And it sounds alright. After I’d tried previously to get this going on a breadboard without success (and after my System 100 VCO woes) it was pretty satisfying to get it going so quickly.

As a bare minimum I found I needed something attached to the range input, or else I got nothing, or possibly just a subsonic rumble that I couldn’t hear.

Being a saw core, here’s the derived triangle with the reset glitch very apparent.

It doesn’t sound too fuzzy despite the glitch. Here’s the reset pulse when zoomed in. I had to take a photo to measure it ‘cos it was jumping about a lot, and my oscilloscope doesn’t do the storage thing, looks like about 2µs.

I’m blaming the pitch wandering on the thicket of wires and power cables taped under my desk. Fiddling with C5 might improve the reset speed, although possibly at the expense of stability. The SH-7 oscillator core looks similar to the 702 and uses a 10pF capacitor for reset pulses of less than 1µs.

Here’s the comparator/reset generator from the System 700 schematic:

Here’s the sine…

With the possibly suspect ua726, and a 10K resistor at R22 (as detailed in the parts layout rather than the 15K in the schematic), I found I had to effectively drop R9 to about 45K to give the width pot enough of a window to get it to track across the octaves.

My current test set-up doesn’t really lend itself to the greatest accuracy – which is a nice way of saying it’s a shit-tip – but after a lot of fiddling I got it to play acceptably in tune across between 0 and 5v, drifting significantly sharp at 6v.

Input voltage	Measured frequency (hz)	Ideal frequency (hz)	Error (cents)
0v	27.7	27.5	+12.5
1v	55.1	55.0	+3.1
2v	110.1	110.0	+1.6
3v	220.1	220.0	+0.8
4v	439.9	440.0	-0.3
5v	879.4	880.0	-1.2
6v	1771.5	1760.0	+11.2
7v	3538.1	3520.0	+8.8

…which apparently in the world of VCOs isn’t that great. I’d put a audio demo in here but it’d make your teeth go on edge.

It seemed odd that it drifted sharp higher up, if anything I would’ve thought the oscillator would go a bit flat.

Some later Roland oscillators swap the 3.3K resistor at R28 for a 4.7K trim pot (referred to as “linearity”), which I reckon compensates for the high frequency error. This is from the SH-09:

I think the 2N5484 I’ve subbed for the rare-as-hen’s-teeth NF510 is better (faster?), which means the 3.3K is compensating too much for the error.

Fixing this means either increasing R28 or perhaps switching the 2N5485 for a 2N4392, which the Jupiter 8 service manual recommends as the sub for the NF510. I need to fiddle further.

I’m having giddy dreams of building nine oscillators for the full ridiculous System 700 experience, but using ua726s isn’t realistic.

To that end I’ve long had a ‘726 replacement heated CA3046 planned out on stripboard, I just need to get round to building it. Annoyingly 3046s are getting harder to get hold of in DIP for now, so it’ll be cheaper to go surface-mount for the final PCB. I’ve never done much in the way of surface mount before so this’ll be another new thing to learn.

Going to need to hibernate for a month or so now while I do some other stuff, but I will return to System 700 fiddling.