roland jc-77 clipping and distortion on clean

[j_and_m]

Hi. To start, I don't really know much about electronics, but I'd like to learn, and I have friends that know things.

Anyway, I have an old jc-77. When I got it (10ish years ago) it was super clean, but now, when I'm playing with any volume, and especially low end, clipping/distorted scratchy noise comes with the sound.

Is there a common issue that causes this, or could it be one of many things?

Thanks

[Enzo]

More than many...many many.

It could be something simple like a loose guitar pick inside the speaker frame bouncing around. (I mean between cone and frame in the back, like you tossed a pick in there and it got in that space) Or some other small item. It could be electrically simple, like tired filter caps needing replaced. It could be involved, like a failing semiconductor you'd have to track down. Everything in the amp goes towards making the sounds, so it could be anything at this point.

The whole process is to isolate the problem.

[j_and_m]

Enzo excuse my ignorance, but what is the process to isolate the problem? Would that be to use a meter at each step of the circuit and then look for a bad value? And how would I know what the correct values are? I have someone who can probably help me read schematics; is this something I could do considering all this?

Thanks.

[Enzo]

SOmetimes a part changes in value, and sometimes that affects the sound. But usually that isn't so helpful.

Isolate: ANy amp has three parts, a power supply, a preamp, and a power amp. Normally I play into the POWER AMP IN jacks, but yours lacks those. SO connect a cord from LINE OUT to some other amp. Does the noise come out over there?

No system works well without proper power supply, so we always want to check power supplies. You start with a schematic. The low voltage supplies are usually 15vDC both positive and negative. Occasionally an amp uses 17v or 18v or less often 12v, but the two will be equal voltage other than polarity. That is what powers the op amps. Is the voltage reasonably close? And is it clean? Clean? you look at it with a scope to see if the nice smooth DC we expect is now full of wiggles.

Above we listened to the preamp out, if that is clean, and power supply is clean, we look into the power amps. There are two, one for each speaker. SO does the noise happen in both speakers or just one?

If you have an electronics friend willing to help, you should work with him.

[j_and_m]

This is helpful, thank you!