[sdiy] Roland SH-2000 VCO question

Adam Inglis 21pointy at tpg.com.au
Fri May 10 02:19:51 CEST 2019

> On 1 May 2019, at 5:42 PM, Oakley Sound via Synth-diy <synth-diy at synth-diy.org> wrote:
> The actual VCO core is based around the SCR Q103. It's a linear VCO with the main timing cap C102. I believe the 555 is being used as a 'semi-fixed' monostable to produce an error CV, courtesy of IC101 pins 1, 2 & 3, that partly compensates for high frequency tracking droop and also allows some control over the overall pitch of the VCO core.

Many thanks for this Tony. I’m not familiar with SCR-based oscillators. I had a look around the net, and found a couple that indeed use a 555, but nothing really like this one. This is more complicated. I poked around with the scope to try to get an idea of what’s happening, I’ve put the new scope shots up here along with the VCO schematic...
The keyboard is 4 octave, F to F. At F0 the VCO is around 1.5 kHz, at F4 around 11 kHz.
You can see there is a big change to the mark-space ratio between these frequencies at pin 3 of the 555. This pulse output is then integrated by the first stage of IC 101, it would seem (see pic)… but I wonder what is happening at the second stage where the negative key CV is introduced?
Any further enlightenment from anyone will be greatly appreciated!

Just for the archives:
Using three stages of a quad op-amp, I’ve managed to get useful external control of this beast. 
1) Pin 8 , VCO board - inverted and scaled 8 volt CV (Hz/volt)
2) Pin 9 VCO board - 10 volt gate here manages to trigger the envelope for the VCA (on another board) as well as the the one for the PWM sweep.
3) Wiper of the Touch Sensitivity pot - gives access to the voltage buss  (0-9v) that can control volume, “wow”, “growl”, vibrato and pitch bend up or down. (Keep the pot above minimum to avoid grounding this voltage).
Roland has very thoughtfully provided a 3 position, 3 pole slide switch at the back (originally to attenuate output levels for external amplification) so this can be commandeered to switch between ext and internal control of the above.

In addition, putting the Volume (VCA control input) of the Touch effect on a switched jack allows separate control of this (range of about 10 volts) and compensates somewhat for the lack of any envelope adjustment. This gives a very useful dynamic range all the way to silence.

Having now got external CV control of the VCO, I see it will go all the way down to zero, so the usable range is more than 4 octaves. It never goes out of tune.
Ext control of the “growl” (VCF modulation by a 30 Hz LFO) allows creation of new metallic timbres I’ve never heard from this synth before.

> Regarding the ladder filter - only the early ones had the transistor ladder, most have the five stage diode ladder. Years ago I converted my old SH2000 to a transistor ladder and although, I thought at the time, most of the presets sounded better some of them most certainly didn't. The flute and bass guitar sounded better with the diode ladder.

Mine is a very early serial number, so presumably has the original filter. That “Bass” preset certainly means serious business!

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