A possible circuit mod solution for Roland Jazz Chorus hiss

[sgosnell]

1.5Mohm is not the same as 150Kohm.

[Surrealmonetskies]

1.5Mohm is not the same as 150Kohm.

That is correct, what are you trying to say exactly? I lowered the gain to make it more bedroom friendly and controllable, since no one reported back with a JC-55, I decided to.
To successfully do the mod on the JC-55 that resistor needs to be changed FROM 1M5 to 180K.

[vintagelove]

If the preamp is the source of the noise, why not just throw an fx loop master volume box in there? They only cost a few bucks to make. Certainly a better idea for inexperienced folks.

[almostjay]

I just acquired an old JC-55 that I love but would love even more minus the hiss.

I have it opened up and I can’t for the life of me even find R46.

I also noticed that it looks to be missing R45 completely:



Is the mod being discussed in this thread applicable to only European models or something?

[jaredmqz]

Hello, first I wanted to thank Jazzfrog for his great work.


I recently acquired a JC-77 and have been servicing it this past week. Replacing electrolytic capacitors (many for improved reliability), cleaning and lubricating potentiometers etc.


I decided to implement the modification Jazzfrog described to improve the noise floor at lower playing levels, but wanted to retain the ability to reach the original maximum volume if required - when playing live for example.


To me the obvious solution was adding a switch to select between two values for R46. A high volume mode with the original 100k value and low volume mode (with reduced hiss). However there were some obstacles to this. I didn’t want to introduce any inadvertent capacitance around R46 in the connecting wires to the switch, so these would need to be kept as short as possible. This would mean drilling a hole for the switch somewhere between the volume and hi-treble control. Not very pretty!


Instead I felt a more elegant solution was to replace the volume pot with a push pull type. I used a Bourns pot, part number: PDB183-GTR01-105A2. Unfortunately this configuration only seems available with a 3/8” thread, requiring the hole to be enlarged slightly. A metric threaded version exists according to the data sheet but I couldn’t find a source online.


The protruding switch body and its contacts meant the pot had to be mounted upside down (contacts facing away from the PCB). Additionally I had to move C24 and C30 to the underside of the board and slightly bend a few components for the pot to fit.


I then simply connected a wire from one side of R46 to the switch, and a 3.3k resistor (a value I had on hand) from the other side of R46 to the switch, such that in one switch position the wire and 3.3k resistor around R46 are connected, forming a parallel resistor network of 3194.58 ohms. In the other position the 3.3k resistor is disconnected reverting R46 to the value of the 100k resistor which is alway in the circuit. The potentiometer connections were connected by three wires to the holes previously occupied by the original volume pot.


Overall this hasn’t seemed to introduce any additional noise into the circuit. There is a very noticeable pop when switching between modes but it doesn’t seem anywhere near loud enough to cause any damage, and it shouldn’t need to be switched very often.


I thought of another approach could solve the pop - replacing the two way power switch with an equivalent three position switch, with the two volume settings in the top and bottom positions, so the amp has to be switched through the off position to change mode. I have seen this arrangement on valve amps such as the orange rocker 32 for half and full power modes. This approach would certainly require a more complex implementation to avoid running the wiring and thus the feedback loop of this gain stage all the way over the mains wiring and back. Perhaps a control voltage switched by the power switch, controlling a relay or solid state switch to modify the resistance near to R46 could work?


Anyway here are some photos of the front panel with the switch ‘in’ (quiet):
Attachment 81012


And ‘out’ (loud):
Attachment 81013




I chose this configuration as it is easier to accidentally push the switch in then to pull it out, and I wanted to avoid accidental large increases in volume!


I’d be happy to post some pictures of the wiring if anyone is interested to try it this way themselves.


Thanks,


CTC

Hey! I know it's been a while since this was posted, but if you have photos of the wiring and/or anything that would help me do this on my own I'd love to see them if possible. Thanks!

[Coconut_Head]

Any update on a mod for the JC-40 to accomplish the same no-hiss goal? I know this mod isn't for JC-40 or JC-120, but maybe someone's come across a mod for them that can remove the hiss.

And I love the sound of my JC-40 more than any other amp I've ever played, and I love the look even more. I find that my JC-40, my 2009 Volvo XC90, and my Minolta Hi-Matics AF and AF2 all have a very similar black retro look that I love.

[lbovay]

This worked for me on a JC22 if you’re willing to sacrifice max loudness.

Roland JC-40 Hiss - Any way to fix this?