JC-120 chorus fix, anyone?

[HoodTube]

Hello folks. Another amp question from me, my apologies.

I've got an old JC-120 where the chorus section has been out of action for a couple of years now. I can't remember exactly, but I think the reason it broke is because I plugged something into somewhere in the back that I shouldn't have and it blew. I think it was a result of sending a cable from the line out into one of the main ins, maybe. I don't particularly want to try it again to find out.

Anyway, I'm on a bit of an attempted repair spree at the moment and I'm wondering if anyone has encountered this, or knows roughly why this mistake would make the chorus non-functional? I think I've ruled out the speakers as being a cause, as they both work separately.

The chorus/vibrato rocker switch makes a pop as normal when switched around the on and off positions, but there's just no effected signal coming through. Both amp channels seem to work fine otherwise, just getting no effects. Reverb works but has a bit of a hum, not sure if that's related.

I've had it in a repair shop in the past for this exact same problem believe it or not, and I think the guy mentioned changing a transistor. But which one might it be?

This is the documentation I found for this particular model - http://www.tangible-technology.com/schematics/Roland/JC-120_3RD.pdf

Any advice would be great, thanks!

Edit - and just noticed these scorch marks on the bottom right here, guessing this could be a clue to something - https://imgur.com/a/yFVqNBd

[Govmnt_Lacky]

If the Vibe is working properly but, the Chorus is not......

Best guess would be (in order) IC3, Q46, or Q47 on the Effects board.

Good Luck! 

EDIT: If you have an Oscope, you could set to Chorus mode and scope the clock pulses at the BBD (IC1) Pins 2 and 12 to see if it is getting proper clock pulses as indicated in the document you attached.

[HoodTube]

Hey, nah the vibe doesn't work either. I should've been more specific with that.

I'll have a look over those three things you mentioned anyhow. I don't have a scope, is there a way to test the chorus chips with a multimeter?

[Govmnt_Lacky]

Hey, nah the vibe doesn't work either. I should've been more specific with that.

I'll have a look over those three things you mentioned anyhow. I don't have a scope, is there a way to test the chorus chips with a multimeter?

Ugg....

If neither is working then that opens the door to a LOT of other possibilities unfortunately.

*** WARNING... TESTING INSIDE OF A LIVE AMPLIFIER IS VERY DANGEROUS DUE TO HIGH VOLTAGES. PROCEED WITH CAUTION. STANDARD ADVISE IS TO HAVE A PRO LOOK AT IT ***

You could do some quick and dirty testing with the multimeter though. Set the amp to Chorus mode and check voltage (DC voltage) at all of the pins on IC1 of the effects board (BE VERY CAREFUL AND MAKE SURE YOUR METER IS PROPERLY AND SECURELY GROUNDED. ALSO MAKE SURE YOU DO NOT TOUCH THE RED PROBE TO ANYTHING OTHER THAN THE PIN BEING TESTED) List them on here with their respective Pin numbers for further analysis.

[HoodTube]

I've actually just stumbled upon a new discovery. One of the speakers doesn't seem to be working properly after all.

With the right speaker (looking from the front of the amp) disconnected, I only get sound output with the effects switch in the OFF position. When I switch to the Chorus or Vib, it's near silence. There's a tiny, tiny bit of output coming through, but something isn't right there.

Any thoughts on that? Thanks for your help so far, by the way!

[teemuk]

There are approximately at least 12 different JC-120 versions with widely varying vib/trem circuits. What applies to one may not apply to others. Would help if you point us to correct service documents...

Im general, the signal path of one of the stereo channels switches to sidechain consisting of series of stages with filters and BBD when either effect, vib or trem, is toggled. No. Reverb is not related. Trace the audio signal through that path. Where it dissappears or gets attenuated to oblivion? Then find out why.

[HoodTube]

Hey, cheers for the reply! This is the correct service manual for my version - http://www.tangible-technology.com/schematics/Roland/JC-120_3RD.pdf

Also noticed that when I turn the volume all the way up with the chorus engaged, there is a slight guitar signal coming through, but it sounds fuzzy and weird, not chorus/vib effect. Would this likely suggest a problem with the power amp on that side?

I'll try a few other things later today. I'm fairly inexperienced with this stuff, truth be told, but trying to learn and gather info along the way. I can see where the orange speaker wire connects to the PCB, and that's the speaker which cuts out when the chorus is engaged. So the fault is somewhere before this? Should I use an audio probe connected to an external amp and have a poke around to see where it starts outputting?

Cheers.

[anotherjim]

I don't know how Roland implemented the chorus/vibe selection, and I couldn't find all 3 poles of SW2 in the schematic!
As it's a 2 channel amp, the chorus could be direct clean to one amp and vibe to the other. Mixed in the air, it would be a chorus effect, while vibe could be vibe only fed to both channels.
If it's very old, I might suspect the switch first as it's a moving part and subject to wear & tear.
You could have a secondary problem with a speaker voice coil with intermittent contact between the terminals and the voice coil which is usually the flexible copper braid falling apart.

[HoodTube]

Hello, cheers for the suggestions! I don't think it's the switch, as that was recently replaced before the first time this happened (around 2017).

Plus the problem started immediately after I plugged the line out into one of the main ins (I'm sure that's what happened anyway). I think I was trying it out as an effects loop or something, I had read a suggestion somewhere that turned out to be disastrous.

I'm just wondering if that could've blown something in the power amp on that side. Would that be easy to diagnose?

[anotherjim]

Ah! If you use any fx loop sockets that haven't been used in a while, the built-in switch contacts that change the routing can prove to be tarnished and not make contact when the plugs are removed. They should all get a good dose of contact cleaner.

[teemuk]

There are at least FOUR schematics in the document, each with different kind of switching setup for vib/trem. NONE of the schematics portray versions with "Main In" feature. So which schematic again exactly depicts your amp?

All I can say is that you should trace the signal through that sidechain. If the amp produces sound normally with the vib/trem effect off (and as said, the effect is introduced to only one of the channels) then the problem is somewhere in that sidechain.

[HoodTube]

It should be the one on page 15, according to the serial number. Really strange how the main ins aren't anywhere to be seen.

This one - http://www.tangible-technology.com/schematics/Roland/SM_JC-120%204th.pdf has the main ins, and seems to more closely resemble my particular amp, even though the serials don't match.

My amp has no separate effects board, whereas in the 3rd edition notes, it seems to say it's supposed to. So I think this 4th edition might be the right one.

Does it become any more apparent what might've been damaged upon plugging the output into one of the main ins?

[pinkjimiphoton]

I've actually just stumbled upon a new discovery. One of the speakers doesn't seem to be working properly after all.

With the right speaker (looking from the front of the amp) disconnected, I only get sound output with the effects switch in the OFF position. When I switch to the Chorus or Vib, it's near silence. There's a tiny, tiny bit of output coming through, but something isn't right there.

Any thoughts on that? Thanks for your help so far, by the way!

sounds to me more likely you blew one of the power amps, which is why the chorus doesn't work. its unlikely the speaker blew, but very likely the amp did from your description, and that would explain why the chorus isn't working as well.

[teemuk]

But didn't OP mention in the first post that "both channels seem to work fine otherwise, just getting no effects"...

So... do both of the stereo channels work and amplify the input signal, except if you toggle the vib/trem (at which point only one channel remains operational)? If yes, then there's intermittency in the vib/trem sidechain.

If one channel is permantly off then the problem is something else. Most amps should be rather tolerant to mild "user errors" (i.e. JC-120 shouldn't fail from plugging generic signal input to "mains in" ..but would likely if one plugs in power amp's output or ..gulp... a mains coord). And I am pretty sure that if one power amp is blown you would know it from smoke, loud hum, and power transformer squeling loudly. Good troubleshooting practice with faulty amps is to power the amp using a current limiter, though. I suppose you are not doing so?

[HoodTube]

But didn't OP mention in the first post that "both channels seem to work fine otherwise, just getting no effects"...

So... do both of the stereo channels work and amplify the input signal, except if you toggle the vib/trem (at which point only one channel remains operational)? If yes, then there's intermittency in the vib/trem sidechain.

If one channel is permantly off then the problem is something else. Most amps should be rather tolerant to mild "user errors" (i.e. JC-120 shouldn't fail from plugging generic signal input to "mains in" ..but would likely if one plugs in power amp's output or ..gulp... a mains coord). And I am pretty sure that if one power amp is blown you would know it from smoke, loud hum, and power transformer squeling loudly. Good troubleshooting practice with faulty amps is to power the amp using a current limiter, though. I suppose you are not doing so?

Ah, that was my error, I'm sure I was getting sound from both speakers when the effects switch was set to "OFF", then one side cuts out when engaging the effects switch. Perhaps I could've worded it better than simply saying "both channels work fine".

I think I may have identified the transistors that were replaced the last time this same problem happened, but I don't know if it would be as simple as just changing them, or if there are other factors that might be affected too.

I've booked it in for a repair now anyway, all of this is a bit beyond me at this moment in time. Still welcome any suggestions though, as I'd like to be able to know what to look for in future.

[kaycee]

Whilst it is nice to fix things, amps can be dangerous things, so putting it in to repair by a professional is a good course of action I think.

[pinkjimiphoton]

its not all that different from what we already do. chase down voltages, follow audio path, probe it, where it disappears, BOOM!
these aren't particularly high voltages inside... enough where ya still need to show ample respect... but its not like working on an svt!
you got the service manual, follow it thru.

one hint... often on amps like this, there's switches in the jacks, preamp outs, power amp ins, fx loops etc... make sure those are clean. one dirty switching jack could kill one side of the output as well.