1978 Roland JC-120 Strange Channel 2 Behavior

[caesparza]

Hi,

Recently I acquired an old JC-120 (1978 serial number). Everything is working fine in the amp, but I have some strange behavior on channel 2 when Vibrato or Chorus Effects are selected.

I attached a little vídeo of the problem and the schematic for the version of the amp, basically you can feel the effect in the speaker until the signal gets to certain volume threshold then it kind of cuts/interrupts (actually I think it is shutting off because I don't see any signal on that speaker). Measuring the voltage at the speaker output if the signal is above 200mA there is no problem at all, but once it goes down to a lower value the problem happens.

The only modification I can see in my amp is the MN3002 IC that was replaced at some point for a MN3007.

I didn't find anything related to this sort of problem on the other threads on the forum, so I decided to start a new one.

I'm trying to clarify the whole schematic since I have never worked on a solid state amplifier before. I can't see clearly how the switching for the effects is done.

Any pointers on where to start to check would be appreciated.

Thanks.

[g1]

When I listen to the sound file, it sounds like both speakers quit when it is doing the fault. So like a noise gate.
Is that what is happening, both speakers quit?

[caesparza]

Just the effect speaker g1, the other one keeps working.

[caesparza]

I found this information in another place: it is supposed to be a feature of the amp and this is supposed to be the way of fixing it.


"I had the same "problem" with no chorus @ low volume. I have searched and searched.....until I found out, that it´s definately a feature. The wire coming from the speaker (terminal MO) goes to a rectifier and switches chorus ON from a certain level on. If you don´t like thios feature - simply install a switch that connects the B3 termoinal (+15V) in series with a 10K resistor and a diode to a WIRED OR connection @ D15. Now the chorus will work ANYTIME !"

I would like to understand the operation of that section.

[g1]

Does your amp have 'bright' switches?

[caesparza]

No bright switches. It's an old one, serial 758920.

[teemuk]

...The wire coming from the speaker (terminal MO) goes to a rectifier and switches chorus ON from a certain level on. If you don´t like thios feature - simply install a switch that connects the B3 termoinal (+15V) in series with a 10K resistor and a diode to a WIRED OR connection @ D15. Now the chorus will work ANYTIME !"

I would like to understand the operation of that section.

The circuit is basically a "noise gate" that disables the vibrato/chorus effect at small signal output where all output of the channel is basically just noise from the bucket brigade delay.

The circuit varies from one JC-120 version to another but here's how it works in the attached schematic circuit:

Speaker output feeds to transistor Q40 that acts as a high gain amp. Signal from Q40 saturates transistor Q39, which acts as a switch that establishes ground reference for the chorus LFO oscillator (and by doing so the LFO can operate). Thus the switch is effectively activated by presence of high-enough output signal at the speaker terminal. Wiring D15 to positive supply (as described) permanently biases the Q40 on.

There is also a LDR switch in the circuit. Its LED part is on when Q39 does not provide ground reference. In this condition the resistor part of the LDR "shunts" output signal from the BBD sidechain to ground and effectively mutes the signal.

[caesparza]

Thanks for the explanation teemuk. I'll read it carefully.

So basically according to your message there are 2 sections that controls the interruption of the effect but doing the proposed change around Q40 I can bypass the noise gate. Am I right? What kind of diode could be used to implement the mod?

I wonder why they made the amp that way, the switching is really annoying.

Thanks again.
CE

[teemuk]

Like I said, the circuit is used for muting BBD noise at low signal levels. (The bucket brigade delay line is a high frequency switching device and as such very noisy). What remains in the signal after the muting threshold is largely just noise and hiss. At that point noise and hiss is more dominant than any guitar signal component - not to mention the amp would be noisy and hissing even without any guitar signal. Therefore "noise gating".

If you follow the modification instruction you notice the diode you ask for is already there in the circuit. The modification just ties a particular circuit point to +15VDC with a current limiting resistor, and this permanently saturates the Q39 switch. Without the modification high positive output voltage from amp Q40 saturates the Q39 - so you have triggering at certain threshold of output signal at the speaker out terminal. Basically triggering at the signal level that is not just BBD noise.

Note that there are several very different revisions of JC-120 so before applying any mods make sure your amp actually matches the discussed circuitry.

Oh, and before "bypassing the noise gate" I would make sure the circuit is actually functioning properly and does not - for example- operate at too high threshold.

[caesparza]

teemuk first of all thank you for taking the time to answer my questions, I really appreciate that.

I'm sure about the model of the amp and the corresponding circuit. I get the purpose of the gate. If I understand your point, I can end up with a noisy amp if I bypass the noise gate, Am I right?.

What I can conclude is that the amplifier is operating properly. But I wonder if I could make that gate more transparent.

In regard to your last comment, what would be the correct procedure to determine that threshold? I guess it can be manipulated to lower the point of activation, but I don't have the knowledge to do it.

[g1]

Why don't you try disabling the gate and see how noisy the chorus is at low signal levels. (connect base of transistor thru 10K to 15V)
The threshold you can check by setting up amp for 150mV RMS output level at spkr 1. Chorus should be engaged. Decrease output til chorus cuts out. Should be about 100mV.

[caesparza]

Hi g1,

Will try on the weekend and report the results.

Thanks.

[caesparza]

g1 shouldn't I connect B3 to D15 anode through the resistor?

[g1]

g1 shouldn't I connect B3 to D15 anode through the resistor?

I think that will work too.

[caesparza]

Ok, yesterday I bypassed the gate. Now I have permanent chorus/vibrato effect which is really nice.

This is the funny thing, if you listen to the attached video (sorry about the quality, I didn't have anyone to help me with the phone) as the guitar sound fades out, you start to ear a low speed oscillating noise (in the last seconds), well, with the gate enabled I can ear it too before the speaker shuts off!!

I reckon that this noise could be a problem on a recording situation, but you can process the signal and try to handle it. But not having the thumping/throbbing effect of the speaker shutting on and off is great, apart of the volume level difference when the effect is on an off (I don't know if that is normal, much louder with effect engaged), for me, an big improvement. I think I'll leave the bridge installed, but maybe I will measure the levels as you recommended g1.

For now I have a solution the the problem, so teemuk, g1 many thanks for your guidance.