System 700 711 output module clone-ish

I’ve had this board hanging around for…about three years? That’s how excited I am by it. This is an old photo, ‘cos it currently has about an inch of dust on it.

It’s all a bit kind of just OK – the phase shifter is alright, the spring reverb is nothing special (nowhere near as nice as the Tombola spring board) and the test oscillator is my own version and that sounds… acceptable, as far as a test oscillator goes.

For the spring, I replaced the TA7136AP with a stripboard adaptor using a TL071 and kept everything else the same, as far as possible.

There’s no specification for the reverb tank as far as I can see, so I can only guess Roland used the Z-3F tank they used in the Space Echo and System 100 103 mixer. Somewhere I’d seen suggested that the Accutronics 8AB2D1A would do the trick, so that’s what I’m using.

I did some underwhelming demos at the time – here’s a familiar-sounding bassline going through the spring:

And a blippy-ish loop, starting from dry:

For the phaser I roughly matched the 2SK30A FETs as far as I was able to using the method outlined by RG Keen in JFET Matching for Effects. They’re not super-cheap, so it’s expensive to buy loads just to match eight.

These demos might be an even greater test of your patience – hold on: here’s some twiddling with white noise:

And here’s an annoying loopy bassline, first with the resonance turned down, then turned up:

For the 1KHz and the A octave test oscillators I couldn’t find any of the large variable inductors, so I went with a couple of phase shift oscillators with a bit of extra simple RC filtering afterwards.

The octave divider chip is an LM3216 – at the time I designed the board, this was fairly rare (£12 from Little Diode), so to avoid using a sought-after IC for such an unexciting purpose I designed the octave divider around the CD4024.

I say “designed”, I probably nicked it from somewhere.

Here’s a sample of the 1KHz test tone – three seconds is as much as I can take

Looking at the spectrum analysis the odd harmonics are accentuated, so I guess it’s a bit squarer than it should be, but it’s OK.

The board still needs a front-panel, and I’ve spent way too long trying to work out whether it’s worth doing a PCB or just a stripboard for the mixer section.

Either way, it’s the summer and while it’s not piddling down or throwing a gale I’ll be out riding my bike.

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LED indicator noise in System 700

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:

CD4001AE in the 703E filter

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.

Mystery solved, put the kettle on.

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100M 140 envelope panel

100M 140 envelope clone

My clone-ish 100M panels were from Schaeffer using their UV print process. Not as good as screen printing but still really good.

Shame about the “env 1 out” on the second envelope out, could’ve been worse though. Would have liked to have it in grey but couldn’t see that as an option, so strangely it looks a bit 700-ish. And pretty much as big, which I realised later, wasn’t really the point of the 100M.

Oh well – it’s built now, and it’s bloody massive.

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