Roland JC-120 mod

[HoodTube]

Btw this mod worked flawlessly. No more lost chorus at low volume 

Thanks again for the help!

Hello folks, I just found this thread. I've had the same problem with the gating chorus at low volumes for years now on my JC-120.

Will the shorting emitter/collector mod work on my version of the amp? It's this one, on pages 2 & 3 - https://music-electronics-forum.com/filedata/fetch?id=969647

In this case, would it be Q26?

Cheers!

[BJF]

Hi there,

Oh thank you for posting that link. I bought aa JC 120 a couple of years back because it is along with JCM900 the most common backline.
Back in the mid 1980s it was common in Sweden too and at the time I would choose the JC 120 over the Marshalls even though I would play mostly entirely distorted guitar, and I would set controls like this Bright off,Treble fully off, midrange fully off, Bass on max and then use a distortion box like BOSS DS1 and set Tone on that at 9oc and the resulting sound was a lot better than the Marshalls and this was loud on stage so and I think I used Chorus only once at a short passage of a song.
Anyway I bought a JC 120 a couple of years back thinking I know exactly how to set this amps so it will be good...
Firstly the chorus wasn't working and the amps was a bit noisy but I admit it being the first time I would play a JC120 at low volumes but also the EQ did not work the same way. So I took the amp apart to service it  but on searching for schematics it turned out that that there was almost a new model per year with very extensive changes to the circuit.
The model I had bought was not the same as the ones I used on stages.
Wading through schematics it turned out that the model I have had a noise gate for the chorus that turns it off at low volumes!
However besides that there were some components that had havoc in the chorus so I defeated the noise gate and repaired the chorus.
Further I went through amp with freeze spray ( freezing suspected components listening for change in noise) and as a result I changed all Electrolytics in the amp which partly lowered noise and hum but also made amplifier stable.
Thes amps will have idle noise because the volume control sits after first stage

However as I said there was a pile of schematics and not that easy to get the corresponding schematic to the one I had in my workshop.
Now looking at what changed through the years it was very interesting.
Early amps had some Jfets and in particular a dual jet right before the power amp to soften transients when playing really loud.
This is an expensive part and it was dropped a couple of models later
Distortion circuit was more behaved in the early ones but with each iteration it acme more fuzz.
At some point a noise gate was put in to silence the chorus output when input signal was too low- in essence chorus only worked when playing loud enough and this was made to cut down background noise and it says so in the manual.
Q26 is part of the output sensing circuit that is supposed to gate the chorus.
I believe I removed the gate Q26 Q25 and put in a bias trimmer to get the oscillator to trigger
You can see through R146 onwards that these two transistors sense the output of the power amp
Then all you have to to is getting a nice stable square wave from the trigger made with one half of IC5
The schematic  MAR 20 1979 you posted has more J Fets than I have seen in other versions JC120 and also Distortion placed earlier in signal chain and it is also a linear stage being overloaded and a little less linear in the JAN 20 1983 but look at the Tone controls how they change!
I did have a look at the reissue of this decade and it is very different

Also here
https://music-electronics-forum.com/forum/amplification/guitar-amps/vintage-amps/4690-need-schematic-diag-roland-jc120
Scroll to post 2

In the 1979 version you can see the dual J Fet that is a limiter preventing overload of power amp so it distorted just below max input to power amp.

Oh yes I have thought about getting a speaker load for my JC120 to explore the fantastic overload ( or mounting a master after this stage) which can be pout at Main in

Have fun
Bjorn Juhl

[HoodTube]

I believe I removed the gate Q26 Q25 and put in a bias trimmer to get the oscillator to trigger

Hi Bjorn, thank you for the detailed reply. It seems that this gate feature is much more common than I thought.

So when you say the part about putting in a bias trimmer, was that necessary to get the effects section to work properly? Would bridging the emitter and collector legs of Q26 alone not be an effective way of bypassing the gate?

Thanks!

[BJF]

Hi there,

Thank you
It was very interesting to see yet another JC120 varaint

Yes looking at the schematic it would seem to suffice to just short C and E but I remember mounting a trimmer on the square generator to get it to work as I wanted but it was sometime ago so I do not recall exactly why it was needed off the top of my head and I also removed the noisegate components while I had pcb lifted from chassis. If your amp is old it can be a good idea to also replace the electrolytics btw

Have fun