Roland Jazz Chorus JC-120

[goldtop5]

I have a Roland Jazz Chorus JC-120 that doesn't pass signal on channel 1 and 2. However, if on channel 2, when the chorus setting is enabled, it passes signal. When the effects control is switched off or to the vibrato setting, the signal shuts down. Did anyone come across a problem like this?

[52 Bill]

I have a Roland Jazz Chorus JC-120 that doesn't pass signal on channel 1 and 2. However, if on channel 2, when the chorus setting is enabled, it passes signal. When the effects control is switched off or to the vibrato setting, the signal shuts down. Did anyone come across a problem like this?

There are 6-7 different versions of that amp, try and find the schematic for the version that you have.

The only thing that happens when the chorus is turned on, is that the signal is sent in stereo to the two power amps. My guess is that when the signal is split by the chorus circuit it actually gets to at least one of the power amps.

I think that some models had mono FX loops some had stereo loops and some had both. Clean and check or bypass all of them and see what happens.

[Jazz P Bass]

Not sure if this applies but the JC series of amps utilizes a normally closed contact on the extension speaker jack.
They are known for going open.

[goldtop5]

The version I have is a JC-120B. Hope this helps

[diydidi]

As Jazz mentioned. Check the rear jack sockets.That would be a good first step.

[Jazz P Bass]

JC 120 'B' does not help as the different schematics do not use a letter annotation.

You have to determine if you have a preamp or a power amp problem.

Line Out to another amp: if that works for both channels, then the preamps are good.

Main In: this jack accesses each independant power amp.
Sending a signal in each separate jack will prove whether or not the power amps work.
This test depends on :
1: each Ext Spkr jack must be 'closed'. You can insert a guitar cord & jumper the tip to ring if in doubt.
2: the speakers must work.

[goldtop5]

The version that I have does not have the speaker jacks on the back of the chassis. Only speaker wires. This looks like an older version. The preamp section seems to work good so the primary focus is on the amplifier section. But I don't know if I have the right schematic.

[Jazz P Bass]

Is this the amp that you have?
1996.
http://lib.roland.co.jp/support/en/m.../JC-120_e4.pdf

[goldtop5]

Yes.

[Jazz P Bass]

I do not have that schematic.

Is your amp that different from the schematics that are out there?

Have you done any signal tracing?
That's what it will take.

[Jazz P Bass]

Roland JC120 UT Schematic

I may be wrong but you may have the JC120 UT.
What sent me off ln that direction was the FX Loop Switch. (parallel/serial)

[g1]

I think it may be the same JC-120UT schematic referenced here :
http://music-electronics-forum.com/t28379/#post249180

[Jazz P Bass]

Whew.
Is it me or is that schematic tough to follow without the amp in front on the bench?

[Jazz P Bass]

I have a Roland Jazz Chorus JC-120 that doesn't pass signal on channel 1 and 2. However, if on channel 2, when the chorus setting is enabled, it passes signal. When the effects control is switched off or to the vibrato setting, the signal shuts down. Did anyone come across a problem like this?

On a hunch, I would check the Jfet's that are being used as function switches.

[52 Bill]

Whether you have the correct schematic or not, have you checked all of the standard things to check?

Are both speakers okay? Are there normal signals coming from the two line out jacks? Did you try jumpering the FX loop with a spare patch cord?

From your description, the straight signal is missing from the output and the delayed signal is present whenever the chorus or vibrato is turned on. Listen to the speakers and see if only the right side is making sound.