Hey Roly,
I finally got a chance to sit with the jc-120's. I inserted a guitar cable into the main out jacks and to my amazement, it worked. Both channels are now working. However, it seems to have developed a new problem(I don't think these amps I got from my friends closed studio got much love). It is making a loud hum(B nat. if my guitar is in tune) that gets louder when I turn the volume up. It turns into a full blown motorboating sound when it gets past half volume. It does not happen when I plug into channel 2 and turn up channel 2. But the general low hum remains.
I'm hoping to get a look at the other jc-120 this week. I appreciate all your help.
Scratchcoat

Hey Roly.
I'm hoping injecting a signal can be accomplished by taking a stereo headphone output and running a guitar cable into the mains. Shorting out a jack plug by o.n.o. ? Unless that is a Yoko Ono I have no idea what that refers to(lol). Can you explain that process? Thanks
Scratchcoat

I got a chance to test my 2 jc-120's today. I plugged my guitar directly into the mains out. Amp number 1 played out of both channels, both speakers worked. In the other jc-120 only one mains out played, the other side did not work. I appreciate the help. I'm looking forward to your responses.
Scratchcoat.

Hey Roly,

I went to test the amp today but was thwarted. My amp(s) are mid to late 70's versions. The back panel has a line out and 2 mains in but no effects loop.

Channel 2 does work, just not the vibrato/chorus or the speaker.

Thanks for the detailed explanation of the diagnosis tree. I'm sure I will be able to put that to good use. Particularly the part about not rummaging around!

Scratchcoat

Hey Roly,
Thanks for posting. I won't get a chance to test amp till later. But am stoked at the process.
Just so I can wrap my brain around the concept. If the channel comes to life is it a preamp problem?
If the channel does not play it's a power amp problem? Thanks for your patience, I always wanted to
be able to rummage around in an amp!!!

Hi,
Hoping someone can help me trouble shoot my jc-120. It just was given to me by a friend but only one side works. I switched the speaker terminals to the non-working speaker and the speaker works. Does this mean a channel or output is blown? Any help would be appreciated, I always wanted one of these amps for the chorus/vibrato.