JC120 chorus vibrato kills one speaker

[rockman627]

I have a JC120 ,sn 571243. My problem is when I engage either chorus or vibrato, sound stops coming out of the right speaker. The left speaker has plain guitar sound. I downloaded the JC120_3rd.pdf schematic,off of this forum, but it doesn't exactly match up with what I see. Is there supposed to be literally a separate effects board? I have a circuit board for the normal channel, one for channel 2 and one for the power supply. My resistors only go to R158. I think I need a better schematic.

[DrGonz78]

What is the serial number on the amp? With that it will be easier to track down the correct schematic.

[Enzo]

His post says s/n 571243

Be aware how these chorusing amps work. They are stereo, two channels. One power amp for each speaker. Normally the dry signal is fed to both amps. IN chorus mode, one speaker still carries the dry signal, while the other channel carries the chorus signal ONLY. ie not a blend of dry and chorus as it would be from a pedal say. Since both sides work in normal mode, the power amp and speaker must be OK. SO your preamp is not passing the chorused signal. And that could be a zillion things, you have to narrow it down to isolate the problem.

[g1]

There are a few schematics called '3rd', in the package attached, yours should be the one on pg.15

[DrGonz78]

His post says s/n 571243

Be aware how these chorusing amps work. They are stereo, two channels. One power amp for each speaker. Normally the dry signal is fed to both amps. IN chorus mode, one speaker still carries the dry signal, while the other channel carries the chorus signal ONLY. ie not a blend of dry and chorus as it would be from a pedal say. Since both sides work in normal mode, the power amp and speaker must be OK. SO your preamp is not passing the chorused signal. And that could be a zillion things, you have to narrow it down to isolate the problem.

Whoops I read too fast. Looks like G1 found it. Nice!!

[glebert]

That serial number could be 1977 or 1985. Does it have bright (BRI) switches? If so it is after 1981 and would be a different schematic than above.

[g1]

That serial number could be 1977 or 1985.

The schematic package I posted is a 1988 Roland printing, it does not mention anything about serial # recycling?

[glebert]

The schematic package I posted is a 1988 Roland printing, it does not mention anything about serial # recycling?

It says Feb 1, 1979 on all the pages. They definitely recycled serial numbers as the first two numbers is the month, starting with the founding of the company (or something) so when they hit 99 they started over again.

[g1]

It says Feb 1, 1979 on all the pages. They definitely recycled serial numbers as the first two numbers is the month, starting with the founding of the company (or something) so when they hit 99 they started over again.

Right-o. The schematic on pg.3 of attached package should cover the later version (1985 with 'BRI' switches).

[rockman627]

Yes, I have bright switches.

[rockman627]

This schematic seems to be the closest to what I have. Roland JC120' 82.pdf . I am a little confused as to how you interpret sw2a, sw2b and sw2c. I think sw2a is the direct path, 2b is the chorus path, and 2c is the vibrato path. If so both the chorus and vibrato paths go through IC5 and I think I will start with replacing that.

[rockman627]

I should mention most of my electronics experience is with Fender and Marshall tube amps.

[g1]

I would verify the switch is working correctly before changing any other parts. It could just be a simple bad switch issue.

[glebert]

This schematic seems to be the closest to what I have. [ATTACH]n946317[/ATTACH] . I am a little confused as to how you interpret sw2a, sw2b and sw2c. I think sw2a is the direct path, 2b is the chorus path, and 2c is the vibrato path. If so both the chorus and vibrato paths go through IC5 and I think I will start with replacing that.

I think IC5 is the gain parts of the oscillator for the chorus and trem. Should be able to probe it with a scope or DMM and see if it is running or not.

[rockman627]

Replacing IC5 did nothing. Reviewing the schematic,R79 caught my attention because it connects to the switch, and is involved in what appears to be a distortion mod. A cap goes from the mid wiper of the distortion pot to R79 and jumper W11 is missing. I restore to the schematic specs. Now both speakers work in any switch position, but there still is no vibrato or chorus. I'm slowly getting closer.