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Thread: JC-120 Distortion malfunctioning

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    JC-120 Distortion malfunctioning

    http://www.tangible-technology.com/s...-120%204th.pdf

    Hey y’all,

    I have. Jazz Chorus that is stumping me. Everything sounds fine until you turn on the distortion circuit. I know these have a reputation for having lackluster distortion, but something is definitely malfunctioning; if you strum softly it’s not so bad, but if you strum a chord with gusto it pushes the distortion into a blatty mess.

    I replaced q5, q6, and q7, and the d4 and d5 diodes with it (sk117 and in4148 respectively), and a couple off spec caps.

    Does anyone have a link to an explanation for this type of clipping circuit? Or would someone who is familiar help walk me through the functions? I know my way around tube amps but solid state circuits are still above my pay grade. I don’t have a scope but I have an audio probe.

    Thanks

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    Old Timer J M Fahey's Avatar
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    Quote Originally Posted by Mr_bibbles View Post
    I have. Jazz Chorus that is stumping me. Everything sounds fine until you turn on the distortion circuit. I know these have a reputation for having lackluster distortion, but something is definitely malfunctioning; if you strum softly it’s not so bad, but if you strum a chord with gusto it pushes the distortion into a blatty mess.
    That´s the way it is.
    I replaced q5, q6, and q7, and the d4 and d5 diodes with it (sk117 and in4148 respectively), and a couple off spec caps.
    But you kept the circuit, so .......
    Does anyone have a link to an explanation for this type of clipping circuit?
    It´s a very poor FET circuit, which gets poorly overdriven, and has not enough gain, ugly clipping, no pre or post EQ, a mess.
    Or would someone who is familiar help walk me through the functions?
    See above

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    Don't forget the joker g1's Avatar
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    Agree.
    Unless you are comparing it to another of the same model and finding it substantially different. The distortion circuit in these is pretty much unusable.

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    Quote Originally Posted by g1 View Post
    Agree.
    Unless you are comparing it to another of the same model and finding it substantially different. The distortion circuit in these is pretty much unusable.
    I am comparing it to another JC-120, and I’ve worked on a few before. It is definitely broken. It’s not just a ‘bad sounding distortion’, It’s malfunctioning; I think it’s a transistor biasing issue. It sounds blatty and uneven, like it stutters as it’s fed signal. I don’t really know where to start because I’m not sure what function each transistor in the circuit performs, should I just look up SK117 operating parameters and check voltages?

    Thanks guys.

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    Old Timer J M Fahey's Avatar
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    I calculated what voltage, signal and waveform yu should have in a properly working preamp.

    Had to guesstimate a lot because Roland does not care to show DC voltages (except at the supply itself) nor even *label* where supply voltages come from

    I am supposing preamp gets +27V straight from supply regulator (not even a regulator, just a ripple filter), so confirm it first; if different all others will change accordingly.

    Click image for larger version. 

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    The very cheesy "Distortion" is just a starved Fet gain stage, Q5, getting about 1/4 of the "27V" so about 7Vdc.
    Its Drain "should" be around 5V DC if stage is biased as a "gain stage which gets too much signal and can´t avoid clipping" .

    As cheesy as can be, for multiple reasons:

    * FETS are more inconstant than a teenager Daughter, parametrs vary all over the place, even same brand and model, same batch, , as much as 5:1
    If you replaced it, good luck trying to get one which works there even close to original.

    * FETs HATE low voltage supplies; the lower the voltage the crazier more inconstant they become; so you may very well have there a FET so misbiased that it´s slammed groundwise or railwise , slimilar to a "broken amp", and so chops and farts when fed signal.

    * to boot such signal has not been preprocessed even minimally, such as cutting bass for clarity (what *every* distortion does) or cutting some treble afterwards (what most Distortions do) to attenuate buzzyness.
    So even if working "properly" this Distortion is a mess.

    Can´t believe they didn´t add , say, a Tube Screamer or any of their other excellent distortions and can only guess this amp was designed by a Jazz lover who **hated** Distortion, Rock, etc. and only grudgingly added one, the worst he could so nobody uses it.

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    Stray Cap DrGonz78's Avatar
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    It's good enough for Jazz... oh wait a moment!! I remember fixing a dead distortion channel on one of those amps. When I returned it to the shop owner I warned him to inform the customer how bad the distortion channel sounds when working properly.

    More on the subject of the thread or a question that came to mind. Is there a way to increase the power supply voltage to these FETS? Of course that opens up Pandora's box. Has anyone ever modded these amps to get the distortion channel sounding better?

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    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.
    Japanese loved them (Yamaha, Roland, etc.) but had to heavily depend on large purchases of Factory selected parts, and even so ...
    They use Fets preselected, measured one by one, and identified by a coloured dot of paint, that´s why you always see a letter at the end, "Y", "G", etc. meaning Yellow, Green, etc.

    They specially designed *one* FET for very low voltage, the one used inside electret capsules, which works as low as 1.5V DC, but which has terrible amplification.

    Similar to tubes, they work better with higher voltages ... but most stand only 25V ... you´ll see those in Japanese amps, and Randalls fed exactly that and no more.

    Not worth modding JC distortion; either use an external pedal or embed a small distortion PCB (say, an MXR Dist+ which is the simplest) and use switching Fets as a mini , dedicated FX loop.

    But in this case check voltages to be able to repair it at least to Roland standards

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    Maybe change C21 and C25 to "tune" the distortion a bit?

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    Wow

    Thanks JM, this is exactly the answer I was looking for, plus a whole lot more. I’ll get to the bottom of it and record what the failure is in case it can help someone else.

    Seriously though thanks for the effort... test voltages???? Amazing!

    Quote Originally Posted by J M Fahey View Post
    I calculated what voltage, signal and waveform yu should have in a properly working preamp.

    Had to guesstimate a lot because Roland does not care to show DC voltages (except at the supply itself) nor even *label* where supply voltages come from

    I am supposing preamp gets +27V straight from supply regulator (not even a regulator, just a ripple filter), so confirm it first; if different all others will change accordingly.

    Click image for larger version. 

Name:	JC120 distortion voltages.png 
Views:	20 
Size:	56.3 KB 
ID:	55000

    The very cheesy "Distortion" is just a starved Fet gain stage, Q5, getting about 1/4 of the "27V" so about 7Vdc.
    Its Drain "should" be around 5V DC if stage is biased as a "gain stage which gets too much signal and can´t avoid clipping" .

    As cheesy as can be, for multiple reasons:

    * FETS are more inconstant than a teenager Daughter, parametrs vary all over the place, even same brand and model, same batch, , as much as 5:1
    If you replaced it, good luck trying to get one which works there even close to original.

    * FETs HATE low voltage supplies; the lower the voltage the crazier more inconstant they become; so you may very well have there a FET so misbiased that it´s slammed groundwise or railwise , slimilar to a "broken amp", and so chops and farts when fed signal.

    * to boot such signal has not been preprocessed even minimally, such as cutting bass for clarity (what *every* distortion does) or cutting some treble afterwards (what most Distortions do) to attenuate buzzyness.
    So even if working "properly" this Distortion is a mess.

    Can´t believe they didn´t add , say, a Tube Screamer or any of their other excellent distortions and can only guess this amp was designed by a Jazz lover who **hated** Distortion, Rock, etc. and only grudgingly added one, the worst he could so nobody uses it.

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    Quote Originally Posted by drewl View Post
    Maybe change C21 and C25 to "tune" the distortion a bit?
    C21 definitely, and R44, so they combined (that´s why we talk RC networks) cut some Bass to make distortion less farty.
    But let mr bibbles repair it first, maybe post some audio, and then improve it *slightly* ; not much wiggle room available.

    FETs all over the landscape is the reason why all those "tube schematics rebuilt with FETs" pedals have one trimmer per FET

    Sort of acceptable for hobbyists, impossible for a Factory.

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    Measurements:

    So the main rail is coming in at 24.3vdc.

    The biggest discrepancy is at Q2/c21 junction. Instead of 160mv it’s a flat 15v. After C21 it settles down to .5

    Also, the 160mv supply is off on he Q2 side of C2. Seems like it’s fine on the q6 side but my meter has trouble that low, bounces between 100-300mv while reading.


    Other than that the 7v and 5v readings near q5 are 5v and 3v respectively. That could be accounted for by he new calculated rail voltage?


    Based on this, my hunch is c2, c21 or q2. What do y’all think?

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    Quote Originally Posted by Mr_bibbles View Post
    Instead of 160mv it’s a flat 15v. After C21 it settles down to .5

    Also, the 160mv supply is off on he Q2 side of C2.
    Numbers in boxes are AC voltages with signal level shown at input jacks.
    Numbers in ovals are DC.

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    Member HaroldBrooks's Avatar
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    Might I suggest using this ? The ST-2 With an EQ in front of it is my "Amp Model" of choice, and it's amazingly inexpensive, and up to around 12 O'clock sounds a lot like a vintage Marshall Plexi. The trick is to not feed it too much treble on the input, and to not expect it to distort like a modern Metal amp and still sound good. For a vintage Lower distortion, it's a fantastic model, and generally I hate amp modeling, but sometimes it's better than other options (poor distortion built into the amp.) It get's some bad press because it's abused and misused, but as an amp modeler, it sounds better than anything in my Eleven Rack, IMHO. Again, I trim back the real highs and real lows from my guitar, and it's pretty good !

    https://www.youtube.com/watch?v=Fg5HLdYBskM

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    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

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    Old Timer J M Fahey's Avatar
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    Looks like I have 2-3 MVac on all leads of q2, at c11, r41, and c2.
    With what signal input?

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    Supporting Member gbono's Avatar
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    Quote 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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    Quote 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.

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    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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    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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    Old Timer J M Fahey's Avatar
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    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.

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    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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    Old Timer J M Fahey's Avatar
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    Quote 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.

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    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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    Old Timer J M Fahey's Avatar
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    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

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    Senior Member vintagekiki's Avatar
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    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.

    EDIT 190921

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

    DOD - 210 FET Preamp schematics
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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

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    Last edited by vintagekiki; 09-21-2019 at 08:16 AM.

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