Roland JC160 no chorus

[blindboybenton]

Good morning forum

I have a fault with this JC160. I only repair amps for friends etc and this one im not sure where to start. I've never repaired a amp with an chrorus fault
All the channels and functions are working along with vib. But no chorus
I have removed the pcb and checked over for dj's. Cleaned the s/w
Roland JC120' 82.pdf all i have is the map for the jc120

Is there anyone who can give me any ponters on where to look/start

Bassman1965

[Chuck H]

I'm just pigeoning my way through the schem. I'm not the most savvy guy with SS circuits. since you have all functions other than chorus I'd start by checking for voltage on IC3 and 4. Look to the schem to follow switch function WRT when voltages are present and where. It looks to be that the switch earths voltages. This should give you some info immediately if there is a problem with the switch or anything isn't getting power. If all looks good there you can signal trace through that circuit. My guess, tracing the signal path through the schem, is that since you have all other functions and the chorus and vibrato look to be in series with the same preamp, that it's probably the chorus is passing signal but isn't chorusing rather than some outside power, switch or supply fault. This should be IC3 and IC4 specifically. That's a hell of a schematic. I think I popped a vein in my head.

P.S. The final at the speakers does indicate the arrangement for the 160 model so I think you have the right schem for the amp and minor differences between the 120 and 160 are either shown or implied.

[blindboybenton]

Many thanks Chuck.. Sorry about the vein in ya head.. hope it gets better soon

Basman

[52 Bill]

The fact that the vibrato works is a good sign, as the chorus is basically a mix of vibrato and straight signal.

When in vibrato mode, do the depth and speed controls both work correctly? Even though you have cleaned the C/V switch, try and check the contact resistances.

[Chuck H]

The fact that the vibrato works is a good sign, as the chorus is basically a mix of vibrato and straight signal.

When in vibrato mode, do the depth and speed controls both work correctly? Even though you have cleaned the C/V switch, try and check the contact resistances.

Looking at the schematic it seems (to me) that the chorus is a delay line modulation circuit comprised of IC3 and IC4. The vibrato looks like it's own thing.

[52 Bill]

Do both power amps work? Plug a signal into the left and right main in jacks and see that both sides are working. And make sure that the main in jacks normally closed switches are closing as well.

The extension speaker jacks are wired in series with the main speakers, be sure that those jacks are working correctly as well.

[blindboybenton]

Hi all

I'm back. Been working hard at my day job. That pays for the steak and fries. I had the wrong version of the schematic this one is correct
JC-60_80_120_160 (p.6-9).pdf bottom left handside

As i said in previous post the amp is working 100% on both channels and Vib is working too Just no chorus. Depth and speed controls are working 100%

Ive checked IC9a/b and there is no signal in or out of the chip. I'm about to start to have another look, This is my first fault of this type and a little help with what and how its works would be great

Many thanks

bassman

[Chuck H]

The file didn't work for me. YMMV.

Since everything is working, and as Bill said, the vibe and chorus sort of work together, the problem can be isolated to the delay circuit. It will probably be a parallel circuit and marked on the schematic as the chorus circuit as well as having a "ms" delay value allocated. Check functions and parameters for that chip and you should be able to find the problem. Standard stuff. Is there voltage, bias, output, etc. It will either be a bad chip or a circuit problem related to whatever isn't happening for it.

[52 Bill]

The file didn't work for me. YMMV.

All I got was a pdf with 4 blank pages.

Most of these use one power amp for the straight signal and the other power amp for the delayed signal. So a dirty break jack or speaker jack or incorrect speaker wiring can all cause the loss of the chorus effect.

[mozwell]

try these schematics
As i recall JC120 has 2x 12" speakers, JC160 had 4x 10" speakers, other than that, the amp was the same

Are you getting clock signals on CP1, CP2 at the MN3002 chip
Do all the chips have relevant voltages on their power supply pins
Is the opto resistor "PC" working correctly.
There are two 125mA fuses, are both of these ok

[blindboybenton]

Sorry for delay. I resolder up the effect panel in the end and it worked. Couldnt see any Djs. Glad to get the thing out of my room

Bassman