DIY Roland System 700 – stripboarding the 703b/c VCF

System 700 703b VCF stripboard from above

For the stripboard perverts, here is a version of the System 700 703a/b/c/d 12db state variable voltage controlled filter, which has high-pass, band-pass, and low-pass modes. It should use matched CA3080s, although I haven’t got round to that yet.

According to analogue-heaven lore, it was the only type of VCF included with the first System 700 models, with the 703e/f/g/h 24db low-pass added later.

I spent a fair while wondering why it was so horribly distorted, before I realised that R33 in the feedback loop should be 1K rather than 10K. Here’s a detail from Yves Usson’s photo of the board:

703VCF R33 resistor

…yes, I have spent a fair amount of time staring at pixels wondering if they’re red or brown, how exciting for me.

The other problem was the orientation of the stereo resonance pot. Having built it up as suggested by the schematic, the level into the filter gets boosted as you turned the resonance up, which makes sense, but it amplifies the input so much that the resonance struggles to be heard over the original signal. The System 100 VCF has a similar arrangement but boosts in a rather more subtle manner. Here’s the relevant bit of the schematic re-arranged a bit to fit into the page.

Resonance level boost

Flipping the wires round boosts signal as the resonance is turned down and lights the overdrive LED for a 10v peak-to-peak input; not the correct behaviour according to the manual, which states that the red LED should light for an input of 13v p-p.

For now I’ve just bridged the wires going to the boost side of the resonance pot. It’s possible there’s something wrong in another part of the circuit, given that I’ve laid the board out manually in DIYLC, but I’ve been over it several times and been unable to find anything different.

Before I do a PCB for this I’ll experiment to see if it’s worth adding a resistor across the boost side of the pot to tame the amplification somewhat. I’ve stared at Yves’ photo of the 703 trying to work out if there’s anything extra going on but to no avail.

When listening to the first demos for this on headphones I noticed some familiar fizzy distortion on the lowpass output, which turned out to be caused by the connected LED level indicator circuit, just as with the 704 VCA. I re-recorded the demo with the LED disconnected, but fixing this should just be a matter of adding the 1.5nF capacitor from BL+ (the positive supply to the U8) to ground, marked as C17 in the track layout in the service manual.

There are a couple of versions of the 12db 703, one with LM301s and (according to the schematic) 1.5nF mylar capacitors, and another with CA3140s and 470pF polystyrene caps. The 301s need compensation capacitors, and the 3140s don’t. I started out intending to build the LM301 version, but in the course of debugging the distortion I ended up swapping to the CA3140s (The LM301s should work fine, it wasn’t their fault).

Here’s a quick demo of the System 100 square wave into the 703 VCF – first is the bandpass, second is the lowpass, all sequenced by the MC-4, no VCA – these were done using 1.5nF mylar caps.

And here’s the sound of the different modes using the sawtooth from a different oscillator into the 703 VCF, with cutoff initially modulated by a dubious MFB Dual LFO and increasing amounts of resonance, and then some fiddling about with the CV from the MC-4 and the clone System 100 envelope, all with no VCA. They’re hardly the most musical of demos (all in the key of predictability) but it’s more to get an idea of the character of the filter. Warning: nasty high pitched resonant sounds!

First lowpass:

Possibly unpleasant highpass:

And the bandpass:

…not really your classic fruity Roland sound, but then you should expect that from the 12db-iness of it, and to my mind that’s a good thing to have a different flavour to the usual 24db sound. It sounds slower to react to the envelope than the 5-stage (30db?) System 100 VCF.

Here’s the layout out of DIYLC, including the C17 LED noise fix, but with the resonance pot boost as described in the schematic – should you build it you might need to experiment with this. I’ve been over it a few times checking for errors but it’s always possible I’ve missed something, let me know in the comments below if you spot anything.

Roland System 700 703b stripboard front

Roland System 700 703b VCF stripboard - back cuts only

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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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