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:
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…
…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.