The Jazz Guitar Chord Dictionary
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  1. #26

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    Quote Originally Posted by jazzfrog
    Aaaah... nice, that's a good point, I didn't see this.
    Another project then. I think it could fit at the bottom of the chassis if you make a small casing for it.
    Thanks, I now have one more thing to do on this amp.

    But we are getting off topic.

    Thank you to the several of you who tried and vetted this modification for op-amp based JC circuits.
    I have not had time yet to try anything related to this on FET (ie transistors) based JCs.
    (but I do have a JC-60 on the way)
    Interesting! I was thinking I may open my JC-77 up again and install sockets instead, so I can change the resistor more easily (not sure why I didn't do that right away, instead of wiring in a temporary pot, etc.)

    I have noticed that, while it is still loud enough for my purposes, it now has less overall volume on tap than my orange Roland Cube 60 (however, cranking that little amp causes the cabinet to rumble anyway...and the reverb also sucks on that one. Ha).
    Last edited by p_wats; 03-18-2021 at 10:34 AM.

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  3. #27

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    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):
    A possible circuit mod solution for Roland Jazz Chorus hiss... current explorations-img_0173-jpg


    And ‘out’ (loud):
    A possible circuit mod solution for Roland Jazz Chorus hiss... current explorations-img_0172-jpg




    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

  4. #28

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    The obvious way to prevent the pop would seem to be to turn the amp off before switching, then back on.

  5. #29
    Thank you for this thread. I have 2 JC77s, so I am going to try this mod on one. A small question... what wattage value should the resistor be? I know the resistance is 2.2, but what should the wattage rating be? Thank you so much!! Peace - Don

  6. #30
    Quote Originally Posted by Supergretsch
    Thank you for this thread. I have 2 JC77s, so I am going to try this mod on one. A small question... what wattage value should the resistor be? I know the resistance is 2.2, but what should the wattage rating be? Thank you so much!! Peace - Don
    Good question!
    Your standard 1/4W resistors

  7. #31
    Hi jazzfrog. I have hissy 1977 JC-60 and was wondering if you had the chance to test this mod on yours yet. Thank you

    Quote Originally Posted by jazzfrog

    Thank you to the several of you who tried and vetted this modification for op-amp based JC circuits.
    I have not had time yet to try anything related to this on FET (ie transistors) based JCs.
    (but I do have a JC-60 on the way)

  8. #32
    No sorry... not yet.
    But the amp is on my bench!

  9. #33

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    I really appreciate this thread. I solved the hiss problem in my JC55 by sustaining high frequency hearing loss. For all I know, the hiss may have cured itself.

    But, I still appreciate someone who is capable of this analysis and intrepid enough to try a few different resistors -- and then kind enough to share the info.

    Well done!

  10. #34
    Quote Originally Posted by jazzfrog
    No sorry... not yet.
    But the amp is on my bench!
    No worries. I appreciate the reply and look forward to what you find!

  11. #35

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    Thanks so much for your work on this Jazzfrog!

    Any insight on how I can hunt this down on a JC 120?

  12. #36
    Quote Originally Posted by sweatervest
    Thanks so much for your work on this Jazzfrog!
    You are welcome, this was a fun experiment, yet a never ending one.

    Quote Originally Posted by sweatervest
    Any insight on how I can hunt this down on a JC 120?
    Speaking of never ending. I am now having some time to look into the transistor based JCs, and taking that on as my Winter 2021 project.

    There is only one JC for which the mod above applies, and it's the JC-120H, the head only that Roland released around 1984.
    I have one, I applied this mod and it works great. That head being a 120W head, the loss of gain is actually very well compensated by the power it outputs.

    There are many versions of JCs as the design changed and evolved over the years (both due to part changes and improvements)
    Which one is yours?
    Check here for more info:
    The ultimate JC-120 thread
    Last edited by jazzfrog; 09-12-2021 at 01:05 AM. Reason: typos

  13. #37

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    Quote Originally Posted by jazzfrog
    You are welcome, this was a fun experiment, yet a never ending one.


    Speaking of never ending. I am now having some time to look into the transistor based JCs, and taking that on as my Winter 2021 project.

    There is only one JC for which this mod applies, and it's the JC-120H, the head only that Roland released around 1984.
    I have one, I applied this mod and it works great. That head being a 120W head, the loss of gain is actually very well compensated by the power it outputs.

    There are many versions of JCs as the design changed and evolved over the years (both due to part changes and improvements)
    Which one is yours?
    Check here for more info:
    The ultimate JC-120 thread
    I seem to be a slight oddball, unless Im reading these wrong. My SN starts with 45, but I don't have the chorus trim.


    3rd Edition (1979) - applicable to

    JC-120 S/N 380100 - 415099

    - Gain stages are discrete, combinations of BJTs and FETs
    - Distortion circuit is an overdriven FET+BJT stage permanently in circuit
    - Features a FET-differential -based "pre-limiter" preventing power amp hard clipping
    - Discrete BBD oscillator driver
    - Separate LF oscillators for vibrato and tremolo. Vibrato /w. depth and speed control in front panel.
    - Line out tap at speaker output

    JC-120 S/N 420600 – 471649
    JC-160 S/N 440100 – 470549

    - Pretty much similar to revision described above
    - Separate LF oscillators for vibrato and tremolo. Vibrato /w. depth and speed control in front panel. Also includes a trimmer for chorus adjust in rear panel.



  14. #38
    Quote Originally Posted by sweatervest
    I seem to be a slight oddball, unless Im reading these wrong. My SN starts with 45, but I don't have the chorus trim.

    JC-120 S/N 420600 – 471649
    JC-160 S/N 440100 – 470549

    - Pretty much similar to revision described above
    - Separate LF oscillators for vibrato and tremolo. Vibrato /w. depth and speed control in front panel. Also includes a trimmer for chorus adjust in rear panel.

    On the rear panel right? The ultimate JC-120 thread 3 picture from the top?

  15. #39

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    Quote Originally Posted by jazzfrog
    On the rear panel right? The ultimate JC-120 thread 3 picture from the top?
    4th from the top, no trimmer.

  16. #40

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    I recently tried a JC-40 (new 2x10 model) and really liked it, but I hear it suffers from hiss. I don't have the technical ability to mod an amp.

    I really wish Roland would make a brand new all analog 2x10 JC model (JC-200?) with zero hiss, excellent reverb, excellent pedal-quality overdrive (just steal from their own OD-3 circuit?), and super light weight (like the Fender Tone Masters). Personally, I'd love if they had sine-wave tremolo as one of their modulation options.

  17. #41

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    Thanks Jazzfrog for sharing this great idea. I own an older JC-120 (1979). I have attached a screenshot from the schematic around the second gain stage. Would it be possible to do a similar mod to this circuit? Any pointers to which values to change would be much appreciated.

    All the best,
    Lasse
    Attached Images Attached Images A possible circuit mod solution for Roland Jazz Chorus hiss... current explorations-22843cf6-5dd3-43f4-b03c-2dbf3e24be4f-png 

  18. #42
    Quote Originally Posted by Lasselakken
    Thanks Jazzfrog for sharing this great idea. I own an older JC-120 (1979). I have attached a screenshot from the schematic around the second gain stage. Would it be possible to do a similar mod to this circuit? Any pointers to which values to change would be much appreciated.

    All the best,
    Lasse
    Hi Lasse,

    I haven't figured out yet how to apply this to transistor-based JC.

    Sorry, I will soon start studying this.
    Meanwhile, you could explore what Payne has done, the link is in my first post. His mod affects transistor based JCs.
    Not sure how applicable to your JC.

  19. #43

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    Quote Originally Posted by jazzfrog
    Hi Lasse,

    I haven't figured out yet how to apply this to transistor-based JC.

    Sorry, I will soon start studying this.
    Meanwhile, you could explore what Payne has done, the link is in my first post. His mod affects transistor based JCs.
    Not sure how applicable to your JC.
    In a wholly simplistic “modular” approach, would removal of the IC and a connection from C113 to R119 be a start? Or simply removal of the IC and a jumper from socket 2 to socket 6? Totally doubt it’s that simple (I’m absolutely clueless about the interactions of the components here) but from a brute force perspective…

  20. #44
    Quote Originally Posted by sweatervest
    In a wholly simplistic “modular” approach, would removal of the IC and a connection from C113 to R119 be a start? Or simply removal of the IC and a jumper from socket 2 to socket 6? Totally doubt it’s that simple (I’m absolutely clueless about the interactions of the components here) but from a brute force perspective…
    Yeah, that IC is actually a preamp, not an op(erational) amp(lifier), I don't think it's that simple.
    In fact, I have not been able to find a lot info on a TA-7122AP.
    This is the only detailed page I found on this IC. It is fairly insightful but still requires some more thinking.

  21. #45

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    You. Are. A. LIFE SAVER!

    I had a JC-77 bought new in the late 80s. Loved it for clean and chorus, but was too into heavier distorted sounds so I sold it. A few years ago I was/am happy with my boutique tube amp, even for most jazz (all-mahogany Custom Shop Les Paul and the amp does clean up nicely). But I wanted that great JC fixed chorus and sparkling clean tone, actually more for rock/pop rhythm sounds. Found a pretty decent one on eBay and it sounds great. But good LAWD is it noisy! I have been scouring Tha Intarwebbz for over two years, hoping that someone either offered a fix or could at least explain how it could BE fixed. As it is, the amp is unusable for recording with all that hiss, and I had figured out that the hiss is there whether anything is plugged in or not.

    So tonight I'm researching a few pieces of gear and I think, "Hey, let's look again for the JC-77 fix." Boom - this thread pops up top of the list. And your explanation and fix make perfect sense. I'm no good with a soldering iron, but I have friends who are. It will take me a few weeks or so, but I WILL be giving this mod a try. In the meantime, I want to sincerely thank you for taking the time to not only find and fix the problem, but to post it here for people like me, who could never in a million years have figured this out on my own. I'll have no problem with it not being as loud, since it's ridiculously loud now. I also don't mind a slightly reduced EQ effect; I'm not into extreme settings anyway. BTW, can you tell me exactly which resistor value you decided was the best, in terms of producing the lowest hiss but not being too low to run the amp properly? I've seen 2K and 2.2K mentioned in this thread. Thanks again.

  22. #46
    Quote Originally Posted by JFL440
    You. Are. A. LIFE SAVER!
    ... Thanks again.
    You're welcome.
    It was a fun project and I am glad it can help other folks

    Quote Originally Posted by JFL440
    BTW, can you tell me exactly which resistor value you decided was the best, in terms of producing the lowest hiss but not being too low to run the amp properly? I've seen 2K and 2.2K mentioned in this thread. Thanks again.
    2k or 2.2k won't be such a big difference between the two.
    2.2k is more readily available, it's a standard value.

  23. #47

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    The difference in resistance between a 2k and a 2.2k is within the tolerance range.

  24. #48
    I've got a JC-120 that I love, but I finally got fed up with the hiss and found this thread and want to see if I can fix mine as well. I think mine uses this old schematic, at any rate, I could see that my R46 is 10k, and I see a bunch of fets or transisters scattered around the board:

    Roland JC-120, JC-160 (J-FET Preamp) Jazz Chorus Schematic.pdf 9365 Audio Schematics and Service Manuals files magnet Collections

    My suspicion is that when they port the circuit over to a new format, they keep all the gain staging the same or similar, so hopefully I can identify that problematic gain stage and set it to unity as well. But discrete amplification circuits are not my forte, so it might take me a bit. If anybody else feels like looking at it in parallel, I'd welcome the help.

    Thanks for this great thread! Definitely excited to imagine having my favorite amp without all that hiss.

  25. #49
    Quote Originally Posted by demcanulty_(dan)
    I've got a JC-120 that I love, but I finally got fed up with the hiss and found this thread and want to see if I can fix mine as well. I think mine uses this old schematic, at any rate, I could see that my R46 is 10k, and I see a bunch of fets or transisters scattered around the board:

    Roland JC-120, JC-160 (J-FET Preamp) Jazz Chorus Schematic.pdf 9365 Audio Schematics and Service Manuals files magnet Collections

    My suspicion is that when they port the circuit over to a new format, they keep all the gain staging the same or similar, so hopefully I can identify that problematic gain stage and set it to unity as well. But discrete amplification circuits are not my forte, so it might take me a bit. If anybody else feels like looking at it in parallel, I'd welcome the help.

    Thanks for this great thread! Definitely excited to imagine having my favorite amp without all that hiss.
    Hi,

    There were many versions and revisions of the JC-120.
    Only the serial number will tell you where yours fit.
    See this article for the details: The ultimate JC-120 thread

    Let me repeat this again: this particular mod ONLY works for opamp-based JCs.
    The JC-120 combo was NEVER an opamp based amplifier (only the JC-120H was, and it's just the head).
    As such, your "R46" is a different component in a different part of the circuit, with a different value.
    I'd be surprised if we are talking about the same R46...

    It is my intent to look into the same equivalent mod for all transistor based JCs (and save the world!)
    But so far, I haven't had the time nor the focus.
    It is a 2022 resolution though

    Thanks for that source of technical data, great content and well organized!

  26. #50

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    Quote Originally Posted by jazzfrog
    Hi,

    There were many versions and revisions of the JC-120.
    Only the serial number will tell you where yours fit.
    See this article for the details: The ultimate JC-120 thread

    Let me repeat this again: this particular mod ONLY works for opamp-based JCs.
    The JC-120 combo was NEVER an opamp based amplifier (only the JC-120H was, and it's just the head).
    As such, your "R46" is a different component in a different part of the circuit, with a different value.
    I'd be surprised if we are talking about the same R46...

    It is my intent to look into the same equivalent mod for all transistor based JCs (and save the world!)
    But so far, I haven't had the time nor the focus.
    It is a 2022 resolution though

    Thanks for that source of technical data, great content and well organized!

    Has anyone tried using an attenuator just enough to lower the noise floor a db or 2 to an acceptable level, then just turn the output up from 3 to 4 1/2?

    Seems like it would be an easy fix. You would also want to contact the maker to confirm the attn will work with the JC. From what I understand, some work, some don't.