Stare lovingly, desperately at the service manual, then wake up and overlay the parts on the tracks.

Draw out the circuit in Kicad from the publicly available schematics. It might be right this time, who knows.

Export the net list in Orcad2 format then convert with a homebrew Java program to OsmondPCB format.

Grep out the list of parts from the Orcad2 net list and choose footprints for each part (“DIP8”, “RCR7”, “TO-92” etc) and then import all that into OsmondPCB to lay it out, using the service manual track diagram as a template:

Use the track diagram to trace from to help eliminate any errors

Check there’s no expensive mistakes being made in the Gerber files using a handy online web tool.

…looks passable, anyway (shoulda annotated all the pins) – send it off to China, and then two or three weeks later –

and then nine months later, when I’ve finished doing all the not-so important things, like moving house

And it worked, first time. Scarcely believable.

The published schematic actually is correct, although only one of the two envelopes is drawn out. It uses an N13T1 programmable unijunction transistor, like the System 100 and SH-5, although I reckon the 2N6027 could be used instead.

The timing capacitor is specified as a 0.33uF tantalum with a 3.3uF cap hanging off a switch for longer times, and it does get pretty clicky and tight – here’s everything set to a minimum, rise time of about 600µs.

Shorting out R101 gets us even faster, but I don’t think there’s much point:

and it all completes in just over 2ms:

Here’s a dodgy demo with some mistakenly applied Quadraverb, all System 100 with the 705 clone envelope:

And it sounds ok, yunno, although I don’t think it’s quite as good as the 100m, but that’s only going by memories ‘cos my stripboard version is buried in the loft somewhere. I’ll dig it out soon and compare – now I’ve got an oscilloscope I’m interested to try and work out why the 100m envelope is so nice.

Either way, another one off the list. I’ve put a big, possibly too optimistic order in with the ever-reliable Futurlec for a load of 30mm slide pots in the hope of recreating the 700 panels, and then hopefully I can stop dragging crocodile clips across my desktop to test a board or two at a time.

Two weeks later and the VCO and ADSR boards have arrived from iTeadStudio, and they look great. Went for 1.6mm rather than 1.2mm thick boards this time, so they look a bit less luminously green:

The dual envelope board is the same size as the VCA, but the VCO board just seems massive at 15x15cm. Here they are next to a Doepfer SEM filter for comparison:

Nearly big enough to eat your dinner off. Once again nearly zero skill or invention involved, just shamelessly copying the same layout as the originals, only with any extra components that would have been bodged in on the originals added as standard.

Apart from the extra silkscreening and a few extra test holes for ground and power, the only other change is that I’ve included holes for a 15-way 0.1″ socket rather than the possibly unreliable edge connector from the original. I’m intending to use something from Molex KK range, if I only I can manage to order the right thing from Farnell.

I’ve started populating a couple of the VCO boards already, 100 resistors soldered last night, some way to go yet.