Roland JC120 - Weak/Distorted Chorus and Vib output

[electroharmony]

Hello, I am new here - thank you for taking the time to read my question.

I have what i believe is an early 80's JC120. 2EXT speakers, LINE out, 3Footswich, 2MAIN in, Toggle brights, Toggle Chorus/Vib.

I am having a problem where when the effects are engaged (Vib/Chrous) the output on that channel is very low and distorted.

Things to point out: I have verified each power amp is in proper working condition. Also, the signal does seem to get processed through the effect path when circuit is engaged.

To describe better, volume output is normal out of both speakers when effect is off. When it is engaged, left speaker volume stays the same and right speaker level drops significantly. While engaged, I can unplug the left speaker to listen only to the effect channel through right speaker. Output comes out very low and distorted, however each effect is discernible.

Neither of the three schematics circulating around match my amp boards. Roland has not been able to help locating correct schematic. It has the MN3004 chip (IC2) with a white trim pot located directly above that (VR9). There are only 3 other ICs on this board are MC140138, MC140118, and 4558JRC. There are 4 D736A output transistors. Also this version has the Chorus adjust pot on the back of chassis.

If anyone has any ideas or suggestions on what to look for, I would be very grateful for the help.

Thank You

[Jazz P Bass]

JC 60 schematic

I have the JC 60 schematic.
Maybe it will help you sort things out.
It has the rear panel pot for chorus adjust.

[doctor]

I have the JC 60 schematic.
Maybe it will help you sort things out.
It has the rear panel pot for chorus adjust.

I hope that at least one schematic will be suitable.

[teemuk]

Archive of several JC-120/JC-160 service manuals can be found from here:
Index of /Audio/Musical/PRODUCTS/ROLAND

Hopefully, one of them matches yours.

[SPARESROLAND]

Hi are you able to supply a part number from one of the pcb's in you JC120?
These part numbers are usually on the edge of the pcb. This will allow me to identify which JC120 service notes you will require.

cheers
steve at Roland UK ltd

[electroharmony]

@Jazz P Bass, that is the closest board i've seen that is in my JC120! All the transistors, IC's, etc are the same. Seems the JC60 of that era and JC120 use similar boards. I've never heard of the JC60, apparently it is a mini JC120. Or to put it another way, the JC120 is a JC60 + Another seperate 60W amp board

If anybody else is having trouble finding the correct schematic for their JC120s, take a look at these (JC60), as they are similar for the effects channel

If anyone is curious about serial numbers, my JC120 that jives with these plans is: 921575

I've been searching high and low for a few weeks, so thank you. When i fix the amp I will update
Sweet!

[electroharmony]

@SPARESROLAND the amp boards are (CH1) AP-113 141-113 ; (CH2/Effects) AP-114 141-114 . If you could find the service notes for these two boards that would be great.

Looking closer at the JC60 plans, it actually is a different board as the JC60 uses AP-116.

Thank you!

[tim glasgow]

Hi, electroharmony. i have the exact same amp with the exact same problem! My serial number is 8810xx and i also have the AP-114 card. It seems this version of JC-120 sits between two different versions that i have schematics for and neither schematic is correct. The JC60 diagrams seem to be very close (and solve the mystery of the white trim pot between the two outputs of the BBD chip) but if you've repaired this problem i'd love to know what you did so i don't have to reinvent the wheel. If you haven't, perhaps i can help at my end.

Did you manage to fix it? Did you find a better diagram?

Thanks!

[electroharmony]

Hey man, no luck on my end. While troubleshooting, i managed to short something out, which caused a new set of problems on my end. Now I have no audio passing when either the chorus of vibrato are engaged. Found and replaced a shorted zener diode, but still no luck. There is a faint trace of the chorus oscillating though when engaged, not audio per-se, but like a "whirl-whirl-whirl". All other components have been tested and seem to be in working order. So the project has been shelved for now untill I get motivated again on it. If you happen to have any ideas that would be great.

I found the correct schematic, and attached it to this reply

[tim glasgow]

Oh no! Sorry about your luck, electroharmony. i'm likely going to try a full re-cap to start with. This thing has lots of weirdness. The BASS and MIDDLE knobs only do anything up to 5; they cut, but don't boost, and the reverb is way too sensitive. Walking across the floor makes it rattle (i opened it up and mechanically it seems okay). Since this thing is indeed 30 years old and the goop on the screws hadn't been cracked, it's par for the course.

Thanks a ton for the diagrams! So much easier to do this without all the guessing!

[Luiggi]

Hi all, any updates on the solution for the vibrato issue? I also have the exact same problem here, as soon as the vibrato is engaged, the output of one of the channels falls of until its disengaged again. Any answers will be appreciated. THis unit I have before me has no trim pots whatsoever that I can see.

EDIT: I resolved this problem by replacing IC5 on the efx board, I even replaced with a garden variety 4558 and all ehll broke loose , now the amp sounds beautiful again.