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jonfowler

Roland JC 120 Unwanted Distortion/Break Up

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I have an early 80's Roland JC-120 and the tone is distorting/breaking up at low volumes with the distortion channel turned completely off. It essentially sounds like the distortion knob is set to 25% at all times. This is very frustrating because the only reason to own a JC-120 is for the totally clean sound. Does anybody know whether this is most likely damaged speakers or could this result from an internal electrical problem? It happens on all the inputs and I have tried adjusting all of the settings and used different cables and guitars. I have searched every gear forum and haven't been able to find any similar threads/comments.

Edited by jonfowler

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You can test the speakers with another amp in order to rule them out. Just connect them to another amp that can produce similar volumes.

you can also detect a warped voice coil by moving the diaphragm manually in and out. If you hear rubbing then the coils overheated and you likely have both an amp and speaker issue with the power transistors going bad and pushing high voltage into the speakers.

 

Its unlikely you'd damage the speaker with a properly functioning amp even if it was running full tilt.

 

My buddy had the same amp and he blew the preamp in it. In his case the bolt that holds the strap in place came loose. The nut must have vibrated loose and it dropped down onto the circuit board below. I diagnosed the problem for him but because the nut landed on a multi legged transistor which was unique to the manufacturer, and because I didn't have a schematic to find a substitute part number (and the cost involved in the repair) he didn't want to spend the money so he sold it off to someone else to repair.

 

If the speakers are good and the problem is the amp there isn't a thing you can do to repair the amp yourself. The amp is fairly complex for even a competent tech. You'll likely need to take it to an authorized repair facility that has access to buying parts directly from Rolland. This is not a generic amp where any tech can scrounge up parts for them. If you're lucky you only wind up spending a couple of hundred getting it fixed. If not you'll have to pay a repair fee.

 

You could pull the chassis and take it in for repair too instead of the entire amp. If the repair is too expensive and you decide against repairing it some shops will take the chassis in place of the diagnostic fees and you'd still have the speakers and cab to sell or modify with another head. Twin cabs with speakers are in high demand and the Rolland cab might fit something like a fender head in there.

 

You could also sell it as is and have someone else repair it. Those would be your decisions to make.

 

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The JC-120 is a stereo amplifier. Can you tell if the unwanted distortion is coming from one or both speakers?

 

One way to test it would be to plug into the effects return (or main in) jacks one at a time. You may need to run your guitar through a Boss pedal (or equivalent) to boost the signal enough to drive the power amp. This will give you some opportunity to isolate the source of the distortion.

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Definitely a internal electrical problem.

JC-120's are practically "bullet proof", rarely any PROBLEMS.

I just dumped a new Boss 100 watt Katana combo for a JC-120.

The katana was just too busy.....it did a dozen things & they sounded just ok.

The JC-120 does 3 things excellent: Clean Tones, Chorus, Reverb.

This JC-120 has never been gigged, it came outta a studio. It's MINT.

Since 1985 this is the 4th JC-120 I have owned, I just keep going back to them......

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Hi John Fowler did you ever work out what the problem was? I have the same issue going on with my Jc120, but not all the time. I’d say it does it 70% of the time. The othe 30 it sounds as it should, nice loud and clean.

 

I took mine into Roland for repair, but sure enough when they plugged it in and played it was clean. When I got it back home, it was clean, turned it off, back on again the next day and the distortion was back.

 

Its T’s annoying, as I was just about to use it to record, but it sound pretty crappy with the unwanted distortion.

 

Did you have any luck fixing this?

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Did they even clean the thing? Oxidation and dust bunnies can cause intermittent issues. With my gear the effects were throttled by humidity.

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I have an 80 's or 90's JC 77 that is doing the same thing. I sprayed contact cleaner on all the pots.  Nothing appears burnt on the circuit board.  Any ideas? 

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I would look at the pre-amp, but specifically? Hard to say.

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