Roland JC90 issue

[STAGEFIX]

Hello everyone, My name is Juan, I'm part of a service center where we repair professional audio equipments. I'm having a pretty big trouble so I came here for help. Let me explain the situation.

We received a Roland Jazz Chorus JC90 for repair. It had noisy potentiometers due to dirt stuck inside. We opened them, cleaned them up properly, the noises were gone, problem solved.
But then the client claimed that the amplifier started doing a noise that supposedly wasn't there when he first brought it in. We received it back and checked, and... there's the slightlest hiss, you literally have to put your ear against the speaker to hear it.
It's very possible that the noise was there from the very beginning but we didn't pay attention since it wasn't declared as a fault and it's minimal. But the point is, the client use this amp in a recording studio and claims that it wasn't there before.

So we started researching, measuring, checking everything... the amplifier is perfect. Newer capacitors and checking different parts of the circuit made no change. And searching about it, all JC have hiss since they have a high gain preamplifier, we even borrowed a JC77 and a JC120 to compare and show the results, but since they weren't JC90, the owner didn't acknowledge the fact.

We turned to Roland, asked them for information on this. They discontinued the product so they no longer have any information on their database. They DELETED it. What the... so no refferences to measure and compare from factory to this amp. Does anyone has a JC90 in good condition, or has measured the noise level of this amp, and could share with us this information?

I'll be waiting good news, thank you very much!

[Jon Snell]

Most JC** amplifiers are inherrantly hissy, always have been due to the type of components used.

[Steve L]

Here is the service manual for one. It has a few voltage measurements in the schematics.

Roland JC-90UT Jazz Chorus (Ed.1, 6-97) Service Manual.pdf

Frankly, I agree with Jon. If all you have is a little bit of hiss in the amp, you should consider it fortunate.
Most Jazz Chorus amps always had a lot of white noise.

Steve

[g1]

I think to compare the hiss in the studio, he would have to guarantee that the microphone placement was exactly the same as before. Many other factors, like microphone type, would also be involved.
Sounds like you are in the unfortunate position of dealing with a difficult customer. Like others here have mentioned, these JC amps are inherently noisy.

[DrGonz78]

Well let me just spray some dirt back in those pots. And put all those old caps back in there. It seems you fixed It up so well that the noise floor has been awoken. You can’t fix something that was designed to function this way.

[x-pro]

Noises or rustling? It's important now.
A customer came to you with a defect of the device - rustling of the regulators during control.
Did he inform you about the noise of the device itself in static mode (regulators are static)?
If not, your conscience is clear, return the device. The thing is that earlier the noise (if it really was) was masked by rustling of regulators. He can't prove it 100%, there were no complaints about the noise during acceptance.
If the customer hasn't serviced the amplifier in a long time, how can he claim that there is no noise in the amplifier? He probably had different quality recording equipment when the amplifier was new. He paid no attention to the natural noise of that amplifier. It is also possible that a number of other devices he was using were already contributing to the noise, such as through the power supply circuitry.

Translated with DeepL.com (free version)

[DrGonz78]

Yeah you would think tinnitus suffering musicians would already have a hiss sound blaring in their head. I sure do.