JC-120 Bypass Chorus "Gate"

[thuk]

Yes, that will work. That end of R144 connects to the +15V line.

It didn't work, unfortunately. It just bypassed the chorus/vibrato completely. Just dry sound with the chorus/vib switch engaged.

This amp truly hates me. I would take it to a local repair shop (again), but I've been to the nearby ones and this problem had them all stumped too.

[g1]

I'm not sure how that could happen. Did you have to remove the board to do it? Often, other problems get introduced during the mod process.
If you remove that resistor now, does the chorus/vib work again? Maybe post a picture with the resistor installed before you remove it.

[thuk]

I'm not sure how that could happen. Did you have to remove the board to do it? Often, other problems get introduced during the mod process.
If you remove that resistor now, does the chorus/vib work again? Maybe post a picture with the resistor installed before you remove it.

Hi again, no I didn't remove the board. I already removed the resistor straight away to make sure I hadn't accidentally blown up the effects circuit or something. Has happened once in the past... But it was fine.

There's been another development though, I used the "shorting collector and emitter legs of transistor" method detailed in this image - https://postlmg.cc/ykhg7d9G (from this thread over here - https://www.diystompboxes.com/smffor...topic=131471.0). On Q26 in my version of the amp. And it worked! I couldn't believe it. A 7 year nightmare with this amp seems to have finally ended. I just hope doing this mod hasn't opened the door for any other problems to arise. The guy who suggested it seems to have come up with a good explanation for how it works in post #14 over there though.

g1, thank you very much again for all your tips and suggestions. I think I've become slightly better at following a schematic over the past few days, which can only be a good thing.

I'll need to make another post soon detailing what to do if anyone else with this problem finds this thread. Cheers again!

[g1]

I used the "shorting collector and emitter legs of transistor" method detailed in this image - https://postlmg.cc/ykhg7d9G (from this thread over here - https://www.diystompboxes.com/smffor...topic=131471.0). On Q26 in my version of the amp. And it worked!

That's interesting. It achieves the same thing as we were trying for so not sure why the first mod didn't work. Especially odd that it turned the effect off completely.
Turning on Q26 by applying positive voltage to the base should have turned on Q26 and given the same result as shorting E to C.
The only thing I can think is the layout picture from pg.2 of pdf. does not match the schematic. Maybe parts are called out incorrectly or something.

Glad to hear you got it to work!

[thuk]

Hello again. Here's the mod, based on Rob Strand's method posted here/pic also attached below - http://www.diystompboxes.com/smfforu...topic=131471.0

This is for anyone who has this same issue/feature and may find this thread in the future (if your JC-120 matches this one - JC-120_160 5º ED (1).pdf )

Locate Q26 (far right hand side, next to the big capacitors). Solder a tiny piece of wire, or make a solder bridge between the collector and emitter legs of transistor Q26. Search for the data sheet for your particular transistor if you need to confirm which legs are which. In my case, it was a C1815, so legs 1 & 2.

It's easier if you take the board out first and solder the underside.

For certain other versions of the JC-120 with the chorus gate, refer to these links -

https://www.diystompboxes.com/smffor...topic=131471.0
https://postlmg.cc/ykhg7d9G

As always, take all proper precautions and if in any doubt, please take to a qualified amp repair technician.

All the best!