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Hey there everyone, just got a jc 120 off of craigslist for $200 dollars. However, two of the pots are broken on the reverb and distortion and the chorus effect doesn't work at all.

Other than that the amp sounds great.

Any advice on how to switch pots/where I can find replacements and on fixing this chorus issue?

Thanks a million.

-Nick

a jazz chorus without a chorus? oh well, at least you got it for a killer price, and it's probably one of the best solid state amps ever. best of luck

Pop a footswitch into the rear jacks, see if they're switched off from there for chorus etc.

Failing that I'd just have a tech do the pots and diagnose the 'why don't I have chorus' for the price you got it at.

Thanks for the help, another issue I'm having is finding a replacement pot, I have the model number from the pots which are : M10KB- 637 and M10KC- 637. But I can't find them anywhere, any help? Also my amp gets a rattle when at high volumes, I can't find where it's coming from, any suggestions?

Thanks for the help I really appreciate it.

Call Roland, should be able to order parts from the support department.

Their parts aren't cheap but they aren't ludicrous either.

Rattle could be speaker rub or something loose in the cabinet.

rfurtkamp wrote:Call Roland, should be able to order parts from the support department.

Their parts aren't cheap but they aren't ludicrous either.

Rattle could be speaker rub or something loose in the cabinet.

Dat.

For the rattle, check if any of those bolt things that line the edges are loose.

I'm guessing the vibrato doesn't work either? I'd put my money on a bad connection somewhere. It's basically just a CE-1 hooked up to a toggle, so if you has a pedal building friend, he might be able to figure it out. Otherwise, you're probably gonna want to tech it up.

VauxhallandNick wrote:Thanks for the help, another issue I'm having is finding a replacement pot, I have the model number from the pots which are : M10KB- 637 and M10KC- 637. But I can't find them anywhere, any help? Also my amp gets a rattle when at high volumes, I can't find where it's coming from, any suggestions?
Thanks for the help I really appreciate it.

The Pots are both 10k, the Reverb pot is labeled "B" because it's a linear pot and the Distortion pot labeled "C" because it's LOG pot.
The 6, 3 & 7 relate to the 6th, 3rd and 7th letter of the alphabet or "FCG" which stands for "Friendly Chinese Girls" because that's
who the pots were manufactured by.

You can buy 10k Linear and LOG pots anywhere that sells pots. Whether or not they are made by Chinese girls, friendly or
otherwise shouldn't matter.

Here's the Schematic if you are interested:
http://www.freeinfosociety.com/electronics/schematics/audio/Roland-JC120-JC160.pdf


*I made up the part about the Chinese girls but the rest is true.

MEC wrote:

VauxhallandNick wrote:Thanks for the help, another issue I'm having is finding a replacement pot, I have the model number from the pots which are : M10KB- 637 and M10KC- 637. But I can't find them anywhere, any help? Also my amp gets a rattle when at high volumes, I can't find where it's coming from, any suggestions?
Thanks for the help I really appreciate it.

The Pots are both 10k, the Reverb pot is labeled "B" because it's a linear pot and the Distortion pot labeled "C" because it's LOG pot.
The 6, 3 & 7 relate to the 6th, 3rd and 7th letter of the alphabet or "FCG" which stands for "Friendly Chinese Girls" because that's
who the pots were manufactured by.

You can buy 10k Linear and LOG pots anywhere that sells pots. Whether or not they are made by Chinese girls, friendly or
otherwise shouldn't matter.

Here's the Schematic if you are interested:
http://www.freeinfosociety.com/electronics/schematics/audio/Roland-JC120-JC160.pdf


*I made up the part about the Chinese girls but the rest is true.

C means Reverse log, not log... A is log. B is lin and C is rev log.

Schlatte wrote:

MEC wrote:

VauxhallandNick wrote:Thanks for the help, another issue I'm having is finding a replacement pot, I have the model number from the pots which are : M10KB- 637 and M10KC- 637. But I can't find them anywhere, any help? Also my amp gets a rattle when at high volumes, I can't find where it's coming from, any suggestions?
Thanks for the help I really appreciate it.

The Pots are both 10k, the Reverb pot is labeled "B" because it's a linear pot and the Distortion pot labeled "C" because it's LOG pot.
The 6, 3 & 7 relate to the 6th, 3rd and 7th letter of the alphabet or "FCG" which stands for "Friendly Chinese Girls" because that's
who the pots were manufactured by.

You can buy 10k Linear and LOG pots anywhere that sells pots. Whether or not they are made by Chinese girls, friendly or
otherwise shouldn't matter.

Here's the Schematic if you are interested:
http://www.freeinfosociety.com/electronics/schematics/audio/Roland-JC120-JC160.pdf


*I made up the part about the Chinese girls but the rest is true.

C means Reverse log, not log... A is log. B is lin and C is rev log.

Could be true...it all depends on when and where it was made.

If the shaft is broken but you can still turn it, just take it out and measure it before you order the new one.
Turn the pot half the way and measure the resistance between lug 1 and 2.
If it's close to half of the pot value, it's linear. If it's way less, it's logarithmic and way more it's reverse log.

That said, I doubt the distortion will get much use once the you hear it anyway.

Everyone thanks so much, the vibrato does work, just not the chorus, reverb or distortion.

I'll look into the pots a bit more, i opened the thing up and saw no loose connections anywhere. :/

it may be a bad chip I'm thinking.

Thanks again everyone.

Your amp is worth more if the distortion doesn't work, trust me I've got a 1985 JC-120, and the Distortion is horrible, it sound like a dying battery through a blown speaker, but really high pitched and piercing, with normal eq. But 4200 is a steal

Personally, unless you have a friend that would fix it for free, I probably wouldn't even bother.

I've got a super early production and I wouldn't even give it the distortion the blown speaker credit; sort of want to equate it to a bad DS-1 actually. But who knows, maybe they've improved over the past 30+ years.

Honestly, I like the vibrato more than the chorus. But then I'm a huge Alison's Halo fanboy, so the "thickening" from the JC's chorus just isn't enough for me. If the vibrato works, I'm definitely putting my money on a bad solder joint on the toggle since the chorus and vibrato go through the same circuit. I'm pretty inept with electronics though, so take that with a grain of salt.

And the reverb is nice, but again, it's not especially wet. You've got 2 reverb pedals as it is, yeah? Because I can barely hear the difference between having the amp reverb dimed and off when I've got a reverb pedal set fairly high.

I wouldn't even give the distortion DS-1 credit; hell, it makes a DOD FX55 sound good.

I used the distortion circuit on my JC50 exactly once in a jam situation, to answer a friend's demand why I never used it.

I left it on for about 20m.

No one has ever asked me to switch it on again.

It's nasal but not even good abrasive nasal, like nothing anyone's cloned or bottled because in one fell swoop Roland found the single worst sound imagineable that isn't even good for abrasive synth stuff.

I do like the chorus though, but the 50 being mono has a slightly different flavor.