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.

9 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.

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