Appears to have original filter caps

4. A bit more noise (higher pitched buzz vs hum)

Otherwise, it’s a 1990 model with EL34s, and sounds pretty good…much better than the SL-X I had recently.

If I took it to a tech, any ballpark idea what it might take to get it right? I realize online diagnosis is limited.

The guy is cool and has offered to refund, but may want to just keep it and get it tuned.

Any advice appreciated.

 

[Jon Snell]

Check your settings firstly and if you are competent to work on high voltage valve equipment;
Pin1 of IC2 should switch between +15v and about +7volts when the pedal is activated. (Maybe the controls are not working; VR3 & VR4).
If it doesn't switch and the pedal, wire (and jack socket) is good, check TR1.
If it does switch, suspect IC2 or its +15volt supply.

 

[JSJ900]

Took my 2100 to a tech a few years ago. Sound was cutting in and out. He had it on the bench and it was singing, cranked w/ no issues. He couldn’t figure it out until he put it on top of a cabinet.

The vibrations from the cabinet were making the 100/50 (pentode/triode?) switch cut in and out. He asked if I wanted to wait for a replacement switch to ship. I asked him if he could wire the switch to just the 100 W position. He did and no issues since.

 

[myersbw]

Ask your tech if he's comfortable replacing an IC if need be. The IC Jon is referring to is a common fail. I've replaced a couple and it requires pulling the board. On a 900, it's just a little tedious. Let the tech know you'd like a socket installed if it needs replaced. Makes any future replacement MUCH less expensive! (I've always installed one when that problem arises.) The part is inexpensive...it's just the labor of diagnosing, any board pulling, etc. That cost greatly varies with any given tech/shop. Good luck!

 

[gkelm]

Thanks guys!