Here’s my stripboarded attempt at the Roland System 700 706B VC-LFO, which is the one without delay, mostly because I didn’t want to use up a CA3080 or have to finagle a 13700 for a substitution.

It’s based on a not-so-common N13T1 programmable unijunction transistor, which I subbed for a fairly similar 2N6027, and a pair of BC560C in for the dual PNP IT132 expo converter. Actually it turns out that Linear Systems now make this part (IT132 datasheet) but I wouldn’t think it critical. The rest is all the usual period 733/945/1458/2SK30AY.

I used Yves Usson’s redrawn schematic from his System 700 page. Most Roland service manuals are available in good quality scans but the available System 700 schematic is hard to read in places. The only difference from Yves’ drawing is that I’m using a 10K resistor at R61 instead of 470K, cos it’s pretty clear on the original schematic.

When flipping the switch for the first time, it didn’t quite work properly much to my slightly perverted joy, because I’ve come to like trying to work out why these things are broken. The sawtooth offset was slightly low, which led to the sawtooth and sine being mangled. The problem was tracked down to an uncut track (boring!).

While I was testing this I was running at +/-14v, initially intending to put it in my infinitely expanding System 100 expander clone. The voltage levels aren’t quite the same, I’m only getting 8.78v out of the saw rather than the specified 10v.

There’s also quite a spike in triangle and sine, presumably down to the usual reset glitch in the conversion from saw to tri.

It’s only audible at the top of a sweep of an audio oscillator as a slight tick – here’s a demo of it sweeping the System 100 VCO.

If we zoom in a bit on the waveform…

…we see it’s about 840mV high, and lasts for – well, I’m not sure where you would normally count to, but you can see from oscilloscope readout that I’ve measured the whole glitch to last about 100µs.

For a little while I wasn’t bothered about this, but then I got irritated by it. Jacking up the supply voltage to 15v didn’t solve the problem, and neither did using an original N13T1.

Reading around a bit, it turns out that parasitic capacitance is a feature of veroboard/stripboard.

“The stray capacitance can be much easier to deal with, but removing any unused copper can help. That said, …I built a prototype function generator on Veroboard, and couldn’t get the square-wave risetime below a few hundred nanoseconds… When we went [to] the PCB version, the risetime was too fast to be measured with a 20MHz scope!”

(from Bodge-o-tronic: Veroboard! thread on vintage-radio.net)

Following the advice on minimising capacitance problems I removed the flux (with a can of Maplin’s flux removal spray I had in the shed, and never used – whoops), moved the timing capacitor closer to the 2N6027 and cut a couple of unused tracks, which brought the glitch height down to 670mV. Better, but still not great.

Peering at the circuit, it seems C3 (470pF) is there to smooth out the glitch – adding a 1nF cap to ground from pin 3 of IC2 helped, but didn’t entirely get rid of it. Too big a cap would start to noticeably affect the waveform.

I’m wondering if re-doing this as a proper printed circuit board might improve matters, though I suspect it won’t remove the glitch entirely. I’m also wondering if stray capacitance was the thing that made my System 100 VCO build so treacherous.

Going back to looking at the System 100 LFO and MS20 LFO it seems they are tri-core, so don’t suffer the same problem (correct me if I’m wrong, I’m sure you will).

Anyway with all that in mind, here’s the layout from DIY-LC. I added a basic LED rate indicator that only shows one half of the waveform. Can’t imagine anyone will fancy building this but, just in case.