We’re into the area of circuits that I should have probably just built on stripboards, but anyway.
IC1 is a TA7504M in the service manual, I just used a 741. I even replicated the legs-in-the-air arrangement for the junction of Q3/C9/IC2 pin 3 as detailed in the service manual, presumably meant to minimise leakage.
The schematic in the service manual didn’t need any corrections, it works fine as it is.
Just now looking at the service manuals I realise the 100m version looks more fun as it includes a slew – if you were looking for one to do, maybe try that one.
I included the 716 mixer as on the original, but I’ve not got round to making that up yet (probably ‘cos it’s not going to be very exciting).
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.
The original envelope on the System-100 keyboard wasn’t really one of my favourite things about it. There’s precious little resolution on the decay slider for tuning in those tight bonky basslines, so I tend to end up moving the slider by infinitesimally smaller amounts to try and get the sound I want.
I should point out that it’s a different circuit to the possibly legendary System 100m ADSR envelope (“the snappiest adsr eg of the world…”). For once the hype is deserved, my stripboard 100m 140 clone felt super-snappy, with satisfying control ranges.
Let’s build the System-100 version anyway, maybe we can use different potentiometers or swap the timing capacitor size for something a bit smaller.
The rare-ish programmable unijunction transistor N13T1 was swapped for a 2N6027, although it has since been possible to get hold of the originals on eBay. The 700 LFO worked better with the N13T1, so that might be case here. I used 2SA733 and 2SC945 transistors as per the schematic, albeit the -GR variant rather than -Q, with 1N4148 diodes standing in for the 1S2473s that are splattered throughout.
The original never quite fully opened the VCF so I’ve added a single opamp to optionally boost the level from 6v to 10v.
Here’s a video of the envelope in action – I left the text on there to keep the camera from trying to going apeshit trying to autofocus on the trace. Not being a storage oscilloscope makes it trickier to track slow moving signals but you get the idea.
In this test I’m using a 3.3uF tantalum for the timing capacitor as standard but smaller pots than the original: 500k for the attack, 100k for the decay (which is too small, really), and 500k for the release, all audio taper. At about 0:43 the sustain pot is turned to 100% which causes the voltage to ramp up slightly rather than staying level. I’ve not looked very hard into fixing this – I’ve just been turning it up to just below maximum.
Here’s the layout and the DIY LC file (without the 6v to 10v boost) should you feel in the mood to torture yourself with some stripboarding.
Having been through all that, if you want tight and snappy then the 100m envelope is probably a better bet.
Due to other things (work, house move, more work…) the System 700 envelope PCBs I made have been left untouched in a box since March, I’m looking forward to getting those going and comparing them to the System 100 and 100m… eventually.