Roland JC-120 - defect chorus

[Humphrey85]

When I play my guitar there is a sweeping sound,but only when I turned on the chorus, it something like "SWOOOOOOOOOSH"

[Humphrey85]

There is a Swoooooootcing sound when the chorus is engaged, but only when I try to play my instruments.

[nevetslab]

The Chorus has a fixed sweep speed, not really a deep chorus like you might find on various pedal units. It's subtle, but normally works. The Vibrato has both depth and speed adjustment to the effect. Does the Vibrato work? Did this present audible character you're describing suddenly change from working to this state, or has it always been like this? The same dual-IC chip set is involved in both Vibrato and Chorus on the later models. The earlier JC-120 used a somewhat different circuit. I don't have one in the shop at the moment, but do have them in our rental inventory.

Internally in the Roland amp chassis, the effects PCB has the two following IC's that produce the Vibrato/Chorus: MN3007, MN3101. There's a dual op amp, 4558 used with this IC that is involved with the Depth, Speed effects for both the Vibrato and Chorus.

[DrGonz78]

There is a Swoooooootcing sound when the chorus is engaged, but only when I try to play my instruments.

So it sounds like a waterfall of white noise when you play? Playing soft passages on the guitar and it's there on the noise floor very noticeably? Play loud and it gets harder to notice the white noise that is engaged when playing? Stop playing and no more waterfall white noise?

Sounds about right for these amps. They have a sort of gate that cuts out that white noise on the chorus when your not playing. Soft passages makes the white noise very apparent and annoying really. It's just how they designed the chorus circuit on these earlier models.

[Humphrey85]

It is not a normal hiss sound like is suppose to when the chorus is engaged, it's louder than that,and irritating, VIB is working normal as it's supposed to...the OP ic it's new.. At first when I bought the amp effects where not working at all, I sent it for repairs effects where working but the chorus was faulty,when I engaged the chorus it had a whizzing sound like a voice of robot,I sent to the other tech that's when it came back with a Swooootching sound.

[nevetslab]

It is not a normal hiss sound like is suppose to when the chorus is engaged, it's louder than that,and irritating, VIB is working normal as it's supposed to...the OP ic it's new.. At first when I bought the amp effects where not working at all, I sent it for repairs effects where working but the chorus was faulty,when I engaged the chorus it had a whizzing sound like a voice of robot,I sent to the other tech that's when it came back with a Swooootching sound.

You may have the older version of the amp, which uses a different circuit than I had commented on. Take a look at this recent post from 9/4/18 https://music-electronics-forum.com/...ad.php?t=47276

The first post included the earlier circuit schematic, and the solution that cured it. Just a thought.

[Amapola]

Chorus mistake on jc 120...uhm not really

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/

Thanx for it...

In fact, with the manual here...

https://mpe.berklee.edu/documents/st...p%20Manual.pdf

... we can see whom the speed and depth pots are availables only with vibrato... I dont know why but i was convinced that chorus could be controlled by the pots ! Wow so many times passed to try to resolve the mistake lol.

For now i have to resolve the mistake of reverb ^^ ; the unit is ok , i tried it into another amp ; but the wet signal of reverb is poor... I have to put the reverb level full to hear a little reverb !! i found the mystery of the chorus lol ; i think i can find for reverb too ;-)

But if anybody have an idea about the mistake of the reverb ; it's welcome ;-)

[caesparza]

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.

Hi,

Reviving this old thread. I recently got a 78 JC-120. Everything is working fine in the amp (effects wise). But I have the same strange behavior that AtomicMassUnit described on his post. I attached a little vídeo of the problem and the schematic for the version of the amp, basically you can feel the effect in the speaker until the signal gets to certain volume threshold then it kind of cuts/interrupts. The only modification I can see in my amp is the MN3002 that was replaced at some point for a MN3007.

The problem happens with Vibrato or Chorus activated.

Any pointers on where to start to check would be appreciated.

Thanks.

[Roteddy]

I have most of the problems mentioned in this thread i one amp: Chorus threshold (effect disappears as chord rings out), unable to adjust speed and depth in chorus, distortion not working.
I changed all the electrolyte capacitors (which got me rid of some noise) and changed the MN3002 (to a MN3003 as the provider on Ebay said it would be OK), but it didn't do any good. According to my serial number my JC-120 is from February 1985.
Before I go any further trying to solve the problems, I would really like to know: On which models (years and months) was it possible to adjust speed and depth of the chorus effect? I really want that feature...
I would be very grateful for that information.

[g1]

On which models (years and months) was it possible to adjust speed and depth of the chorus effect? I really want that feature...

When you want to adjust speed and depth, you switch it to 'vibrato'. That's it. With the switch in 'chorus' position, it is never adjustable by the front controls, no matter what year.
There is a gate that turns off the chorus as the note decays, that is normal. It should only happen at very quiet levels.
If your chorus cuts out at louder volumes, then that is a problem.
Otherwise, it sounds like the amp may be normal except for the distortion not working. It sounds horrible anyway, so you're not missing much.

[DrGonz78]

^^^^^what G1 said

Not to mention the distortion is typically working as good as it gets. Just happens that as good as it gets makes you think it’s broken. It’s probably the worst distortion channel ever created on such a great amp. I did once have some luck cleaning the distortion pot and it made the signal stronger. That’s about it.