Jc120 channel 2 not working - extra switch?

[Ems71]

I recently bought a jc120 and the channel 2 input jacks do not work. Along with that the left speak does not work either. Have an ohm meter, basic electronic experience and wondering if Its a fatal attempt and going to cost more time and money than its worth or possible a simple solution.


Mid/late 70s model
An extra switch installed next to the power amp 1 which looked like a potential kill or phase switch for the speaker or an external jack.... Not hooked up so have no idea what the mod was.

Nothing on channel 2 works including both jacks, the effects. But when turning the knobs on channel 2 there is slight his and light pot noise.

Where should I start.

[Jazz P Bass]

The schematic can be found here: http://music-electronics-forum.com/t4505/#post56289

[Ems71]

Thanks for the schematic link. Just wondering If based on my description someone had some ideas.

[Jazz P Bass]

It appears that there are two separate & distinct power amps associated with either input jack.
So you need to narrow down where the signal is failing.
Try going in the Main In jack.
That will bypass the preamp.
If that does not work, then there is a problem with the power amp circuit.
First item to look for is a dc voltage on the speaker out.
If there is a voltage there then the power amp is bad.
Remove the speaker connections on the failed side until you sort the problem out.
At that point you could test the speaker also.

[Ems71]

thanks for the guidance. so voltage to speaker. Both speakers also "move" when turned off and on. Completely deal with all inputs to channel 2.

So, it sounds like amp's work bit something is bad in the chain prior to the power amp. I had asked about this in the first post but there was a switch with 4 wires off it that looked like they went to the speakers. my gues is some way to bi pass the speakers when a cab was plugged in??? It doesn't look hacked up (clean job) but I was wondering if somewhere some one changes the wiring and unless the switch is hooked up it wound work???

For now I start with the ohm meter checking for connection through the input jack's and pot's right?

Thanks

[Jazz P Bass]

I do not have a clue what the switch did or does.
'For now I start with the ohm meter checking for connection through the input jack's and pot's right'
That is not going to get you anywhere.
This is a pretty complicated amp.
The fact that the speakers thump when the amp is turned on means that they are hooked uo.
To what, you will have to find out.
The circuit boards have markings on them that coincide with the schematic, as to which wires go where.
It would be nice to know, from the start, whether or not the amp is wired correctly.
As far as testing the amp, you will need a number of tools & some skill sets.

[Ems71]

Thanks. Going to check the schematic's and if I cant figure it out, off to the repair guy. Anyone know of a good tech in the bay area?