seven-hundred progress

System 700 LFO clone and a messy desk

Still going, just about. Not sure what I’m doing with that speed switch yet, whether to go faster or slower, hmm.

While I scratch my chin about how to reinforce these daft plastic panels, I’ve been enjoying reading through Fumitaka Ansai’s page on the System 700 –

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 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.

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100 vs 100M (clone) VCA and VCO

system 100 vs 100m clone fite

Prompted by a comment on my System 100 VCA stripboard post, for as much use as it is here’s a very quick comparison of the original System 100 VCA and a clone System 100M VCA, built on stripboard with a BA662 clone for the OTA.

No envelope tweaking, just a bit of VCF variation from CV2 on the MC-4, and then full-gas into whichever VCA.

Here’s the stripboard clone 100M VCA, behold:

100M clone stripboard VCA

This is the (non-clone) System 100-only demo:

And this is the System 100 played through the clone 100M VCA (set on linear response), using the System 100 envelope as a CV source.

The System 100 has a lower output envelope than the 100M, so the 100M VCA is super-quiet when triggered from the 100 when set to exponential response. Flipping the VCA to linear gives you a usable audio level to play with.

The VCOs in my 100M clone are a couple of Curetronic boards, both using a heated CA3046 in place of the ua726. I’m not convinced the scaling is totally right on either VCO, so try and ignore that. Here’s a quick test of the VCO waveforms played through the System 100 VCF and VCA. You’ll hear:

  1. System 100 square, low VCF, mid res
  2. 100M square, low VCF, mid res
  3. System 100 sawtooth, low VCF, mid res
  4. 100M sawtooth, low VCF, mid res
  5. System 100 PWM’d square, open VCF
  6. 100M square, open VCF
  7. System 100 sawtooth, open VCF
  8. 100M sawtooth, open VCF

and if you can take it, here are some sweeps, 100 then 100m, in the order

  1. sawtooth
  2. square
  3. PWM square
  4. triangle wave

That’s too much excitement for now, I’m going for a lie down.

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JX3P keyboard fixing

After a quick attempt to fix my Midiverb II again (and failing) I pulled the JX3P out again to have an attempt at fixing the dodgy octave.

Thinking that it was possibly the keyboard decoder, I replaced IC45, which took a fair amount of time to do it cleanly with the desoldering vacuum.

JX3P - removing IC45

While I’ve got the board out…

Crinkly JX3P circuit board

not sure I really like the look of these crinkly paths on the back. Anyway onwards.

Changing IC45 made zero difference, so it wasn’t that. And anyway, I could see the signal appearing on the connector on the keyboard. It took me a while to work out what was going on, and it definitely made it easier yanking the keys out.

JX3P keyboard diodes

This is part of the dodgy section of the keyboard, and it looks a lot like my unhappy Poly61M from the same time. The rubbery circular things are pressed down by the keys – inside the dome is a carbon disc that presses down onto the board and completes the circuit.

One side of the switch connects to the top of the diode below it, with the other half connecting to the decoder IC45 via the keyboard connector at the far side. Pressing the switch should make the signal from IC45 appear on the other side of the switch (the top of the diode).

I can’t believe you’ve got this far, but aaaaanyway, the signal wasn’t coming up on the diodes for the broken section.

The soldered track on the far right here should have given the game away

JX3P keyboard connector and track fix

…essentially the track had failed for some reason. Scratching out the soldermask from either side of the break and soldering a wire across it sorted it. It wasn’t actually super-obvious where the break was from just looking at the track, it took me a couple of test scratches to work it out.

There was also one intermittent key which just needed the rubber switches refitting, and that was sorted too.

I still have lots of screws missing (in the JX, that is…), and the transformer hums like an electricity substation, but at least it all works now.

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