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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9 comments

  1. 3rd January 2020kamiel says:

    Hi, I recently started building from this layout, and just noticed that the track above the one labeled gnd/ground, albeit having multiple green wires and numerous resistors connecting to it in a way that make it appear like it should be connected to ground as well. While it’s nice that the possibility of using a jumper, or perhaps even a switch, to be able to use both “configuration options”, i would prefer to know wheter these 2 tracks should be linked or not. 🙂

  2. 6th January 2020ua726 says:

    Yes, the green line above the marked ground line is ground as well, I admit it’s not obvious – if I can find the original file I’ll change it. I was going through a period of devoting two tracks to ground, which was probably a good thing. The green wires in my layouts do indicate a ground wire generally.

    Hope it works for you, although to be honest you might want to save yourself some bother and build one of the PCB clones that are now available.

  3. 7th February 2021Mike Fiction says:

    Did you ever figure out about the resonance volume boost. It sounds ok with it bridged but obviously the volume compensation is defeated entirely – sounds terrible unbridged…

    I suppose trying out resisters between rather than bridging it entirely is what I should try.

    also on mine the bandpass is a bit quieter than the HP or LP, did you see anything like that?

  4. 7th February 2021Mike Fiction says:

    Now that I’ve tested it further, I notice it’s actually entirely quieter than the 703E. I haven’t measured it, but it’s noticeable by ear. LP and HP are quieter, with BP even quieter still. I’ll have to play around. I didn’t match the CA3080s.

    This is why we make things right, fun times, lol

  5. 7th February 2021ua726 says:

    Hey Mike – thanks for your comments. Embarrassingly, I struggled with boxing my System 700 modules up (partly cos of the plastic front panels I had made), so I’d put it all together in a badly made temporary open rack for testing and then put it away right at the top of one of my shelves, waiting for the day for me to get back to it.

    Anyway, I got it down to check what I’d done with the 703bc module, and by the looks of it I just wired it up as mentioned in the service manual. And there is quite a large jump in volume when turning the resonance up; it’s probably not ideal, but I got bored of faffing. The band-pass is quieter than the high-pass and low-pass – I can see that this might be expected.

    Maybe we need to persuade some friendly System 700 owner into poking round the back of their 703bc.

    The comparative levels of each filter vary with resonance but they’re in the same ballpark. Dunno, they don’t seem so bad.

    After having a bit of a play with it (once I’d knocked the wires around a bit to get it back in tune) I concluded that it sounds absolutely awesome and I need to get it into service sooner rather than later. The 703bc is nicely vicious sounding. It’s odd that the 100M never included a multimode -12dB filter. Maybe I need to get the panels re-done in something more solid.

    (I should say also that I ended up making PCBs for everything – I didn’t trust my stripboarding in the end.)

  6. 24th December 2021Maik says:

    Hi there,

    and Merry Christmas 🙂
    I’m soldering the filter. I noticed that R50 exists twice! Is the R50 (1M) connected to R18 and R14 totally superfluous, or should it be occupied with something other than 1M? The other R50 is connected to IC8, which is ok ?!

    Regards

  7. 24th December 2021ua726 says:

    hello! merry christmas Maik

    …and good luck with this one; I had it working but I don’t think I was ever completely convinced by it, hence making a PCB for it.

    And sorry about the mistake; can’t remember if I mentioned this at the time but the R50 physically next to R16 and R17 should actually be a 1/2W 10M. Having said that it’s actually crossed out on the layout, and crossed out on the schematic with a *2, which suggests it was probably binned on the second version of the board.

    Hope that makes sense, and fingers crossed your board works

  8. 27th December 2021Maik says:

    Hi there,

    I finished building it! Nothing gets hot and no capacitor has exploded :).
    Everything works, unfortunately not as good as your audio demo, maybe something is wrong with my stripboard. I had tried it with LM301 with 15NF as with CA3140 with 470pf. The resonance and frequency do not work together harmoniously. The resonance quickly becomes shrill. Too bad the filter sounds otherwise really good.
    But C17 should be 15N (0.015uf) not as you wrote 1.5N, right? Which pins on the resonance pot should I bridge to tame the boost? In Japan under: “www.pharmasonic.jp” there is a dealer who sells the 703 as a PCB! do you have experience with the dealer, is he trustworthy? If you sell PCB yourself I would buy from you too.

    Thanks in advance

    Greetings Maik

  9. 27th December 2021ua726 says:

    Hello Maik

    Yeah, sorry – on re-reading it’s not terribly clear which pins I was bridging – essentially I think I just defeated the level boost as you increase the resonance, so res pot 1 pin 2 to ground. Having said that, I feel as though the way it’s set up (and the way I describe it), the way it is right now should actually damp the resonance somewhat as you turn the level up; in this case it was doing this too much, so the resonance never really poked out enough.

    C17 should be 15nF, yep. I discovered later that a 4001BE in the level detect subcircuit will induce noise in the rest of the circuit, so check you have a 4001UBE in there. Otherwise just pull it out, you don’t need it just to get the filter to work.

    I might’ve mentioned this elsewhere but I built up my circuit board version as mentioned in the manual, and it worked ok, but there’s a jump in volume when turning up the resonance, which isn’t that unexpected considering the schematic. Not having a real 700 handy it’s hard to tell if this is the way it should actually work though.

    On the Pharmasonic front, there’s just recently been a thread on the Analogue Heaven mailing list suggesting that people have not been receiving the things they ordered (including some expensive vintage items), so I can’t recommend them.

    The Frequency Central State 700 filter is apparently inspired by this version of the filter, but as far as I know there’s no published schematic for that one; people seem to like it though? That one at least has the benefit that it uses commonly available parts.

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