Roland JC-120 - defect chorus

[Bombarde]

I have one last Question:

I own the older JC-120 without an Effect loop.

Can I put my Zoom GFX-707 Effect into the main-ins on the rear panel?

Or is the Zoom GFX-707 output signal to strong for the main-ins?

So, is it possible?

[Mars Amp Repair]

Hello Bombarde,
You're on the right path with the signal level thoughts. Most 'Amp in' signals are at the very least 'line level' & not guitar input level inputs. if it were a power amp, the level would be even higher circa 1v p-p.

You can't hurt anything by trying, so that is probably the best test here. glen

[Bombarde]

Thanks for your reply!

I will try it later.
If it will not work, I `ll try to find a Multieffect that has a line out...

[Brystkassen]

Hello,
I have the same problem with my JC-120. Have you found a solution?

[AtomicMassUnit]

Bump for this problem. Same as the OP.

My friend has a very early JC120 ('79 or '80) that is doing the same thing. The chorus works, but when you let a chord or note ring out, there is a whoosh type noise and seems to get a little louder, then the right side stops abruptly. It's not noticeable until you let the sound ring out.

Also, if the amp volume is louder than 3-4, everything seems normal. This only occurs with the volume relatively low. Is this just a design flaw of the very early ones?

I have tried the chorus adjustment on the back, and the trim pot on the effects board, neither one will stop the problem. The speakers are phased correctly afaik, the original solder joints have never been touched.

I have a scope and signal generator, and can dig in to it. I'd love to solve it for him.

[leroy1]

ARG!! I had one of these on my bench last week. The complaint was that the speed and depth controls did not effect the chorus. SIGH. Great amp though.

[DrGonz78]

I only have half of this amp>>> JC-60. Not a true chorus output on the JC60 (only one speaker), but the vibrato sounds great. Just like that complaint that leroy1 was given

the speed and depth controls did not effect the chorus

.
"IS" the same complaint I have after bringing this amp back to life. That will be my next conquest and calling roland to get the right schematic for my exact model. I would bet that some of the germanium transistors might need to be looked at closely.

[oc disorder]

I had one here recently an early model JC80 with one 15" speaker.
The vibrato worked fine but the chorus sounded as though it was not moving ie "stationary chorus syndrome " !!!
{imagine like a waa waa half cocked}.
It had the MN3002 & CA1458.
Turned out the CA1458 was faulty.
There seemed to be some sorta slight change at the end of the note so I thought the oscillator was working but it was the noise gate shutting , - again an early version with an LDR and LED.

I noticed the Boss CE1 (the di-cast footpedal version) chorus is very similar but with a different noise gate circuit.

Incidentally it seemed to have more gain overall than the JC120s I could remember.


'79 cct Ignore the red highlighted pre-sets...

[Mark Black]

the speed and depth controls did not effect the chorus

I believe that is normal for the JC amps - chorus speed & depth are preset, and only the vibrato gets to use those controls.

Check out this thread for more info:
http://music-electronics-forum.com/t10147/

[arizonajill]

For a novice can you explain how to properly phase the speakers? I took my JC 120 apart to clean the pots and when I put it back together the Chorus/Vibe didn't work. Is it just a matter of having both speakers hooked up properly or is there more to it?

[Humphrey85]

I have the same sweeping sound the effect of chorus is there but the sweeping irritating sound...

[nevetslab]

For a novice can you explain how to properly phase the speakers? I took my JC 120 apart to clean the pots and when I put it back together the Chorus/Vibe didn't work. Is it just a matter of having both speakers hooked up properly or is there more to it?

The Vibrato should work even if the two speakers are out of phase. I think even the Chorus would, though I've never intentionally swapped one to check. I can't recall now if there is a small (+) marking on the speaker frame by the terminals or not. You can identify polarity by using a battery to connect to the speaker. The forward cone motion will occur when the battery (+) is connected to the speaker's (+) terminal. If no marking, then you can mark it accordingly. If memory serves, the four speaker wires are all different colors. Blk, Red, Brn & Org, I think. BLK and BRN should be common to the amplifier's ground (check with ohmmeter). RED I think is the Left Spkr, ORG is the right spkr.....checking my schematics to confirm that...... No, it's not shown on the schematic.

But, if after reassembling the amp and the Effects Module isn't working any more, then maybe it was mis-connected? Though I think each of the connectors are a different pin count, which would NOT allow you to mis-connect it. If that's the case, and still no Chorus/Vibe functions, then that module needs to be checked. Usually it's IC8 (4558 dual op amp).

[nevetslab]

I have the same sweeping sound the effect of chorus is there but the sweeping irritating sound...

Welcome to the Forum. Not sure what you mean "but the sweeping irritating sound...."

[nosaj]

Welcome to the Forum. Not sure what you mean "but the sweeping irritating sound...."

Like maybe one of the speakers has a rubbing voice coil?

nosaj

[Leo_Gnardo]

I think even the Chorus would, though I've never intentionally swapped one to check.

Be assured, the chorus sounds just fine with the speaker polarities reversed. When the chorus/vib is off, that's when trouble is apparent. Similar with Crate, Fender & other brand amps that feature this kind of "stereo" chorus.