Roland JC-120 - defect chorus

[mip59]

I have, unfortunately, bought an old JC-120 in almost mint condition but with a defect chorus/tremolo. The amp itself seems ok when the tremolo/chorus switch is off. When the chours is switched on it works and sounds quite ok BUT only on the right speaker (if looking from the front direction of the amp) and the chorus sweepes with a "SWOOOSSCCHH" sound. When I do not play on the guitar it is silent but as soon as I start playing the chorus starts and go from silent to the "SWOOOSCCCCH" sound according to the speed cycle.
The defect is the same if I switch to the Tremolo.
I have detached the CHORUS/TREMOLO PCB and resoldered it but with no effect. The serial number is: 574613 and I guess it is an early 80s amp?
I am not that skilled in electronics so even if I found some schematics (and there are several versions.. ) I do not know what could be the fault.
I appreciate all ideas and support!
Thanks in advance
Mikael
Sweden

[Enzo]

If you know very little electronics, it will be extremely difficult to lead you through a repair. You will need to know how to identify the parts, take voltage readings, and even use an oscilloscope to look for waveforms, not to mention remove and replace solderded in components. Plus you would likely need to find sources for your parts. I can help a novice, but asking you to look for a little black thing that says U123 next to it and then explaining how to use a meter to test it, turns into a pretty large challenge.

When the chorus is off, both speaker amps rin on the main signal. WHen yu switch on the chorus, then one side continues to play the main signal, while hte other plays the signal that was processed through the chorus. If the chorus is faulty, nothing wil come out that side.

Your serial number will help determine which version you have.

Since both sides work normally, I'd say the problem lies on the effects board. it cuold be a bad MN3002, a transistor or two, a small component, etc.

[mip59]

Thank you for your answer.
Regarding my electronic skills I know perhaps more than I expressed... I have built some guitar effects and I know the difference between resistors, IC:s, transistors etc and I have and can use a multimeter. I can also look at a schematic and find the components on the PCB. However I do not own any oscillocope. I can e-mail you some picutures of the PCB if you are willing to give me any hints - on MY OWN risc of course!
Regards
Mikael

[mip59]

Hi again ENZO
As you mentioned the IC MN3002 I just wanted to mention that yesterday when I was dealing with the PCBs I found that the the two small electrolythic caps around the MN3002 produced a lot of noise/hum when I touched the top of the alu shell. No other cap produced that hum when touching.
Is this just a conincidence or an indication that the fault could be the MN3002 or perhaps the caps?

Regards
Mikael

[oc disorder]

check the rear power supply board

Hi I think you'll find that it is normal for these caps to be sensitive
to the touch .. have a look at the power supply board near the
transformer... you may have to disconnect a wire from the transformer
to flip the board over. One I saw had dry joints around the wire wound ?
big resistors and also signs of dry joints at the regulators...
other than that as Enzo said

If you know very little electronics, it will be extremely difficult to lead you through a repair.

http://www.freeinfosociety.com/electronics/schematics/audio/Roland-JC120-JC160.pdf

Thats the schematic up to certain serial numbers
good luck

[mip59]

Thank you for your support.
When I got the amp the big white 10 W resistor ont the main supply board was hanging completely loose so I started by soldering that into place again.
I will flip the board and check the rest of the solder joints.
I read the schematic you attached. I suppose that is for a newer/older? revision and I see that these are built around the MN3007 and not the MN3002 that is in mine.

Once again.
Thank you
Mikael

[Enzo]

I have at least four versions of the thing, what is your serial number?

[mip59]

Hi
My serial number is: 574613
Do you know aproximate manufaturing date?
Thank you
Mikael

[mip59]

More tips?

Hi
I managed to buy the IC MN3002 from US at a low cost but after soldering it into place I found that it was not the reason of the defect chorus.
I also switched a lot fo the electrolythic capacitors (I had some around) but the fault is still there. It says SWOOOSCH and it stops to chorus before the guitar signal ends.
Anyone having any other idea were the fault could be?
Some of the transistors? The other IC:s?
Any tips appreciatet!
Thanks in advance.
Mikael
Sweden

[Mark Black]

Just a thought - does the left speaker work?

In other words if you disconnect the right speaker do you still have sound at all?

Reason I ask is I think the dry sound always comes out the left speaker and only the right gets the chorus/trem delay (when engaged), which would sound pretty strange without the dry signal mixed in.

Oops - I just realized Enzo kind of pointed this out already. So much for skim-reading...

[mip59]

Jc-120

Thanks for your answer.
Yeas the right speaker works and both the tremolo and the Chorus works but not really as it should.

Could someone with a working JC-120 verify to me that their chorus effect is working as long as there is a guitar signal and do not cut off before the guitar signal ends? Mine cuts off before and there is only the dry signal in the end of the signals...

Thanks
Mikael

[Bombarde]

I have exactly the same problem with my JC-120.

My Serial Number is: 196814

Please, can anybody help? I need my chorus!

[Mars Amp Repair]

Just as an addendum,
the speaker need to be properly phased, or your chorus effect will essentially cancel itself out. I'd guess that's not the issue here....glen

[mip59]

Still broken Chorus

I have exactly the same problem with my JC-120.

My Serial Number is: 196814

Please, can anybody help? I need my chorus!

Hi
I have not been working with my amp for half a year and not been here either.
However - I still have the JC-120 with the faulty Chorus and I would appreciate any help.
Thanks.
Mikael

[Bombarde]

@Mars Amp Repair:

Thank you very much! You are right, the speaker wasn`t properly phased.
I changed the speaker cables and now I have the amazing chorus!

Excuse me, my guitar is waiting for me...

Whooooooohooooooh!