DIYing a Roland System 700

Roland System 700 outline diagram

Who wants to stripboard a Roland System 700? Your mum would’ve loved one for Christmas, I bet.

Given how rare they are – rumoured to 40 or 50 full sets ever made – there is zero chance of finding one for buttons in a car boot sale, or at the local auction house (“large home keyboard, only makes wobbly sounds“) or on ebay. The only chance I’ll get to have a go on one is to build one from scratch.

It’s ridiculous given that the 100m is similar-ish to the System 700, and I’d bet that Roland will do an emulation as a plugin for their System-1 or other new analogue modelling AIRA product this year. And looking at the front panel, there’s nothing really that unusual about any of the modules, but mostly I like the fact that it’s massive, there are sliders on the inputs, LED indicators for level (“is it making any sound?”) and that the modules are normalled to each other, making it possible to get sounds without having to trail patch leads everywhere.

I’m looking at just doing the main console initially – the sequencer might be a step too far. Forget about trying to Eurorack-ify everything, I’m just going to try and build it like the original, at the same size. I’m going to struggle with the sliders (50K dual audio for the 703 resonance pot?) but I’m going to attempt to keep it the same.

I’ve already had a crack at stripboarding the 706B VC-LFO and the 703B 2-pole HP/BP/LP filter which both work ok, write ups coming shortly, and I’ve just done my first proper PCB layout for the 704 VCA. Depending on how that goes, I might switch to doing layouts for the rest of the circuits.

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DIY stripboard Roland System 100 VCA

System 100 VCA Panel

Continuing my suddenly obsessive System 100 stripboarding with a boring bit, the voltage controlled amplifier.

System 100 102 VCA schematic

I wasn’t expecting this to be amazing sounding, being not much more than a CA3080, but I thought it was worth a go as I’m trying to replicate the original as far as I can.

Here’s the original VCA, with the blue marking on the 3080 presumably to show that it has been selected for low CV feed-through. I’ve seen circuits for matching OTAs, but I’m not clear how best to select for low CV bleed. Maybe it’s just a case of trying a bunch of 3080s to see which is the least clicky.

System 100 VCA circuitboard

I wanted to keep the mix input from the 102, so I built the thing around Q337. Also not having a TA7136 SIP opamp to hand,  I just subbed in the usual (non-inverting) TL072-based CGS DC mixer on the end, which was a mistake.

As I understand it now, the System 100 VCF inverts the signal from the VCO, and the VCA inverts it again, so it’s all back the right way up. As it stands, my layout inverts it twice, so input to the VCF eventually ends up inverted.

Incidentally I can’t imagine the TA7136 will make much difference, but now I’ve just had some arrive in the post, I’ll build a version with it.

To give an idea of my general confusion, I spent an evening wondering why the output from the VCF was crazily offset by a number of volts, before realising that this would be sorted out by the capacitor C324, which is on the VCA board. Just because it seems all nice and modularised doesn’t mean that it is.

Here’s a demo with the envelope on the System 100 101 keyboard modulating the VCA cutoff.

It gets clicky like my SH-5 with the envelope on a short release and the filter cutoff low, it might be that I need to find a better 3080.

One mod that might be interesting is to boost the input to the VCA from the VCF to see how it overdrives. Maybe messing with R378 on the input to the CA3080 might do the trick.

Here’s a stripboard layout for the inverting version using a TL071, any corrections or improvements joyfully received in the comments below, taaaa.

Roland System 100 VCA stripboard layout

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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.

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