Roland JC120

[NebulaZero]

hello Folks
first post here and new to the forum.

i just bought myself a old school JC120 for cheap because it wouldn't power up
well i fixed that and now i'm only getting sound from the right speaker and no chorus or vib, it'll work sometimes when i crank it and play my guitar really hard then it works 100% but only sometimes i'm thinking it might be the 2 3300uf 50v caps but not sure??? i've read post here with people having similar problems.
i was wondering if anyone might know what the problem might be, like i said its the old model with the cho/vib toggle switch.


Thx
Dan

[Alex R]

Doesn't immediately sound like a problem with those caps.

A separate preamp and amp runs each speaker output. One preamp has a variable delay - it's the way that 'wet' output interacts with the 'dry' output that gives the effect. You can hear the wobble on the 'wet' output so you probably have lost that one.

Since it's an intermittent problem you have a chance it's just a bad connection somewhere. There are loads of wires coming off around the edge of each board. Run a wooden chopstick around the posts etc and see if you can bring the sound back and thus find the bad joint.

If that doesn't work then we are starting to get into some complexity. There are many versions of these amps, with quite different circuitry, and many of them use obsolete and hard-to-find parts. So have a GOOD look for a bad connection!

[NebulaZero]

yeah i went in there and poked around with a plastic probe while i strummed the guitar and still nothing, all the wires seem to be intact. i'm just going to touch up all the solder connections and check for any broken/braking traces. those big a$$ 3300uf caps kinda scare me is there any danger working around them? is there any way to discharge them? i put some tape under them so they dont touch or short out anything.



thks for your help.
D

[Alex R]

You can discharge the caps with something like a 10W 5K resistor across their terminals. Attach probes to either end and hold across the terminals for 30 secs or so.

If it's not an obvious bad joint... next thing would be to check the output transistors. Can you find equal plus and minus voltages (broadly in the 50vDC area) supplying them? Has a fuse been opened on one side? Are the transistors shorted?

Failing that, try to trace the connections from the preamp boards to the two separate power amps. Which one isn't sending a signal? Then trace backwards...

[52 Bill]

And don't forget to check any break jacks like the FX loop for dirty contacts.

[Alex R]

He said:

...those big a$$ 3300uf caps kinda scare me is there any danger working around them? .
D

...then he went quiet...

[NebulaZero]

sorry about the late reply guys, had to go out of town and didn't have access to a pc or internet well when i got back i dug into the amp deeper and found the depth and speed pots were kinda hacked onto the board and non original so i replaced them with some new 100k pots and the amp seems to be working better it at least works 95% of the time!
whoever replaced the pots before me did a crap hack job and lifted and broke some pads and traces, i was able to fix them pretty good!

i really appreciate all your help! i think the more i use it the better it might work
it sat around as a piece of furniture for like five years!


thx
D