Guitar Amp "Always on Ten"

Hey guys,

Going through a batch of stuff from a friend of a friend's estate, I came across a Roland JC-120 Jazz Chorus.  Always wanted one, so I figured i'd check it out.  Anyway, it seems like no matter what the amp settings, both channels, the volume is always on TEN and the volume knobs don't make one bit a difference!  Before I get it on the bench taking away valuable space (it's HUGE AND HEAVY) from other projects, any ideas of possible causes?  Things to check first?  If I get it on the bench, I want to get it taken care of as soon as possible!

-James

edit:  Adding a schematic from an '84 - Mine's an '86 (I THINK), but it can't be TOO different!

> both channels, ...and the volume knobs don't make one bit a difference!

There's only one channel? (Two jacks, same channel.)

I can only find one Volume pot?

> the volume is always on TEN

That's no darn good. It has to go to 11.

It's your Basic Volume Pot. One end gets full signal. The other end gets dead ground. The wiper selects a variable amount between "full" and "dead".

If the ground connection has gone bad, it won't go to dead zero.

(However in *this* circuit it should go to "half", because the 0-1Meg of the pot works against a 1Meg at the next stage.)

Find out why the pot bottom isn't dead-ground. Busted-off wire. Corrupt corroded ground lug. Bad solder joint to the PCB (bad when new because of tarnish, or bad from years of heavy use.) Cracked wafer inside the pot.

That should get you 0-10.

To get to 11, short R42. Now the Hi-Treble knob won't do much at 10 or 11, but if anybody can still *hear* "high treble" when playing up there, you aren't playing loud enough.

PRR said:

There's only one channel? (Two jacks, same channel.)

I can only find one Volume pot?

Click to expand...

Thanks for the reply.  I must confess, I didn't look at that schematic - Just pulled it off the web.  The amp definitely has two channels, though, so (now looking at this schematic) it looks like this schematic is for only the "effects" channel.  (One channel is normal - with high/low inputs and the other is the effects channel - also with high/low inputs)  I'll have to pull it out of my trunk, but i'm 90% sure that BOTH channels exhibited this behavior - which is why I didn't jump to the volume pot being bad.

Here's a picture of the amp, you'll see two separate channels:


It's also of note that this amplifier actually has two separate 60watt amps in it - making for the stereo effects.  At any rate, i'll probably have to get it on the bench anyway.  It's just so damn big!

-James

Love me some JC!  The chorus effect actually happens in the air in front of the amp, instead of electronically.  You think the amp is heavy now?  Load it with JBL E120's like I did.

Anyway.....  JC's are notorious for circuit board problems.  Cold solder joints and such.  The pots can also get very nasty if the amp doesn't get used for a while.  Check the solder joints on the pots, exercise them, and maybe spray some de-oxit in there if you can.  It's totally worth trying to salvage.  These amps may be one trick ponies, but they do that trick very well.

Hello,
You could also controll r46 i think it s a Feedbackresistor. If this get s much higher You loose your negative Feedback, that can result in full Gain. But I am not sure.
Good Luck.
Greetings
Lothar