Roland JC-120 recapping?

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kdawg

Well-known member
Joined
Jun 9, 2004
Messages
129
Location
California
Hi all... I have a Roland JC-120 Jazz Chorus amp that I love, but is quite hissy. I have the schematic (Apparently there are about 8 different Revs) and was trying to decide what to replace/upgrade to get the noise floor down.

The schematic is temporarily up at http://www.jnd.com/kdawg/my_jc120.pdf

Noise floor is about the same on both channels, so I thought maybe its power supply related. I put a scope on it a while back though and it didn't seem too bad. I'd love to get better noise floor transistors but I'm not very good at cross-referencing these japanese transistors.

Any help is greatly appreciated - I actually made a list of all the caps and there are 100+. I don't want to spend a fortune on parts but would like to replace key elements to get this thing going again. Thanks

-kdawg
 
Compare with another one to see if you have excessive noise in your unit. If the noise is standard, there's probably little or nothing gained in a recap job.
 
I have one also, an older one, and it has always been hissy to the point where it's too noisy to record with. I like how it sounds but it's almost unusable. If there's a fix I'd love to know about it.
 
My amp building experience (not much...only built one) has shown me that noise performance is (assuming a decent design) first and foremost about layout and sheilding. From there, it's resistors that may dominate noise performance (particularly in a high impedance high gain amp). What are the resistors in the JC120 made of. High value carbon films may be subbed with metal films for practically no cost...but replacing alot of resistors is a pain. Don't forget about pots as resistors...high value volume pots may be a big source of noise. Consider swapping out for conductive plastic ones. I just built an 18W Marshall with metal films, some select carbon comps, conductive plastic pots, careful attention to sheilding high impedance lines, and I have defintitely the lowest noise amp I've ever heard.

Replacing caps doesn't usually improve noise performance...except if caps are really failing. I took a quick look at the schematic, and didn't see many electrolytics in there, so there's probably not much of a cap problem.

Maybe you can bypass some sections, like the distortion (who uses built in distortion of a JC120)...try bypassing the reverb. See if the noise is coming from one particular stage.

Maybe IC's can be replaced with lower noise variants....

Cheers,

Kris
Kris
 
JC120's were always hissy. I remember in the 1980's when they were the hot ticket. They were bright, jangly and hissy. Re-capping won't do a darned thing for it.

Keith
 
It's a steady white-noise-ish hiss, which increases while turning up the volume knobs for each channel. With both volume knobs off, it is acceptable. No hum. I'm pretty sure it's gaining a lot of noise from the preamp sections because I can plug directly into the "MAIN-IN" an don't have the same hiss problem.

-kdawg
 
Gain entails noise in general. But mistakes can be made in the design to make it a lot less than optimal.

Without a schematic there's not much to be said. But for the most part, a better transistor is not going to make that much difference if the majority of the noise is, say, from a poor choice of impedances in a very early stage.

It is true that some resistors have excess noise when current flows, but it is unusual for a resistor alone to be responsible via that mechanism for excess noise---not unheard of, but unusual. And poor impedance choices may mean higher thermal noise, or higher noise due to opamp or transistor current noise developing a noise voltage across them. Otherwise-decent bipolar op amps like LM833s or 5532s will make a lot of noise with high external impedances. If that was the problem and there were a drop-in replacement FET amp with comparable voltage noise then you might get a big improvement, but it is a crapshoot.

Again---gotta see a circuit diagram. It may be obvious.
 
can someone explain how resistors become noisey? Is it they they go out of tolerance?

I feel like this is the problem I am having with my Wurlitzer 200 - its super super noisey, and filled with old resistors that have certainly done some duty. I was considering replacing them with modern ones. Maybe even up the wattage since the power resistors get hot still. (sorry for the thread-jack, but this may help you too!)
 
[quote author="Milkmansound"]can someone explain how resistors become noisey? Is it they they go out of tolerance?
[/quote]

The old carbon resistors can drift pretty dramatically, usually to higher resistances.
 
"Look above... schematic is here."

Thanks.

Well, for openers, the first thing that you hit is at least a 33k +1k resistance, so the input noise voltage spectral density is already no lower than that thermal noise, which is about 24nV/root Hz, or 3.4uV rms in a 20kHz passband. Then there is the FET noise, probably small in comparison, and ditto the thermal of the FET 1k source resistor. The stage gain is limited to not a lot, < 21dB, and then the next thing you hit is a lossy tone control with high impedances and hence high thermal noise--- throwing away some of the preceding gain, contributing more noise. Then a 1 meg volume control, with more thermal noise (worst at about the halfway point) and making the next FET stage noise something to be considered at least.

High noise is not surprising! A total redesign----don't waste your time swapping component types. As the black Lectroid says in Buckaroo Banzai: "A very baad design."

At least from a noise perspective anyway---it may have other sonic virtues.

At least you know that the noise is coming from the stuff ahead of the volume control, so don't bother with the parts after that. The fact that it is hiss tells you it's not power supply decoupling caps etc. gone bad, unless something were oscillating. The latter is unlikely with simple FET stages with only local feedback, as well as stopper R's in series with the gates.
 

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