JC-120 Bypass Chorus "Gate"

[thuk]

Hi folks. I've got an early 80s JC-120 that gates the Chorus/Vibrato effect at lower volumes.

I've had this problem for years, and finally found an example of someone who had the same issue managing to mod theirs successfully, over at this thread (https://music-electronics-forum.com/...nel-2-behavior)

What I'm wondering is, how could I find the equivalent points of D15 and B3 in my version of the amp? Or if it's even possible to do this mod to mine in a similar way. My version is this one - https://music-electronics-forum.com/...etch?id=969647

Any help or advice would be much appreciated, thanks!

[g1]

Still need more info. There are 2 different schematics in that pdf. Do you have the one on pg.3 or on pg.7 ?

[DrGonz78]

Still need more info. There are 2 different schematics in that pdf. Do you have the one on pg.3 or on pg.7 ?

Well one common point might be D12 on either of the schematics. Looks like same circuit in the older 78 model but it is labeled D15.

[g1]

Well one common point might be D12 on either of the schematics. Looks like same circuit in the older 78 model but it is labeled D15.

Yes, D12 in both corresponds to the D15 from the other thread. Down in the lower right area of schematic off to the right of the Speed control.
For the mod, the anode would get connected through a 10K to the +15V line.

[thuk]

Still need more info. There are 2 different schematics in that pdf. Do you have the one on pg.3 or on pg.7 ?

Hi, thanks for the reply. For my amp, it's the first schematic, the one that applies to both JC-120 and JC-160. And I think the second one is the same as the guy's on the previous thread I linked.

[DrGonz78]

So we know bypassing the circuit is a known solution. However I am a bit curious still how to understand the circuit better. Perhaps there’s a good reason why the circuit is not behaving and it seems to be a common issue. I agree with the idea of bypassing the circuit but how noisy is it without the gate?

[thuk]

So we know bypassing the circuit is a known solution. However I am a bit curious still how to understand the circuit better. Perhaps there’s a good reason why the circuit is not behaving and it seems to be a common issue. I agree with the idea of bypassing the circuit but how noisy is it without the gate?

Yeah the gate seems to be a built in feature in some old versions. I'm not sure how noisy it would be without it, but unless it was something ridiculous, I'd probably put up with it just to be able to use the amp as it was intended at reasonable volumes sometimes.

It seems as though with the chorus activated, the right side speaker only kicks in when there's a note being played of a certain velocity. Anything below that and it stays off, or during sustained notes shuts off early. For example, if I have the volume at 2 or 3, which is more than loud enough for playing at home/at night, the chorus will only kick in with very hard strumming or plucking.

Something else of interest is the part on page 6 of the schematic which says "SENSITIVITY Chorus starts to effect with 100mV rms output at channel 1 speaker".

As an alternative possible mod, would there be any way to change this so that it kicks in at a lower threshold?

[g1]

Something else of interest is the part on page 6 of the schematic which says "SENSITIVITY Chorus starts to effect with 100mV rms output at channel 1 speaker".

As an alternative possible mod, would there be any way to change this so that it kicks in at a lower threshold?

Looks to me like R188 for your version is knocking down the level sensed off the speaker. I would try reducing it from 15K to 10K and see what that does for it.

[thuk]

Looks to me like R188 for your version is knocking down the level sensed off the speaker. I would try reducing it from 15K to 10K and see what that does for it.

Hi, thanks for the suggestion. I don't think mine has R188 though, my amp's the one on page 2. Sorry for the confusion! Do you happen to know what the equivalent would be?

[g1]

Hi, thanks for the suggestion. I don't think mine has R188 though, my amp's the one on page 2.

Sorry, I got it backwards as to which schematic you had. Should have just looked at post #5.

For your version, R146 is the resistor you would want to decrease. Here they use 47K, you could try reducing to 3/4 or half of that. To avoid having to remove the board, you can just tack another 47K across the existing R146. That will make it about 23K, which should give you an idea if you want to go more, or less.

[thuk]

Sorry, I got it backwards as to which schematic you had. Should have just looked at post #5.

For your version, R146 is the resistor you would want to decrease. Here they use 47K, you could try reducing to 3/4 or half of that. To avoid having to remove the board, you can just tack another 47K across the existing R146. That will make it about 23K, which should give you an idea if you want to go more, or less.

Thank you for the suggestion! I'll need to pick up some resistors, then I'll try it out and report back.

[thuk]

Sorry, I got it backwards as to which schematic you had. Should have just looked at post #5.

For your version, R146 is the resistor you would want to decrease. Here they use 47K, you could try reducing to 3/4 or half of that. To avoid having to remove the board, you can just tack another 47K across the existing R146. That will make it about 23K, which should give you an idea if you want to go more, or less.

Hi again. I've tried decreasing R146. I started with one 47k resistor, and there wasn't a huge difference. So I put another one on, and it's definitely better now.

But with the vibrato effect in particular, it's very abrupt and obvious when it cuts off on sustained notes. Is it safe to keep going lower? I'm guessing removing the resistor from the circuit completely isn't advisable, but what's the absolute minimum I could get away with? I'd prefer to be able to have the chorus permanently on when activated, with the gate disabled completely.

Cheers!

[g1]

If you go too low, you end up with a clipper circuit at the amp output.
The original mod is a better way to disable the gate.

[thuk]

If you go too low, you end up with a clipper circuit at the amp output.
The original mod is a better way to disable the gate.

Thanks again for the advice. I'll leave that as it is for now, and try the original mod.

So when you said in your other reply "For the mod, the anode would get connected through a 10K to the +15V line." I'm a little unsure about the "+15V line" part. Would this work? (the green line being a 10K resistor) - https://imgur.com/a/XRXBCx3

Apologies for all my questions and lack of knowledge. To call me a tinkerer would be a bit of a stretch!

[g1]

Would this work? (the green line being a 10K resistor) - https://imgur.com/a/XRXBCx3

Yes, that will work. That end of R144 connects to the +15V line.