Early JC-120: chorus works, controls don't - help?

[jzr85j]

Hello all
I too am having a chorus related problem with a JC-120,
the vib.works fine but the chorus effect is inop with the toggle switch in chorus mode.This particular JC 120 uses the MN3007 BBD (bucket brigade device) and the MN3101 clock generator .I have been searching the internet for day's in hopes of finding the theory of how the circuit functions,where the circuit picks off the dry signal and where the dry and chorus signals mix.I have the schematic but I'm having a hard time figureing out the signal path.

[drasp]

But anyway, someone should really make a dedicated site about these amps.

Well, I agree 100%. I'd like to help with a project like this, maybe even start the site. Just need to find more info. I've started threads on a number of forums (Haunting Mids, HCFX, here, etc.) to start compiling info & pictures on the JC amps. Thanks for posting the pictures you did - if you have any more, I'd love to see them! I'm also trying to find more gut-shots so if anyone has a JC amp of ANY version, please post up anything you've got!


I feel like a real goober for looking so hard to solve a problem that was actually a feature. . .

[teemuk]

I think the problem is that there are so many JC-series amplifiers and so many revisions of them. Someone should make a fan site with compiled service info, manuals and model/revision information according to serial numbers. Those revisions are, by the way, stated in the form of JC-120-XX where the XX is a letter code that marks the revision. This thing should be found from the badge at the rear. Some of the revisions are very different from one another - and I don't just mean the addition of the FX loop. Most differences you can't see unless you sketch out the circuit or you're shown the circuit diagram.

Now, the basic JC-120 has a fixed chorus, like this:

The dial reads: “VIBRATO-OFF-CHORUS”. But without manual it is indeed quite impossible to realize that the controls affect the vibrato only and that the chorus settings are fixed. In some revisions the selector was a rocker switch instead. Like in this older specimen:


However, the chorus/vibrato thing in JC-120H (the head version) is a lot smarter:

(I think this labelling was also used in the JC-77). There's a “MANUAL-OFF-FIXED” rocker switch but no vibrato feature. ...Or is there? Now, what was originally vibrato mode is now the manual mode. The fixed mode is the ordinary fixed chorus mode. Did the operation change? No, just the labelling but it makes a lot more sense now. Those heads do have some real differences too, like a potentiometer to replace the bright switch and different kinds of jacks at the rear panel.

But anyway, someone should really make a dedicated site about these amps.

[oc disorder]

Hey that would be a good one for April 1 drasp !

When in doubt read the manual...................................

[teemuk]

Manual from 1996:
http://www.nuci.org/store/manuals/Ro...orus%20120.pdf

Quote: * Neither the speed nor depth of the Chorus effect can be changed.

---

I think it's been this way since day one.

[drasp]

At least in some circuit revisions the chorus mode (if I remember right) disables both of the potentiometers (speed and depth) completely and replaces them with a constant resistance value or something. That’s just a basic “feature” of those amps. I believe it’s like that in all of them. The controls are not even supposed to work in chorus mode.

WHAT??!! If this is the case with my amp, then there is NO problem at all. The chorus sounds heavenly! I may have done a poor job of describing the problem in my original post - the only issue with the chorus on my amp is that when engaged the speed & depth controls don't function or change the chorus, it just "is". That said, it's heavenly, deep, lush, etc., I just assumed I should be able to control it in the same way as the vibrato. . . Any ideas on how I can confirm that my amp shouldn't have control over the chorus?

THANK YOU!!!

[teemuk]

What would cause the pots to work correctly with vibrato & not chorus?"

At least in some circuit revisions the chorus mode (if I remember right) disables both of the potentiometers (speed and depth) completely and replaces them with a constant resistance value or something. That’s just a basic “feature” of those amps. I believe it’s like that in all of them. The controls are not even supposed to work in chorus mode.

If that remark was just off topic then here's a one final mention about something worth of checking out: In many revisions of the chorus circuit the LFO is switched on with a semiconductor switch, triggered by the output signal of the power amplifier. If the semiconductors switch (a transistor) or the trigger signal’s circuit path from the PA has failed the LFO won’t oscillate and the clock driving the BBD is locked on to a constant frequency.

[drasp]

Unfortunately, the rear-mounted, user accessible pot doesn't have any affect on my problem. I marked it and then ran it through it's full range and while it does seem to have some affect on the chorus, it doesn't make the controls functional. Still, good to know what it does - 'focus' the chorus.

[oc disorder]

Another point, the replacement speaker in my amp (a Pyle) is much louder and 'edgier' than the stock speaker when the vibe/chorus is off, but the two seem much better balanced when the effect is on. Is this just a product of the way the stereo effect works or is this another symptom of my problem?

Although the odd speaker is not ideal its just the way it works.
One side is altered with respect to the other (straight signal)

I would try adjusting the rear control after carfully noteing where the control is.
In the forum Vintage Amps under the heading "Need schematic diag Roland JC120 " vintagekiki posted 3 schematics.
One of them is close to yours "Roland JC120&160' 79.pdf" uses the MN3002.
http://music-electronics-forum.com/s...56289#poststop

Recently I had a "Starforce USA" made in Korea by a fly by night manufacturer
which had a chorus and echo built in..echo worked chorus didn't.
There was a little trim pot which when adjusted brought the chorus into focus.
I'll attach a picture of the section (sorry little large 441kb) and it shows a control VR6 ? (little hard to read) connected to V1 V2 and a 4.7k resistor which your photo shows and its called (drum roll) "Chorus Adjust" !

So I suppose a rear mounted user accessible control in an amp from a pawn shop is a very strong contender !

The control may need cleaning (Caig Fader Lube or similar) and also I noticed
a 50k preset hanging off the MN3002 which I also highlighted which hopefully can be left untouched.

Could it be that simple ?

We wait with bated breath...

[drasp]

I presume you have read this thread ?
http://music-electronics-forum.com/s...ead.php?t=4492
The smalll board connected to the power transformer wiith wire wound resistors and transistors for regulation was riddled with dry joints in one I looked at ages ago.

Flipped this board yesterday & found that it'd been re-touched by someone in the past. None of the joints looked obviously bad, but I re-touched a few for good measure. No change to the chorus control issue I'm having. Anyone got any other ideas or tests I can run? This problem is really starting to frustrate me. I can find posts about this in a few places but no one that has a resolution. One person did say they'd cleaned the switch and it'd started working, but that didn't change anything in my case.

Another point, the replacement speaker in my amp (a Pyle) is much louder and 'edgier' than the stock speaker when the vibe/chorus is off, but the two seem much better balanced when the effect is on. Is this just a product of the way the stereo effect works or is this another symptom of my problem?

[drasp]

I presume you have read this thread ?
http://music-electronics-forum.com/s...ead.php?t=4492

Just spotted some early cmos 4000 series in your photos..these are so cheap and prone to failure
possibly worth changing..snip em out clean up the debris and fit 2 x IC sockets

I had read that thread a few times, but since the OP is complianing of weird noise instead of chorus/vibe I assumed it was a different issue entirely. Will flip the small power board tomorrow to check for problems - thanks for the tip!

Your suggestion about the CMOS 4000 series chips is a bit over my head, but I'll look in to it - do you have a suggestion for a correct replacement?

I'd still REALLY like to know what the single pot accessible by the small hole in the back of the chassis controls. One purple & one green wire going from it to 'v1' and 'v2' on the chorus PCB - shown in my pictures above. Any guesses?

[52 Bill]

"The chorus and reverb now work quite well with one exception = the rate & depth pots have little to no affect on the chorus, which is strange as both pots measure as working and affect the vibrato in a normal, linear fashion. What would cause the pots to work correctly with vibrato & not chorus?"

Is this possible? I haven't worked on one of these for a while, but I thought that the chorus and vibrato controls were controlling the same thing in either position of the switch, the difference being whether or not there was straight signal mixed with the delayed signal or not.

[oc disorder]

I presume you have read this thread ?
http://music-electronics-forum.com/s...ead.php?t=4492
The smalll board connected to the power transformer wiith wire wound resistors and transistors for regulation was riddled with dry joints in one I looked at ages ago.Everything worked except the chorus which I suppose
shows the "chorus area" is the most sensitive area to minor power supply
faults.To flip the board to inspect it have to dissconnect the short wires from the transformer..thats of course if yours has one !
Just spotted some early cmos 4000 series in your photos..these are so cheap and prone to failure
possibly worth changing..snip em out clean up the debris and fit 2 x IC sockets

[drasp]

Thanks. I'll have a chance to check it on Monday & report back.

Well Enzo, just as you said, probing the pins on the switch for continuity was a breeze. The switch has two rows of 8 pins for a total of 16. With the switch in the up or 'vibrato' position, there is continuity between the top pair and the pair immediately below it. In the middle or 'off' position, there is continuity between the top pair and the 3rd pair down. In the lower or 'chorus' position, there is continuity between the top pair and the 4th pair down. This same pattern is repeated with the 5th through 8th pairs. On all pairs, there appears to be essentially no resistance. I assume that this describes the function of a 'good' or working switch. Just to be sure, I re-touched the solder joints on the switch, but they already looked good under a magnifier. Where should I look next?

[drasp]

Never think up reasons not to check something

Thanks. I'll have a chance to check it on Monday & report back.