I’ve never been able to get the LED indicators to work well in any of the System 700 circuits I’d made. Even with no input, the green light would stay on and horrific screechy noise would get injected into the output.
Vowing to finally get it sorted out I connected up my 703E VCF clone, got the expected horrible noise and stuck my oscilloscope on the +14.4V LED power line. It showed that the power line was oscillating at about 32MHz, at an amplitude of about 200mV.
Then I tried some stuff like bunging different capacitors on the LED power line and different power supplies, but nothing did the trick.
Some of the schematics suggest that CD4001AE is the right version of the IC to use, although confusingly, not all of them. Also all the pictures of the boards I’d seen have AE or UBE versions of the 4001 in place. Found some old RCA AE variant chips on eBay and gave them a go:
DAMMIT IT WORKS.
I wasn’t keen on relying on 35+ year old CMOS chips, so I ordered a few 4001UBE, and they work too.
Assuming that Google Translate is accurate (my obsession with the System 700 isn’t quite going to stretch to learning Japanese) it’s interesting to find out that the original prototype had internal speakers, like the ARP2600! Also, it seems that users had been asking for a voltage-controlled envelope generator so they’d planned to release one but never got round to it, even in the later 100M.
I wonder if it was designed and never released? The owners manual for the 700 lists the modules, with gaps at 719, 722, and then 724 – 728, almost as if they intended to fill those gaps at some stage. Actually, maybe the prototype speakers were numbered 719.
Anyway, there are some fun demos on the anz.yokohama site too – if you have a look at the patch samples and then scroll down to Sample Chart 5, you can hear a nice example of using a voice to control a squealing oscillator with the 714 Interface. I’m half-tempted to build that one but it’s a big board, and I’m not sure if I’d just spend ten minutes laughing into it and then never touching it after.
Someone asked on electro-music about subbing a LM13700 for a couple of CA3080s in a Roland-style filter, so I thought I’d have a go. I levered out the 3080s in my 703e clone and wired up three LM13700s (two for the four poles, one-half for the resonance control) and to no-one’s great surprise, it works.
Here is some quick fiddling with an out-of-tune clone 700 oscillator into the 703e VCF.
It sounds a bit overdriven to me – I was trying to avoid any red lights, but being in a hurry to get something down it’s possible I got something wrong along the way.
Now I think about it, it could be that I didn’t recalibrate the DC offset trimmers from when it was set up with the 3080s.
The wires strewn all over the board probably aren’t helping either, so I’m tempted to build up another 703e and make some LM13700-to-CA3080 adaptors to test this further.