OT: Roland JC-120 pre-amp (?) noise

[kdawg]

I have a Roland JC-120 amp (circa 1982) and it would be great, except for the incredible noise level most JC-120 amps have. Now I can understand noise as you increase the gain, but I'm talking about just having the amp ON with all the volumes off (either channel). Like high gain op-amp hiss, no hum.

Now I've tried the Main In which bypasses the preamp, and voila - it's pretty dang clean. So I know it's gotta be possible to make this thing better.

I got the schematics from Roland and will share them here in hopes of some input on the best way I could drop down the noise level to something more reasonable: http://www.jnd.com/kdawg/my_jc120.pdf

Op Amps on the input stage: 2SK117,2SC2240
Power Amps: 2SA970,2SD845

Thanks
-kdawg

[Mike Burgundy]

I've never heard *excessive* noise in a JC120 - but then again I've only encounterd one or two. All amps hiss when the're turned up, some more than others - but they shouldn't do it *excessively*.
This suggests there's some kind of failure in either the opamps (so a failure, not a design mistake), noisy transistors, preamp power supply (Vref resistors?) or a resistor in the preamp, I think.

With the volumes closed both channels are noisy, when using mains in the amp is quiet. That tells us the fault is introduced between the volume pot an mains in - that bit runs on a different powersupply (30V) than the poweramp!
Since both channels hiss, and the supply is the only thing they share in the suspect area, it's likely to originate there.
Replace  trannies Q11 and Q31, replace R18, R19 and R15 1, see what that does. Using metal film resistors helps, and don't forget to stick in the raight rating (at least 1/2W)
hih