Early JC-120: chorus works, controls don't - help?

[Enzo]

Look underneath the switch - the other side of the board, that is. There are two parallel rows of contacts. It is nothing more than a fancy version of the slide switches like Fender uses for Bright switches. If you can visualize how those work, you can visualize this. For every three legs down a row, the center one switches to the leg on one side of it or the other side. Try a couple with a meter, you'll figure the pattern out simple enough.

Just check them all, don't bother tracing which exact ones go where. It won;t take but a couple minutes.

Never think up reasons not to check something

[drasp]

Thanks for popping in!

I didn't bother probing the switch as I don't know what correct operation would look like. I guess I could try to trace the legs of each pot to the switch to see if it's something really straight forward. . .

[Enzo]

CLean/schmean, did you TEST the switch contacts with an ohm meter? Are all of them functioning?

[drasp]

Well, pulled the chassis back out this morning before work and found that the underside of the board, including all the solder joints on the chorus/vibrato switch look great, clean, etc. Cleaned the switch w/ some contact cleaner, no change. Pots still work perfectly on the vibrato & not at all on the chorus. Any suggestions for what to look at next?

FYI - I've read a few posts here about early JC-120s with the little "mystery pot" inside the rear panel. It has two wires, one purple & one green going to terminals marked 'v1' and 'v2' on the chorus board. I noted the original position and then swept it through it's range. Not sure what it does. . . any help here?

[drasp]

More pix:

[drasp]

Howdy! I recently picked up an old JC-120 at a local pawn shop. It's a little worn, but that's fine by me, I like 'em that way. One speaker has been replaced with a Pyle (which is a poor match as it is more efficient & has earlier and more aggressive break-up), the ground prong was broken off so it hummed like crazy, and the 'verb & modulation weren't working. First fix was an easy one, I scored a 'construction orange' 3-prong plug from Home Depot and the amp is now very quiet, even cranked well past noon w/ the distortion up.

I've been keeping my eyes peeled for an early Roland speaker to match the original, but they don't seem to pop up very often (ever?) so I may go with a pair of something similar.

The reverb and modulation seemed at first to be another easy fix as the blue wire going from the main fuses to the board was hanging loose, not able to tell if it was sloppily de soldered by some previous owner or if it just broke off. The 125mA fuse looked to be of the same make & age as the others, and I've now been using the amp for more than a week with no problems, so I don't think it was purposefully disconnected. The replacement speaker must've been wired out of phase with the other as the chorus wasn't very strong until I swapped leads on that speaker. The chorus and reverb now work quite well with one exception = the rate & depth pots have little to no affect on the chorus, which is strange as both pots measure as working and affect the vibrato in a normal, linear fashion. What would cause the pots to work correctly with vibrato & not chorus? Any suggestions for first steps in trouble-shooting this would be greatly appreciated!

I've got all 3 of the common schematics that float around, but I think my amp is pre-'79. The chorus board has a totally different layout than the one shown in the '79 diagram, but still uses the MN3002 unlike the later ones I've read about. I talked to Roland USA, but they don't have a service manual for anything earlier than '79.

Last thing to note is that my reverb tank must've been replaced at some point. It's an O.C. electronics 'type 60' unit, "Manufactured by beautiful girls in Milton, Wis. under controlled atmosphere conditions". See the picture below - I got a kick out of it!