DIY Roland System 100 VCO

System 100 VCO Panel

I’ve read loads on the various forums about fat, vintage-y sounding voltage controlled oscillators. What is the secret? ask the people with too much time on their hands.  “Let’s measure it” say some. “There’s some undefinable mojo” say others.

Does the System 100 VCO sound vintage and fat? What does that mean anyway?

Enough of all that, here’s a schematic.

Roland System 100 102 VCO schematic

My System 100 sounds generally lovely. The oscillator sounds slightly different to my Analogue Systems RS-95 oscillators, apparently itself a “vintage-y” sounding oscillator. I’d prefer to quantify such woolly terms with measurements, but when I started building this, I lacked test equipment beyond my laptop/audio interface inputs and a really cheap DSO Nano oscilloscope. Which actually turned out to be a problem.

System 100 VCO circuit board

(It must be vintage – it’s brown and the opamps are in cans).

‘Cos it’s bigger than the things I’ve stripboarded so far – and I couldn’t get it to work on a breadboard – I was super-careful about laying it all out and checking for shorts before plugging it in.

The expo converter at the heart of the System 100 oscillator uses this…

Roland System 100 102 VCO expo schematic

…the Fairchild uA726 heated matched transistor pair. It was apparently always quite costly, and has been officially out of production for a long time, which means that it’s even more expensive and harder to get hold of.

There are other ways to skin the expo converter cat, but I’d hoped to stick with the uA726. Mainly because it’s easy, it’s what the System 100 had, and I didn’t think I was clever enough to work out how to adapt the circuit for a tempco or a heated CA3046 (like on the Curetronic version of the 100m VCO). Also I fretted about the possible power draw of a heating up a 3046 to working temperature.

All this hand-wringing lead me to searching eBay, where a seller in Hong Kong was offering a pair for fifteen quid with a no-questions-asked refund policy.

With my usual timing, I read all the comments in the analogue-heaven archives about dodgy uA726s after I’d clicked buy-it-now, so it seemed quite likely I’d ordered a pair of fakes.

Here’s one of them – they’re outwardly identical.

ua726-photo

Before I plugged the uA726 in, I bunged in a matched pair of BC547 with no temperature compensation just for testing the circuit.

Here’s the arrangement I used for testing, with a 10 pin DIP socket in the uA726 position.

BC547 transistors in a ua726 socket

I used some electrical tape just to stop the legs of the left hand transistor from touching.

Testing uA726 with BC547 on stripboard

I flipped the switch, the power supply lights glowed happily, and the VCO made some sounds like it was trying to oscillate, but not quite managing it. When waggling the pitch knob it would cut out, and then distort and glitch out in a mad FM sort-of way. This is as good as I got it.

After a lot of head scratching, on a whim I swapped the comparator CA3130 at IC205 for a CA3140, which made it oscillate without glitching. With the BC547 matched pair in place, the pitch drifted in a comical way.

On installing the 726, it warmed up nicely and the oscillator tracked reasonably. I’ve still got some fiddling to do to see if I can get it tracking as well as the oscillator in the 101 keyboard.

Other people have had success with the Korean 726s, Ramcur on Flickr tested some of the Hong Kong UA726HC on his Minimoog clone board and verified them to be working.

Although the CA3140 worked, it didn’t seem quite right. On taking tuning measurements, I found that the pulse width increased with the pitch, from 50.3% at C1 to 51.3% at C6. I measured the pulse width on my System 100, and that stayed steady up the octaves.

I checked my photographs of the 101 keyboard, and there really are two CA3130s there – here they are in the bottom left corner of the shot – so there must be something else going on.

CA3130 in the System 100 VCO

Boringly I spent a couple of months (no really…) to get the CA3130 working with no luck. I looked at the SH-5 for inspiration and tried a bunch of different things, including swapping the CA3130s, checking the value of R227, using original 1S2473 diodes, and checked all the voltages, but nothing worked.

After some moping and a lot of swearing, and I came across a thread over at Muffwiggler on the Roland Jupiter 8 VCO, which mentions the length of the reset pulse being set by a capacitor from the comparator output to input. Here it is marked in blue:

Roland Jupiter 8 VCO core

Then I started having a look at the VCOs from the Rolands from around the same time and found an indication of the length of the reset pulse in the SH-1 service manual, again with using a small picofarad capacitor on the integator feedback:

Roland SH-1 VCO core

Slinging in a 10pF capacitor across pin 3 and 6 of IC205 made it work. I was so glad, it was pathetic.

About this time I finally got a decent oscilloscope (which would’ve really helped with tracking down the problem), so here’s a video of the reset pulse with extra cap, dancing about like a four year old at a wedding after too many sweets:

And here it is with a 10pF capacitor on the integrator feedback, just a picture because it stays still. Although the trace glows brighter as the pitch goes higher.

System 100 VCO reset pulse 10pF

3µs is a bit long, so trying again with a 5pF capacitor we get:

scope-reset-pulse-5pf

which seems pretty close to the SH-1 spec. Going smaller with a 2.2pF capacitor it still resets happily with a pulse length of 1.3µs, and it’ll probably go lower than that.

I’m still interested to know why it didn’t work the first time round, or even more how my 101 keyboard works at all.

I note that the SH-5 and the System 100 have a similar arrangement of diode + resistor round the integrator, but most of the Roland VCO cores that come after use a low picofarad capacitor. Reading the 1S2473 datasheet, it seems like there would be some inherent capacitance there, maybe it’s not quite enough in this case to keep it cleanly resetting.

I made a more basic mistake when comparing the DIY VCO frequency with my System 100, wondering why it was wobbling around like that. Looking at it on the oscilloscope I could see the square wave flexing in and out in a suspiciously 50Hz-ish kind of way. I realised that stringing a long wire across the room to my (switched off) MC-4 for pitch CV was a bad idea – turning it on or removing the wire mostly fixed it, with the rest of the wobbliness coming from the unshielded pitch offset wire coming from a pot on a breadboard.

While researching pitch wobbliness, I stumbled across a post in the AH archives: Ritchie Burnett did some testing on analogue synths to check for oscillator pitch drifting, and found that the VCO in his SH-09 was modulated by interference from the nearby mains power cables. There are some power lines routed directly underneath my table which probably don’t help, so I’ve placed a grounded metal place underneath the circuit board in the hope that’ll cut down on the interference.

The voltage levels are close enough to the original, and the waveforms all look pretty similar, apart from the triangle which suffers from a much larger reset glitch, presumably from the capacitor I’ve added for lengthening the reset pulse.

Here are some octaves of C, all taken through the filter fully open, then through my version of the VCA, which unfortunately inverts at the moment. Not ideal but you get the idea. Warning – the tuning isn’t perfect… Saw:

Hello square:

And a triangle – the glitch makes it fuzzier than it should be:

And some random PWM fiddling:

The scaling measures as ok until it gets to the sixth octave where it goes sharp. Adjusting the scale has sort-of minimised it, but I’m wondering if increasing R224 (resistor in the integrator feedback loop) to the SH-5 standard of 3.3K or adding a trimmer here might help – I note there’s one on the SH-2 at this point which controls “linearity”. Starting off from C0 isn’t terribly realistic so it’s not as bad as it might seem.

The high notes don’t quite sound as pure as the original System 100, which I’m putting down to a 50Hz modulation being picked up from the mess of cables on my desk.

I had a quick go at just comparing a couple of loops from the original and from my clone. Here’s the original, sequenced from the MC-4 with resonance set about half-way:

and here’s the clone

And the original with a bit more resonance

and the clone

The pot positions are different on both, partly because some of the pot values are different on the clone (lower resistance pots = more control over the snappy area, especially on decay) but also some of the rotary pots on the clone don’t have any knobs on yet.

Despite having left both on for half-an-hour before I started this, the clone had drifted upwards slightly inbetween takes, which wasn’t too great.

This thing has been sitting on my desk for over a year now while I fret about sliders vs. rotary pots for a front panel for my 102 clone, and I’ve changed my mind about the uA726. Initially I thought it was a real one but it seems unlikely that they are exact clones given how expensive it would be to start making such a component again, so I wonder how it came to be. At the very least it’s some similar arrangement of transistors shoved into a can with a possibly dodgy heater circuit.

Now I’ve got a bit more confident with changing circuits, I’m leaning more towards remaking it with a heated CA3046 (looking at the Doepfer A110 for inspiration, see below for the relevant part of the schematic), or an LS318 matched NPN pair with a tempco.

Doepfer A110 schematic - heated CA3046

But anyway, here’s the stripboard layout, and rather more helpfully here’s the DIYLC file in the rare case that you are masochistic enough to want to build this and you’ve somehow happened upon a bunch of cheap uA726.

Roland System 100 102 VCO stripboard layout

If you find anything wrong with it or if you know why my oscillator needs the extra capacitor to stretch the reset pulse out I’d love to know.

19 comments

  1. 21st January 2015Ben says:

    I’m working on a similar Roland VCO problem but don’t know how to get in touch- send me an email if you get the chance!

  2. 21st January 2015ua726 says:

    email sent…

  3. 21st January 2015ua726 says:

    I’ve been peering at photos of System 700 VCOs over the last few weeks (‘cos I know how to have a good time), and it seems likely that some versions, possibly earlier ones, went with the diode on the integrator feedback, with later versions going instead for the small value cap. I might experiment when it comes to making the 702 boards.

  4. 19th December 2017Rob @ AMSynths says:

    Hi, just got my SH-5 VCO clone running, which is the same as the System 100. It uses a contemporary expo generator with tempco and dual matched BJT to replace the ua726. After an unsuccessful attempt at 640kHz, I solved the issue and it oscillates nicely in the correct range now, although I haven’t managed to get the square wave working yet. It oscillates with or without the capacitor in the integrator loop, and I left 15pF in. If anyone wants more details or PCBs to build one, drop by http://www.amsynths.co.uk. Best Regards – Rob

  5. 19th December 2017ua726 says:

    Nice, although weird that the square wave doesn’t work, I don’t remember having any problems with that bit. I think there are some minor differences between the 100 and the SH-5 VCOs, but probably not enough to make any difference. My SH-5 and System 100 sound similar enough.

    Coincidental timing; earlier on today I finished laying out a PCB version of the System 100 VCO, swapping the keyboard section for the sub-oscillator out of the SH-101 but otherwise as close to the original track layout as possible.

  6. 26th December 2017Rob @ AMSynths says:

    Square wave is working – duff LM301. I have DN819’s in stock for my Jupiter 4 VCO, which make a quick sub oscillator although SH101 uses a 4013. Regards – Rob

  7. 12th April 2018ua726 says:

    Finally built up my clone System 100 boards, and the VCO doesn’t need the small pF capacitor I added to my stripboard version. Tt could be there’s an error in the stripboard layout or there’s something about the stripboard that throws it off.

  8. 15th September 2018Rob says:

    I found that the original can CA3130A works but only the later 3130EZ version works rather than 3130E. The VCO is sensitive to the type of 3130 chip used. The circuit is as per Roland schematics with no extra cap. Hope that helps anyone trying this project. I also used +/-8V rails to power the 3130’s using linear regs, so they have a symmetrical power supply.

  9. 16th September 2018ua726 says:

    Interesting – just checked on the working PCB version I built up in April, and it seems I used a CA3130E (and no additional cap). Easily possible I just got lucky though, and it’s been a while since I had it running, still need to sort out a panel for it.

  10. 1st July 2020timothy ward says:

    still bummed that frequency central replaced there system 100 pcbs with 100m pcbs. the roland 100m sounds thinner more sterile and more 80s then the system 100 or 700 but why? is it the vco the filter or what? why?

  11. 1st July 2020ua726 says:

    The FCUK System-X stuff was always based on the System-100M, I think?

    I doubt that there’s much difference between the VCOs. I don’t think the VCA has much to do with either, although I’m a fan of the 100M exponential setting for punching a hole in the wall.

    The 100 VCF is ace tho, full recommend making that. Pharmasonic were doing PCBs, they might still have some. If the System-500 505 is close to the SH-5 as they suggest, then that probably has the sound of it too.

    The envelope is possibly part of it as well, it has a much lower max output than the 100M envelope. The one or twice I’ve controlled the 100 via the 100M-clone envelopes it’s sounded a lot sharper.

  12. 15th December 2021Maik says:

    Hi there,
    I would like to build the VCO, but I have a few questions about it, hope you will answer.
    1.) outside on the edge of the stripboard is R243 what does the resistance refer to, what does that mean?
    2.) There is a yellow cable with “Fine” written on it! Does this belong to the Fine Tune controller? If so, on which pin of the potentiometer? And where are the other two connections for Fine?

    I would be extremely happy to receive an answer.

    Regards

  13. 15th December 2021ua726 says:

    Maik! I think I can answer your questions

    1. I think this is a random bit of text going for a walk, I don’t think you have to worry about it
    2. “fine” is shorthand here for the fine tuning pot, yes – this is the wiper/middle connection, the other ends are at -15V and +15V (have a look at the schematic, it should be fairly clear).

    but! on the whole I’d really recommend you don’t put yourself through this and get a clone 100M oscillator PCB or something. I still fret that this version needed an extra component to oscillate in a stable fashion. I ended up getting a circuit board made up for the oscillator, although that had errors that I introduced that I still needed to hack.

    Pharmasonic in Japan were doing circuit boards for the System 100 VCO but I can’t vouch for them either way.

  14. 10th June 2022Stacey says:

    I pulled 6 ua726s off an old device (3 made in Hong Kong 2 made in Korea) and am looking to build a Roland 100 VCO based on this design. Although the CA3130 EZs were a bit difficult to find, I’ve found the transistors have been the most difficult to track down.

    Are you aware of any good sources for purchasing these transistors (or better yet substitutes that could be used in their place) or perhaps their descriptions and key characteristics?
    2sk30a-gr (Silicon N Channel FET ?)
    2sc945q (driver stage of AF amplifier?)
    2sa733q (PNP general purpose amp?)

    Are the above, parenthetical descriptions for these components accurate?
    Also, the following ICs were somewhat difficult to find:
    301AH (Opamp? Is this the same as the LM301AH?)
    CA14586G
    I did mange to pull some CA1458E chips off said machine. Will they work in lieu of the CA14586G?

  15. 10th June 2022ua726 says:

    Wow, 6 ua726s off one item? You’re not lurking round airports nicking them out of radars, are you?

    You should be able to sell the lot and retire (Alright, maybe not quite).

    Like I say, I’m not sure how far I trust my layout above, although I’ve got plenty of evidence to show it working successfully.

    Can’t advise on any sources for the -Q suffixed transistors. I managed to get hold of some of the 2SC945Q ages ago, but couldn’t find any 2SA733Q – I used a 733P instead. I’m not sure that the hFE is that important in this particular application, but I can appreciate wanting to try and make something as close to the original as possible.

    The Fairchild KSC945Y available at Mouser falls into the same hFE bracket as the 2SC945Q, but the pinout is different (base and collector is swapped).

    Roland themselves swapped in different transistors over the life of some products; the 303 used 2SC1815/2SA1015 and also 945/733 (and possibly others), so it may be that the (later?) 1815/1015 will do the trick here as well. There are DIY Eurorack clones of the 100M VCO that use standard BC- transistors, and that doesn’t seem to matter to people.

    I used a 2SK30A-GR for the capacitor discharge switch in mine; a -Y from somewhere like Tayda might do the trick. Otherwise there’s a GR version of the (surface-mount) 2SK208 that I imagine could work?

    Yep, CA1458G is the one; anything else was a typo. A CA1458 would do the job.

    Also 301 is LM301, I’ve got some can versions lurking around somewhere but I just used a DIP version on mine. Looks a bit less vintage, but stays put in a DIP socket.

  16. 13th June 2022_Stacey says:

    Ha. No (I’ll have to keep an eye out the next time I’m in an airport though [shifty eyes] JJ).

    Stacey, here. (Tried posting this earlier but it didn’t take so I tried with a different email to be sure.)

    I got them off what appeared to be a giant printer / photo editor (pre photoshop) in the form of a giant metal desk. There was some kind of camera obscura in the center with solenoid powered shudders and solar cells with RGB filters over them. The company that manufactured them is Polielettronica and they’re based out of Italy, It had lots of 6800 series MCUs as well so it’s probably from the 80s.

    I’ve found most of the other parts needed for the VCO on this same machine as well. It contained a lot of CA1458 ICs; although the didn’t end in the G suffix. Instead it contained CA1458E, MC1458P and UA1458TC ICs. Could any of these be substituted for a CA1458G or plain CA1458 (Not sure what the suffixes mean or if they’re merely manufacturer specific codes)?

    For 1S2473 diodes––I was able to salvage seven glass diodes with a yellow band that resembled the photos I’ve seen of these online; although, I’m sure how to test them to be sure (perhaps by chicking if the reverse current matches the datasheet or using a scope to probe for some other key characteristic..).

    I’m only needing the BC547 used to test the circuit without temperature compensation. (I’m guessing this was only to make sure everything was in working order without damaging the UA726s, no?)

    I did notice in one of the posts that the Roland System 100 is powered by a +15v and -15v power buses. Is this correct? My current Eurorack supply is +12v, -12v, +5v (powered by a 19v wall adaptor). So, if this is case it sounds like I will need to build a +-15v power supply (Happen to know any good schematics/online resources? Or perhaps I would be better off checking eBay.)

    Lastly, do you recall what size of Vero board you used? Seems like the strip-board route you took might actually be less of a headache than procuring a PCB (feel free to correct me if you disagree).

    Thanks, for your response and thanks again for documenting this!

  17. 15th June 2022ua726 says:

    Ta, it’s interesting to hear what the ua726 was used in. I got the impression they went obsolete right as far back as the end of the 70s, but I could be wrong. Roland were certainly using them into the early 80s tho – split could be the Jupiter 4, which definitely used them, which I think was made up until 1981/82ish, and the 1982 Jupiter 8 which used CA3046s.

    Yep, for the suffix it’s one of those “it depends” things that up to the manufacturer; a lot of the time the suffix can indicate package type, sometimes it indicates some kind of quality about the part (I’m thinking of TL072AC vs TL072C, where the input offset is better on the latter).

    There’s also been over forty years since these things were made, so if you were going to use modern parts, there’s a chance there could be some difference, maybe certain parts have got faster over time.

    Short answer is “I bet it’ll be fine” tho, and you could hedge against it being a problem by just using sockets for the ICs and just trying what you’ve got for now.

    I’ve done loads of ferreting around since building mine and ended up managing to find some 1S2473 diodes, although if you read around it seems like the common-or-garden 1N4148 is regarded as a close equivalent, and I think this is what I used at the time. I’m generally saving my 1S2473s for the resonance clipping in VCFs, although I’m not convinced it makes much difference. You might be able to squint (or a take a photo with your phone) to work out if there’s any numbers on the side of the diodes you’ve got. FWIW my 1S2473s have a yellow band on one end like you mention, with a thin silver-ish band torwards the middle.

    Correct on the BC547s, they’re just for testing.

    The power supply on the System 100 is a slightly odd +14V/-14V. I use an older version of the Oakley power supply board for all my testing – it’s designed for 15V operation but it seems happy enough wound down to 14V. It has a current limiter – the power lights dim on mine whenever I’ve done anything stupid so I know when to quickly unplug. It’s run off a hefty Yamaha PA-20 mixer power supply, which is a sealed box, so I avoid having to play with 240V mains electricity.

    It’s worth getting something reliable for your power supply ‘cos it can cause a lot of annoying problems if you’re using something iffy. I made a +12/-12V supply based on a 12V AC wall-wart (documented elsewhere here) and I never really liked it – the 808 bass drum circuit I made didn’t work so well off it.

    The stripboard size for the layout above was 100mm x 160mm, or near enough.

    I don’t know if anyone’s selling System 100 PCBs; I’m not sure Pharmasonic are still in business, and I don’t think they’re generally recommended at the moment because of some customers not receiving their goods. The version that I made has some errors on it that need fairly brutal hacking, and I’ve not got round to correcting them.

    Like I say, if you want to make it a lot easier on yourself you could just go with one of the Eurorack System 100M clone VCOs, like from Frequency Central. I just spent five minutes playing my old Curetronic 100M VCO clones through my original System 100, and then flipping back to the System 100’s VCO and it’s hard to tell any difference. Also they’re going to work on +12V/-12V power supplies.

  18. 27th March 2024Stacey says:

    Aside from the capacitors you have marked as electrolytic is it safe to assume the remaining are all ceramic caps? If not, which of the remaining are a type other than ceramic (tantulum, polyester?)?
    Are some substituted with ceramics in your build?

  19. 27th March 2024Stacey says:

    Regarding the DIYLC file and the yellow inputs on the left of the board–when they specify pins I assume these are the pins to potentiometers where pin 2 is the middle/wiper pin, correct? I know you mentioned, in a response to a previous comment, that yellow wire 31 is for the fine freq tune and it goes to the the middle/wiper pin off a pot (pin2?) with it’s outer two pins (1 or 3?) connected to +14v and -14v. Is the case the same with yellow wire 37 connected to “PWM manual min set” or does this refer to something else on the original front panel (e.g., ADSR | Manual | LFO switch) or does it simply connect to another multi-turn trim VR used to adjust something like the min[imum] pulse width?

    The schematics for the System 100 VCO I’ve looked at online appear to be the same one listed here which doesn’t show the complete connection to the pots, jacks and switches on the front panel. Are you aware of any documentation that shows these connections or at least lists the pot values and types of pots used (e.g., linear or exp.)? (For the jack input like LFO, S&H, and sync I just assume the yellow wires go to the tip and then their sleeves will be connected to the ground of this circuit.)

    From the schematic I’ve been able to correlate most of the circled number I/Os to the left-side yellow wires on the DIYLC diagram; but there are two I/O wires on the schematic I haven’t been able to identify (nor locate their counterparts in the DIYLC). They are lines (33) and (42). Do you happen to know what they connect to and/or where I might find them on the DIYLC board?

    In the DIYLC file there is also a “that300p” part and section. What is that section about? (And I assume the 2 quantity on the ua726 is just indicated that there are 2 transistors in the metal package even though there is only 1 9-pin ua726 used in this circuit.)

    Lastly, in the bill of materials I see there are 3 D204s listed yet I can only see one (axially mounted) in the DIYLC diagram of the board. Is this just a typo or are there 7 total, instead of just 5, 1s2473(/1N4148) diodes?

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