Roland JC-120 fuse blowing

[Lebowsky]

Hello,

I have a Roland JC-120 on which the fuse blew for some reason. First thing I did without checking anything else, was to replace it. So.. I replace it, turn the amp on.. and get almost shocked huge green lightning comes out of god knows where, and one of the spot under the switch board is fried. So, I don't know if the problem came from the switch board that was defect and caused the fuse to blow again, or if there is a problem elsewhere in the circuitry.

I have explained the problem to friend who's an electronic engineer, and after what I explained, he seemed to say that the transformer that goes between the power source and the rest of the circuitry should prevent the rest of any damage.

Does anyone have the electronics schematics for this amp, or have any suggestions on where to look for them? Google doesn't bring anything up.

And if anyone has a an idea of what might be the problem, feel free to give some insight

I'm a noob in electronics, and my friend isn't really a specialist in amps, which is why I'm looking for the schematics for him. Can't really afford to have it repaired atm

Thanks!

[mykey]

Hello,

I have a Roland JC-120 on which the fuse blew for some reason. First thing I did without checking anything else, was to replace it. So.. I replace it, turn the amp on.. and get almost shocked huge green lightning comes out of god knows where, and one of the spot under the switch board is fried.
I have explained the problem to friend who's an electronic engineer, and after what I explained, he seemed to say that the transformer that goes between the power source and the rest of the circuitry should prevent the rest of any damage.

Thanks!

you have a blown rectifier in the power supply, or blown output transistors,
and the transformer does not protect any of it.
in many cases it was just the bridge rectifier, which is easy enough to replace.
the schematic can be ordered from Roland corporation. in solid state amps,
the fuse blows after the damage is done and there is no fuse fast enough to protect the transistors or rectifier from shorting out.
To order parts or product literature, please contact Customer Service at 323-890-3700 ext. 5305.
but don't hold your breath, Roland is rather famous for putting customers on hold for very long
periods of time, while you are forced to listen to recorded advertisements.

[Mark Black]

I might add that you should have the serial number available when you order the service docs as there were at least two revisions of the JC-120 circuit which had quite a few differences...

[Lebowsky]

Thanks for the replies guys.

I found 2 different online schematics after all. Gotta see if it fits my amp. I'm in Europe, so calling in the US isn't and be held on voice messages isn't really an option

I'll forward what you told me to my friend mykey, thank you

[mykey]

Thanks for the replies guys.

I found 2 different online schematics after all. Gotta see if it fits my amp. I'm in Europe, so calling in the US isn't and be held on voice messages isn't really an option

I'll forward what you told me to my friend mykey, thank you

sorry Lebowsky, did not realize you were in Europe.
glad to help if I can.

[Lebowsky]

no problem mikey

damn, lately I've been wishing I had studied electronics instead of stupid computer science

[mykey]

no problem mikey

damn, lately I've been wishing I had studied electronics instead of stupid computer science

you might be surprised to to learn that even though i have repaired
hundreds of guitar amplifiers over 34 years, i never studied electronics really.
i just started one day and learned as i did it, now Ive become pretty good at it.
and i learned computers the same way...
i did however have some electricians training and safety training, but if i did it so could you.

[Enzo]

Not to state the obvious, but have you inspected the mains wiring to see if any wires have come free. Usually when components are shorted on the secondary side - rectifiers and outputs for example - large green sparks don't fly, the fuse simply blows. Loose mains wiring though will often provide green sparks from evaporating copper.

[Lebowsky]

you might be surprised to to learn that even though i have repaired
hundreds of guitar amplifiers over 34 years, i never studied electronics really.
i just started one day and learned as i did it, now Ive become pretty good at it.
and i learned computers the same way...
i did however have some electricians training and safety training, but if i did it so could you.

yeah, I actually started reading a bit about the basics last night, thanks for the advice

[Lebowsky]

Not to state the obvious, but have you inspected the mains wiring to see if any wires have come free. Usually when components are shorted on the secondary side - rectifiers and outputs for example - large green sparks don't fly, the fuse simply blows. Loose mains wiring though will often provide green sparks from evaporating copper.

It didn't seem so, but I'll check it this out as well, thanks!

[Enzo]

Are there any burn marks or soot marks from this flashing inside the chassis?

[Lebowsky]

Are there any burn marks or soot marks from this flashing inside the chassis?

well I don't have the amp right here, and I didn't unscrew all of the boards, only the switch board

one of the "weding spots" (?, sorry my technical english is a bit lacking) under it is burnt. I googled to find a similar image, looks a bit like this: http://images.tomshardware.com/2002/...ing/brand1.jpg

what I don't know, is if this was already the case before changing the fuse, or if it burnt after I replaced the fuse.

if I have the time, I will try to check under all of the boards tonight

[Enzo]

Well, that photo is an excellent example of a burnt board. if yours looked like that, I'd say you found your scorch mark. WHY it burnt is the next question.

[Joe Meslin]

hello. a bit late on this reply but i have been having similar trouble with my jc 120 minus the green sparks. theres a pop and a spark right where the LED is (which is where the fuse is located). is it possible this could be happening due to a non dedicated outlet?

[Xenrelic]

Hello,

I have a Roland JC-120 on which the fuse blew for some reason. First thing I did without checking anything else, was to replace it. So.. I replace it, turn the amp on.. and get almost shocked huge green lightning comes out of god knows where, and one of the spot under the switch board is fried. So, I don't know if the problem came from the switch board that was defect and caused the fuse to blow again, or if there is a problem elsewhere in the circuitry.

I have explained the problem to friend who's an electronic engineer, and after what I explained, he seemed to say that the transformer that goes between the power source and the rest of the circuitry should prevent the rest of any damage.

Does anyone have the electronics schematics for this amp, or have any suggestions on where to look for them? Google doesn't bring anything up.

And if anyone has a an idea of what might be the problem, feel free to give some insight

I'm a noob in electronics, and my friend isn't really a specialist in amps, which is why I'm looking for the schematics for him. Can't really afford to have it repaired atm

Thanks!

Most likely the output transistors on one side have failed. I get a lot of these that have a pair of failed transistors on the right side.

Unplug the amp and test the rectifier first with your multimeter on diode setting and then check the pairs of transistors to make sure they are not shorted.