Roland JC-120H Hiss

[Mongo]

Hi all,

I just picked up a JC-120H and notice a constant hiss with the volume all the way off. It increases if I turn up the volume and/or if I turn up the distortion. This is with no input. The hiss sounds like the ocean. It's before the chorus because the chorus effects it.

Is it possible there is something simple that has failed?

I think this is the schematic for it.
http://www.schematicheaven.com/newam...azz_chorus.pdf

Thanks,

Don

[darrellcheng]

check reverb section, tank and cables

[Mongo]

check reverb section, tank and cables

I'll check for connections,etc in the reverb circuit.

If this helps, the reverb does work very well and the noise is consistent even if the reverb is completely off.

[oc disorder]

These amps do have a certain amount of hiss.
Is it noticable when playing through it ?
It makes sense to me that the fault is before the volume control
in which case it would not be the reverb although it sounds like there could be a seperate issue with the reverb.
Two things spring to mind.
1. The input jacks are not grounding the input with nothing plugged in.
2. The first IC often a M5218L a sil IC (single in line) 8 pins in a row as opposed to 4 either side.. is noisy. (see attachment)
This chip Roland use a lot and is readily available.
To test for no 1. turn the amp on till you hear the hiss then plug your guitar
(passive) into the input with the volume turned right down (off).
This will simulate the input being grounded. Try both jacks.
If that makes no difference have a look near the input jacks on the board
and see if you can spot a M5218L .. they have used other IC's.
That is what I would replace.
If you are not sure can you post a picture of the board at the input end.
They mention on the diagram alternative chips so it is possible it is some
other type. To replace it the knobs have to be taken off the pot nuts removed and the board has to be flipped over to de-solder the chip and replace it.

[Mongo]

OC,

Thanks for the reply. The hiss is not noticeable when playing through it. The noise floor where the hiss is, while still bothersome, is pretty far below playing volume. As you may know, this amp can seriously blow your hair back!

My initial concern was that it just wasn't dead quiet with the volume down. If I ever want to use it for recording, I'll have no choice but to gate my signal above the hiss level.

I'll pull the chassis back out of the case and take a look at the board and see if any grounding issues as well as verify any change once plugged in. I'll take a look at the IC and post which one I find.

Thanks,

Don

[Mongo]

I just got off the phone with Roland and most of the parts are available to bring this baby back to top shape. The M5218L was listed on their parts list and runs $4.60.

I'm probably going to be ordering a full set of pots in addition to cosmetic stuff like knobs, corners, badge, etc... I'll be sure to run whatever tests we need to before putting in an order to be sure I get any op-amps I may need...