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JC-120 Distortion malfunctioning

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  • #16
    Originally posted by J M Fahey View Post
    FETs are wonderful amplification devices, biased like tubes, similar impedance characteristics, clip very similar ... pity rhey are such inconstant b*tches so sooner or later everybody stopped using them.
    These folks still build FETs and do a very good job ..........http://www.linearsystems.com/

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    • #17
      Originally posted by Mr_bibbles View Post
      Looks like I have 2-3 MVac on all leads of q2, at c11, r41, and c2. I replaced c11 (now .022) and Q2 in case it was faulty, no change
      With what signal input?
      I suspect you are measuring while injecting no signal, in that case you are not actually testing it.
      Those low mV values may be hiss/hum/whatever picked by meter leads acting as antennas, and mean nothing.

      Schematic shows 2 kinds of voltage measurements:

      * Circles: DC voltages.
      I had to estimate them, because Roland does not tell even the supply voltage, let alone at internal test points.
      These show GROSS errors, such as no supply - shorted/open FET (or tracks/resistors) - no/excessive bias - etc.
      What you measured so far looks somewhat reasonable, so for now we leave it there.

      * rectangles: Signal voltages, which are AC.
      But they will appear only if you inject some signal, so Roland suggests injecting 22mV into the High input or 72mV into the low one.
      Your generator will probably put out "too much" so you must tame it down.
      Suppose it puts out 1V RMS .... connect it to a 10k resistor in series with a 680 ohm one (using standard resistor values) , now you have about 68 mV out ... close to 72 mV suggested.

      Inject that into the Low input and then measure the Audio voltages suggested in schematic, where I added a couple.

      The distorted Audio is nothing more than the heavily overdriven Fet output, that´s why it will vary as I suggested with different Distortion pot. settings.

      * since distorted signal at Q2 Drain must go on along various points, I suggest testing as far as the next stage , after Q6 and Q7 switchers.

      But we know we are pulling straws here, the *true* test is scoping the signal reaching the tone stack.

      We already know the Clean signal is fine, we can only check that the distorted signal passes on, but not its (suspected) uglyness.
      Juan Manuel Fahey

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      • #18
        Thank you so much for this detailed info! Unfortunately... I have neither a scope nor a generator.

        I was testing it without input, but I plugged a guitar in to at least have some signal passing through it. I realize that as a qualitative test goes. This might not be useful in the slightest, but I got a range of 150mv to 2v while strumming the guitar from ‘very soft’ to ‘heaven handed’ at the AC test points.

        Edit: I just pulled a vintage Heathkit sine/square wave generator out of the closet and it works! It has a .1v and 1v settings, I’m going to hook it up to a resistor as suggested and run some better testing.

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        • #19
          So the Generator is a little spotty but I gave it a lil’ tune up and it was giving me a stable steady signal of 22mv (it has an amplitude adjuster, .01v/.1v/1v/10v, and frequency selector....) I injected it into the high input (measured at the inserted tip to be sure, 22mv still) and did some testing.

          Here are the voltages I got through testing:

          1. Q2/c21 junction: .130vac

          2. R41 / r42 junction: .130vac

          3. Q5/r45 junction - lowest distortion setting: .347vac- 1.3vac (highest)

          4. R51/r50 junction- .342Vac to 1.3VAC with sweep

          5/6: q6 and q7 are giving a range of 104mv-397mvAC, identical within a couple MV

          To try and understand the basics, my analysis is q1/q2 as basic amplifier for channel 2, q6/q7 as switchers and q5 which is our crappy distortion unit.

          I don’t really know how to proceed further, but all things considered, are we trying to adjust the bias on Q5? I don’t have a scope but I can use an audio probe to find the exact point of crossover-esque distortion....

          Thanks as always!

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          • #20
            Ok, then I have to give you *terrible* news

            Everything checks normal, so your distortion stage, Q5, is not *broken* , but works as expected.

            That it puts out sh*t by the bucketful is another problem though.

            MAYBE it blocks under drive and produces a shitty waveform, won´t be able to check that until some kind of scope is borrowed or you get a working PC and can run scope software so by now there´s not much to do.

            All voltages, both DC and Signal look fine, within tolerance.
            Juan Manuel Fahey

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            • #21
              Haha well, terrible news is... educational?

              As an alternative, with a signal generator and an audio probe, could I check each stage individually by ear? Or once I did get a scope (there’s a phone app for free... and a nicer one for 9.99...) what would be the process?

              And lastly is there a way to just shotgun this? I have several 2SK117’s on hand, and although I was eager to learn this isn’t my amp and I’m sure the fella wants it back sooner rather than later.

              Either way I can’t thank you enough! This has been enlightening so far. I’m going to

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              • #22
                Originally posted by Mr_bibbles View Post
                Haha well, terrible news is... educational?

                As an alternative, with a signal generator and an audio probe, could I check each stage individually by ear?
                You already have an audio probe : simply select the distortion and its output is connected to a clean power amp and speaker.
                Which works fine because the clean channel sounds fine trough it.
                You will hear nothing different no matter what you use.
                Sadly we are beyond the plain listening stage, only "news" could be visual.
                Or once I did get a scope (there’s a phone app for free... and a nicer one for 9.99...) what would be the process?
                Unless you have a $5000 phone with separate microphone and line inputs, not sure how could you connect probes to it and test various circuit points.
                No, some day you´ll get a working PC or maybe one of those tiny Scope modules; I´ve seen one which is just a PCB with a small screen on it, not even a case.
                And lastly is there a way to just shotgun this? I have several 2SK117’s on hand, and although I was eager to learn this isn’t my amp and I’m sure the fella wants it back sooner rather than later.
                Shotgunning what?
                The circuit works properly, according to measurements, it just sounds crappy.

                Plan B: if it were mine, I would build a matchbox sized Distortion pedal, say an MXR Distortion+ , no switching, no case, just a small PCB with "a 741 and 2 diodes" and mount it there , driving it with signal from C21 and feeding its output into C25 as a good distorting stage, mounting its own distortion pot in TR5 hole, not using MXR volume control since there´s no space for it, and using Q6 and Q7 as "effects loop switchers" ... but not worth it complicating things that much; it´s simpler and better to ignore internal distortion and plain use a good pedal.
                Juan Manuel Fahey

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                • #23
                  Well thank you for all your help! I’m going to go buy a couple chickens and a dagger; my only hope now is a sacrificial ceremony.

                  All the best,

                  Bibbles

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                  • #24
                    Buy two chickens and sharpen a dagger, sacrifice them and then, either make a ton of Italian sauce chicken stew to accompany your pasta or broil them after being covered with butter, smoked paprika, chili flakes lots of pepper and lemon juice or ....

                    That plus a couple bottles good red wine will immensely improve the Distortion quality
                    Juan Manuel Fahey

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                    • #25
                      JC120 is great clean amp.
                      Before any action on the JC120, try plugging the guitar through some linear power booster preamp (similar EHX LPB-1) to raise the signal for distortion.

                      LPB-1 Linear Power Booster Preamp

                      [ATTACH=CONFIG]55281[/ATTACH]

                      https://hotbottles.wordpress.com/2012/02/14/building-an-electro-harmonix-lpb-1-from-scratch/
                      EDIT 190921

                      DOD - 210 FET Preamp (Portal requires registration)
                      http://www.freestompboxes.org/viewtopic.php?t=12099

                      DOD - 210 FET Preamp schematics
                      Click image for larger version

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                      It's worth trying replace C21; C25 (.033uF) with .1uF; Replace C23 (1uF) with 10-22uF
                      If still not enough distortion, in parallel with R51 (220k) connect 2 diodes and resistor as desired max to 1k (red on schematics)

                      Click image for larger version

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                      about JC-120 Distortion

                      https://www.youtube.com/watch?v=AaeSF-ZNHjk

                      https://www.tdpri.com/threads/roland-jc-120-distortion-question.430949/

                      https://www.thegearpage.net/board/index.php?threads/roland-jc-120-distortion-mods.636445/

                      https://www.diyaudio.com/forums/instruments-and-amps/291757-roland-jc120-distortion.html
                      Attached Files
                      Last edited by vintagekiki; 09-21-2019, 08:16 AM.
                      Who does not know and knows that he does not know - teach him Confucius)
                      Who knows and does not know that he knows - wake him Confucius)

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