Roland Jazz Chorus JC-20

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brianhujung
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Post by brianhujung »

Good day L & G,
I was looking for suitable candidate for my bench amp and I found schematic of Roland Jazz Chorus JC-20 from this japanese website :
https://juggbox.blog.jp/archives/6226648.html.
Looks like this is the one and only JC-20 schematic on the interweb. Anyway, from his article, I got impression that the guy who trace it are surprised that this kind of circuit are released into final products, so probably there's something weird into this circuit.

I did my research based on several another article & gutshot pictures of the actual unit and this is additional information that I found :
1. Secondary transformer are 15V
2. The schematic looks complete (there's probably several non-critical parts omitted when I compare it with gutshots pict), but missing the type of diodes. After reviewing several gutshot and another jazz chorus schematic those are probably 1S133 (small glass silicon diode with yellow stripes). However I can't determine the zener value.

Before I proceed to build it, there's some question I would like to ask :
1. What is the purpose of the block contain 2x 2SK381 on the pre power amp section? The only datasheet I found for 2SK381 are in japanese and all blurry. Unfortunately I have all the parts except this 2SK381. Does the FET selection on this block are critical? or can it be substituted with another FET directly? I know Roland/Boss use 2SK117 or 2SK30 intensively on their products.. can it be a proper substitute on this case?
also can anybody help me to estimate / guess the value of the zener D6?

2. The chorus section using MN3207 - MN3102. The power supply are taken from Vdd, so Is it ok to use 8v2 or 9V1 zener on the D4?
JC20-Poweramp-PSU.png
Oh, also some other web also mention that they don't have a problem replacing uPC1238 8with TDA2030 which is still available right now, so I guess if everything checked, this is a great candidate for clean practice amp.

TIA
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JC-20.pdf
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ppluis0
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Post by ppluis0 »

Hi Brian,

The purpose of the circuit section you marked is to mute the inputs of both power chips when powering the unit to avoid any transients at the output speakers.

Perhaps any jfet can be substitute for this application, but I'm not sure...

Cheers,
Jose

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brianhujung
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Post by brianhujung »

Thanks Jose,

So I guess that section are not too critical. I thought it was some kind of limiter to prevent power amp from clipping.. I can always throw some speaker protector module later if it's necessary..

I'll go ahead with this one then. I'll post my progress later..

Cheers!

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Manfred
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Post by Manfred »

Hi Brian

I found a list in which the J113 is indicated as a replacement type and an unblurred data sheet.
Attention, the pin assignment is different for the J113.
2SK381 datasheet.pdf
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2SK381 replacement.jpg

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brianhujung
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Post by brianhujung »

Thanks Manfred,

That's really good news because I do have J113 on my stash! I'll start redraw the schematic and double check it with some gutshot that I have collect for the past few days..

I'll experiment with the zener values once I manage to produce the layout

Cheers,
Brian

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Post by brianhujung »

5 minutes into action and I already scratching my head because of this unusual arrangement on the power supply section.. is it supposed to be like this?

C4 basically went from ground to ground.. and this is not the only one.. there's also C1 & C7 which both lead connected to ground.. :scratch:

Top view :
roland_jc-20_009.jpg
roland_jc-20_009.jpg (75.7 KiB) Viewed 4082 times
Trace view from component side :
power-section-1.png
Sch :
power-section-1-sch.png
Cheers,
Brian

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mauman
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Post by mauman »

ppluis0 wrote: 28 Sep 2023, 10:09 ...The purpose of the circuit section you marked is to mute the inputs of both power chips when powering the unit to avoid any transients at the output speakers. Perhaps any jfet can be substitute for this application, but I'm not sure...

Cheers,
Jose
Good call, Jose. I found an explanation of how this muting circuit works, along with recommendations for JFET selection. Apparently the RDSon value is important. (see paragraph 2, JFET) https://sound-au.com/articles/muting.html

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Post by brianhujung »

I finished the tracing of main board, picture are attached

Disclaimer :
1. I don't have the actual unit to be traced so the tracing are based on limited gutshot picture from internet. Some of it have adequate resolution, but mostly don't.
2. There's 2 different version of JC-20 board and I tried to make some sense from these 2 against the schematic from Mr. Nagata. Whenever I lost the trace, I trust and use schematic as reference.
3. I traced only the main PCB. The daughterboards for power chip + headphone can be constructed based on schematic from Mr. Nagata.

Result :
1. Only one minor correction : R14 on the schematic should connect to pin 5 instead of pin 1 at the IC1.
2. Some components numbering need to be corrected, but the most of them are good and consistent between Schematic vs PCB.
3. The components with white letter are not appear on the schematic, hence the values are unknown as for now.
4. Diodes looks like 1S133, zener values are unknown.
5. The model/part for chorus switches (SW2) are unknown so the switching scheme for chorus section can't be verified as well.

I only have access to free version of Diptrace, so it's impossible right now to draw full schematic & PCB for final verification due to it's limitation (max 300 pins only).
I will edit the schematic using graphic editor later on and that's probably as far as I can go..

Thanks again Jose & manfred for your help!

Cheers,
Brian
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JC-20_traced.png

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Yazoo55
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Post by Yazoo55 »

This is a very interesting project. I’ve used your trace and the schematics and started to put it in Kicad. I searched the web for gutshots, like you did. The pictures I found are generally not detailed enough apart from some which show capacitor values clearly in a couple of sections of the board. Are you going to go ahead and build this?

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Post by brianhujung »

Yep I'm going for it.. slow progress because I'm struggling to learn kicad in the last 2 weeks.. TBH at this point I really considering to go back with diptrace using simplified design..

However if you're willing to share your board later on, then I'll be happy to build and test it..

Cheers,
Brian

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Yazoo55
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Post by Yazoo55 »

I would be more than happy to share my Kicad files with you. Would you like to collaborate? I have just about finished doing the schematics.

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Post by brianhujung »

Sure thing, let's do it.. :thumbsup
Can you please make it single layer so I can etch the PCB later on.. and please use the normal 16mm potentiometer footprint if possible (5mm lead spacing)..

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Post by Yazoo55 »

Before I layout the pcb, there are a few queries. I’ll send you the Kicad schematics. Some of the parts values are unknown and a couple relate to the biasing of Q2, R41 and C54. I’m not an expert so I don’t know what these should be. R51 and R49 connect to the MN3207. Looking at the guide circuit in the datasheet, a guess for these would be 100k. It would be very useful to have access to the real thing or have good quality pictures.

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Post by brianhujung »

R49 & R51 Looks like 68k based on this picture :
R49-R50.png
C54 electrolytic most likely 4.7uf just like the others
I can't confirm the values for R41 but I wouldn't be surprised if it's 68k
R41.png

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Post by Yazoo55 »

Thanks.

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Post by Yazoo55 »

I’ve decided to try breadboarding the chorus section from C52 to point B. I’ll get back to you when I’ve done this.

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Post by Yazoo55 »

I’m making progress. I’ve breadboarded from the input up to “A” of the preamp. This has thrown up more queries though, R6 on IC 1 section 1 is listed as 68 ohms but the feedback resistor R8 is 68K. If I have got this right, this would give an amplification of 1000! I’m wondering whether R6 should be 68K giving unity amplification.

Similarly, IC 1 section 2 resistor R15 is 680 ohms while the feedback resistor R12 is 68K which would give amplification of 100 which can’t be right.

IC 2 section 1 looks similar for R20 and R 19 with an unknown value R18 in parallel.

Do you have any more pictures which would identify these values please?

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Post by brianhujung »

GainRes.png
This is the best image that I have,
R6 = 68R,
R8 = 68k
R15 = 680R (I saw a hint of brown on third band)..
R12 = 68k

I don't have clear picture for R20, R19
R18 is empty / not populated..
These are 2 different boards, probably one is Italy version and other is Japanese version..

First board :
Top views shows R18 is not populated
X-R18-3.png
X-R18-3.png (529.37 KiB) Viewed 2660 times
Same boards, bottom Views for R18 covered with solder but I didn't see any components leads sticking out..
X-R18.png
Second board :
Top views shows R18 is not populated
X-R18-2.png

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Post by Yazoo55 »

Thanks!

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peterc
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Post by peterc »

This is an exciting project! Thanks and bookmarked.

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