Roland JC-120 Repair Question

Don Downes · Post by **Don Downes** » 7 Feb 2022 12:57 pm

Can anyone instruct me as to how to remove the "electronics" section from the case on a mid-70s JC-120?

I'm having issues with the Chorus/Vibrato/Reverb section. My hope is that the knobs and switches are dirty and just need a good cleaning. Also some of the Molex connectors will need to be cleaned as well.

I believe that the electronics are functional as I can hear subtle changes with switch settings. I can also get "some" kind of effects sounds from different switch positions. Additionally, I have checked both speakers independently and they ~ 8 ohms/ea (out of circuit).

Sounds silly, huh? There is not a vid I can find to inform me.

John Larson · Post by **John Larson** » 7 Feb 2022 1:30 pm

I have a JC-40 assuming they are put together the same there should be bolts on the top you can remove and then slide the metal tray with the circuit in it out. Disconnect the speaker or it won't come out.

Don Downes · Post by **Don Downes** » 7 Feb 2022 2:28 pm

Thanks, John. I found the 2 screws on the top of the cabinet. I also found 2 "screw holes" on the right side of the cabinet. I say holes because there an no screws holding the amp to the cabinet.

I laid the amp on its back and douched the knobs and switches with DeoxIt. I'm hopeful that gravity and capillary action will get it to the wiper. Let's say I'm cautiously optimistic that it will work.

I appreciate your help. This has been driving me crazy

Larry Dering · Post by **Larry Dering** » 8 Feb 2022 5:27 am

Don, StewMac made a slick tool for spraying pots that can be duplicated by slipping a small piece of tubing over the pot shaft and spraying cleaner in the hose. Good idea to lay the amp on its back. I haven't tried this method because it's best to have access to the pot. Make sure you give it time to dry before firing it up. Those pots need to be rotated while cleaning to do any good. I'm sure the amp chassis can be removed. I haven't personally worked on Roland amps because they are rare in our area.

Larry Dering · Post by **Larry Dering** » 8 Feb 2022 5:33 am

As for removing the chassis, you first need to remove the upper back panel. Then the head will be held into place with screws on the top and on the sides of the amp. These usually are black phillips head screws, with a head size of about 3/8". Be sure the amp is unplugged from the wall outlet.

Before you remove the screws, you will need to loosen the speaker wiring bundle. Normally the wire is bundled together and twist tied to the baffle board.
I found this on the Music Electronics Forum. It's an 80s JC 120. Should be similar.

Stephen Cowell · Post by **Stephen Cowell** » 8 Feb 2022 11:26 am

Don Downes wrote:Thanks, John. I found the 2 screws on the top of the cabinet. I also found 2 "screw holes" on the right side of the cabinet. I say holes because there an no screws holding the amp to the cabinet.

I laid the amp on its back and douched the knobs and switches with DeoxIt. I'm hopeful that gravity and capillary action will get it to the wiper. Let's say I'm cautiously optimistic that it will work.

I appreciate your help. This has been driving me crazy

When you do that, you should run the pot back and forth several times... helps with the de-oxiting!

ajm · Post by **ajm** » 9 Feb 2022 8:09 am

Just so you know, unless there was a different version of this amp along the way, the chorus effect is only on one speaker.
The other speaker remains dry at all times.

Don Downes · Post by **Don Downes** » 22 Feb 2022 2:09 pm

Just an update on this project.

I got this guy up and running last night. I replaced the reverb tank with an identical unit (Accutronics, 8AB2D1A, Medium Decay, 3-Spring). I was able to check the old unit and did find that the output transducer was bad. I also found that the input/outputs to the tank were reversed. FIXED.

I also found the there was a wire connecting the reverb circuit to the chorus circuit had degraded and was not connected. FIXED.

The reverb and chorus are now working, as are both speakers. Several things that are missing are the HUMMM and the HISSSSS!!! Unbelievably quiet for a JC-120.

I've plugged in my Strat and Tele just for a test drive. It sounds like a dream. The Strat, with the SD pups, and the volume at about 6 was enough to send my poor dog to hide under the bed. This amp is badass loud. I was really surprised. I've never seen anyone get feedback from a Tele. Now I have

I appreciate all of the help and suggestions and information I've gotten from you all.

Per Berner · Post by **Per Berner** » 22 Feb 2022 11:51 pm

My old 80s JC-60 was unbelievably scratchy, every single pot was extremely noisy. I tried removing all screws I could find, but the chassis wouldn't budge. So I just laid the amp on its back, loosened the pots a bit, sprayed with electronics cleaner and rotated the pots back and forth several times. And it worked! Dead quiet, good as new, ready to sell – got $400 for it.