It samples! It holds! Here’s an aggravating demo.

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).

Another board done, onto the next…

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.

My re-do of the stripboarded 706 LFO I did last year, only this time I found something else to fret about.

Last time I was kvetching over the reset pulse; this time it was the fact that it drifts all over the place. With my test BC560C transistors as a replacement for the IT132 in the expo converter, after setting the LFO to an initial frequency of 44Hz it drifted to 33Hz over three minutes, when it finally stabilised.

Yep, it’s an analogue synth and I expected some general general temperature movement with a pair of unmatched transistors but… even so, that’s a bit drifty even for me. I went back to my stripboarded 706 and it did the same, so uhhhh… I just hadn’t noticed it previously.

Next I tried some Vbe matched BC557B transistors, and they were better, but still drifted 3Hz or so.

Micross in the UK distribute the Linear Systems version of the old IT132 dual PNP transistor, so I had that to fall back on, but as luck would have it I found someone selling some original ICs in the US.

They arrived yesterday to my pathetic joy – once I’d finished the near-endless pre-Christmas cleaning, I plugged in the new (very old) transistors to test.

When being set to 20.23Hz (swept down from the maximum preset frequency of 33.58Hz), after three minutes it had only drifted to 20.28Hz.

Trying again from the minimum frequency (got too bored to to count, least a minute per cycle) sweeping up to maximum, it hit 33.32Hz and drifted down to 33.30Hz.

There must be some property of the old IT132 that makes it so much more stable – by matching my BC557s I thought I’d removed that from the equation, but it’s possible that they weren’t matched closely enough.

Looking at other Roland schematics, they used the IT132 as the expo converter in the CV clock speed control input of the System 100 104 sequencer, but with an SDT-1000 thermistor to provide some sort of temperature compensation.

…obviously they didn’t think it was worth it in the 706.

I’ve got enough of the old transistors now to make a small pile of 706 LFOs, so I’m satisfied for now. I’ll have a go at the functionally similar 100m LFO at some stage for comparison.

Here’s the 706 schematic with the (minimal) corrections derived from the track layout.