JC-120 Low Volume

[teemuk]

Wow... I spent hours messing with it today, and started going through more threads here, and your post confirms what I read. That is a really dumb feature... holy crap, why not have it adjustable too?

While Vibrato mode has always had fixed settings the very first JC-120 amps actually used different modulation waveforms (and separate LFOs) for vibrato and chorus functions, which meant they actually had a slightly different tone to them. In circa 1982 they downsized it all to just one LFO and one modulation waveform style, which basically meant they removed an entire feature from the amp. (Considering all the mods they have done to JC-120s along the years that was actually a pretty minor mod.).

Anyway, in some JC-series amps, that including for example the JC-120H (head version), the control is actually more accurately labelled: "Chorus: Manual/Off/Fixed". So, theoretically "no vibrato" in them, and I don't really know why they kept up with the old format although really removing the vibrato. But as we should know at this point, the "vibrato" has since 1982 been the same thing as chorus dialled to particular settings.

[g1]

In defense of the Roland repair center, you still have a very strange problem with that control line for the mute fet's. It does not make sense that removing and reinstalling the diode fixed it. You may still have to spend a fair bit of time, there may be cracked traces etc. The repair center may have suspected a cracked board and decided that board replacement was the most cost effective/reliable repair. Hopefully you will get lucky and find the problem without having to spend to much more time.
As far as measuring Fet's with a meter, it can be difficult. Depending on the order of when you put which probe where, you can be turning on the fet and it can give different readings till it gets turned off. So to compare the two side by side, you must do your measurements exactly the same way, in the same order. Hope that makes sense. Then on top of all that, they can read fine out of circuit, then leak with voltage applied.

[52 Bill]

...and there was some terrible soldering going on here too, which I cleaned up.

This amp was given to me as a project, and has been sitting for a long time. A friend of mine sent it to a local tech who resoldered and hot glued a bunch of stuff, and then he sent it to the state Roland service center and they wanted to just replace the whole main amp...

For me the worse amp to have to repair is one that has been "repaired" by someone before it comes to me. Generally, if the tech is qualified there is very little disturbance to the board and any replacement parts are fit in a clean and workmanlike manner. When I see this I usually feel that the previous repairs were done correctly and I can proceed with whatever I need to do to fix the amp.

When I see obvious signs of bad soldering, sloppy parts replacement and general disorder, I have to spend time double checking the previous work done before I can go forward with repairs. It may have been some sloppy soldering that you cleaned up that fixed the mute voltage. Replacing the fets should bring the amp back to 100%.

And for me, if you had mentioned the facts about the amp earlier on it would have changed my thinking about how to proceed on the repairs. Unless I know that the amp has been worked on before, I assume that the amp has just broken down and was working and unmodified before the posting.

In any case, I'm glad to hear that you got it up and running.

[AtomicMassUnit]

For me the worse amp to have to repair is one that has been "repaired" by someone before it comes to me. Generally, if the tech is qualified there is very little disturbance to the board and any replacement parts are fit in a clean and workmanlike manner. When I see this I usually feel that the previous repairs were done correctly and I can proceed with whatever I need to do to fix the amp.

When I see obvious signs of bad soldering, sloppy parts replacement and general disorder, I have to spend time double checking the previous work done before I can go forward with repairs. It may have been some sloppy soldering that you cleaned up that fixed the mute voltage. Replacing the fets should bring the amp back to 100%.

And for me, if you had mentioned the facts about the amp earlier on it would have changed my thinking about how to proceed on the repairs. Unless I know that the amp has been worked on before, I assume that the amp has just broken down and was working and unmodified before the posting.

In any case, I'm glad to hear that you got it up and running.

I replaced the bad Fet, reinstalled the other that I pulled and all is well with the amp. Works great!!