[sdiy] Roland SH-2000 VCO question

Adam Inglis 21pointy at tpg.com.au
Sat May 11 10:05:22 CEST 2019

Ack, I’m cross-eyed.
The keyboard is 3 octaves not 4. 
F0 is approx 1.5 kHz and F3 is approx 11kHz.
The usable range of the VCO is just over 5 octaves.
> On 10 May 2019, at 10:19 AM, Adam Inglis <21pointy at tpg.com.au> wrote:
>> On 1 May 2019, at 5:42 PM, Oakley Sound via Synth-diy <synth-diy at synth-diy.org <mailto: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...
> https://mezzoauto.blogspot.com/2019/05/roland-sh-2000.html <https://mezzoauto.blogspot.com/2019/05/roland-sh-2000.html>
> 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!
> cheers
> Adam
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