Roland System 700 708 noise/ring modulator clone

Roland System 700 708 noise/ringmod

An oddly tantalising brown box arrived in the post just before Christmas including some boards and this one, my attempt at the System 700 708 noise/ring modulator.

Looking at the ring mod schematic it looks very similar to the System 100 (and 100m) version. Here’s the 1496 at the heart of it all:

System 700 ring mod - 1496

The only suspicious resistor is the highlighted R7, noted as 56k in the service manual, but peering at the photo on Yves Usson’s site it looks as if R6 and R7 are both the same, and R6 is 5.6k.

Given the sketchy nature of the 700 service manual I’ve been flipping back and forth between other Roland schematics to check for similarities, and the System 100 uses 5.6k too, so I’m convinced that the (not so tidy) 700 service manual is wrong here.

Here’s a selection of noises from a couple of System 100 oscillators bunged through the 700 ring mod, and then back through the System 100 filter and VCA – and as you’d expect it sounds a lot like the System 100. Can you sit through two minutes of ring modulated VCOs…?

And the noise is… noisy. Picking a 2SC828 at random it makes good noise type sounds. If anything it sounds brighter than my 1977 System 100, but that’s not unexpected.

Here’s the pink noise…

and this is the white noise:

Apart from the issue with R7 the schematic checks out.

Oh, and the TA7136. I’d ordered a bunch from somewhere before, putting them to one side ‘cos I knew I’d be using them at some point. Then I started on this System 700 silliness and noted that there was a few in there, so I ordered a bunch from Utsource, and oh dear…

TA7136 vs fake

…the working one is on the right, the fakes – which caused the power supply to curl up and die – are on the left. It’s hardly going to be important for this, given that 7136s are in reasonably short supply I’d probably just replace this with a 741 in an adaptor, or something similar.

Looking again at the 100m circuit I’d say if you’re were ever tempted to recreate the System 700, for the ring/noise (and probably the S&H too) it’d just be cheaper and easier just to buy Curetronic’s 100m clone and have done with it… that probably goes for a lot of the 700, to be honest.

I think I knew that at the start but the mania for cloning the thing has overtaken me, I’m beyond help now.

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2 comments

  1. 24th March 2019ChrisF says:

    I get that this is 2 years too late! Just came across your site looking for the elusive ua726, I’ve got some 700 work underway. Just wanted to confirm for you that on a 708A(149-055) R6 and R7 are 5.6k the PCB layouts in the Service Note also show that as correct, but if you are using the schems out there, they are quite poor quality, the originals are blue prints and after several generations of copying lots of these issues start to emerge, do you have other mysteries that need clearing up?

  2. 25th March 2019ua726 says:

    Excellent, thanks for confirming that! Sorry about the lack of actual ua726, maybe those chaps in Latvia will sort us out, they seem to be doing everything else.

    I’ve seen a couple of versions of the scans and I’ve been able to get all of it working so far, there’s only been a couple of questionable things.

    In my version of the 702, the sawtooth output pitch is fractionally (maybe less than a semitone) lower than the others. For now I’ve just bunged in an LM301 as a unity gain amp on a piece of stripboard on the end of it and called it a day. I’m using a CA3140 at IC3, as specified, and a 2N5485 for the FET – the NF510 being unobtainable. Every other Roland oscillator I’ve looked at has a separate buffer for the sawtooth out so I’m a bit mystified but not that bothered about it.

    The other thing is that the gain boost on the resonance pot of the 12db 703a/b/c VCF is odd – it gets dramatically louder when turning up the resonance, such that I wonder if there should be an extra resistor across the pot or something to limit it. I thought it was my misunderstanding of the pot wiring on the schematic, but a user of a popular Eurorack 700 filter clone reported the same behaviour over on electro-music.

    Everything else seems to behave itself, as least as far as I can tell, although I’ve still got a few things to build up to fully check yet (yes, even after all these years )

    In the things-I’d-like-for-Christmas-please-Santa pile, I’d quite like to see the tracks for the new version of the 700 VCA, which is this half-700/half-100M thing with a BA662 instead of the 3080, and I’ve read the second version of the envelope is more like the 100M too. Neither seem to be documented anywhere – at least that has fallen into public hands, anyway.

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