[sdiy] Roland SH-2000 VCO question

Adam Inglis 21pointy at tpg.com.au
Wed May 1 05:49:56 CEST 2019

I’ll cut to the chase for the busy people. I have a question about this VCO.
Could some clever type be so kind as to cast their eyes over this schematic of the VCO board, 


and explain in simple terms what is going on with the circuitry between pin 3 and pin 2 of the 555 chip? (See upper third of the graphic). The usual CV in pin for a 555 is number 5, and here it is still being used, but only for vibrato and pitch bend, not for key CV. I’m guessing the extra stuff is what makes this so useful as a music VCO?

many thanks

(further rambling follows…)
I’m in the process of looking for the best points to add external control, having now got it fully operational. 
Initially there were some baffling problems that had me chasing them from one PCB to the next…. of course, most of these disappeared when I finally noticed and replaced the dodgy electrolytics on the negative 15 volt supply. This synth needs a robust negative supply, so that all the FET switches (of which there are many) stay turned off when they should, otherwise all sorts of random stuff happens with footages, waveforms, noise, envelopes etc etc.

I’ve been poking around further with a scope and consulting the schematics, because I’m gobsmacked at just how good it sounds. Sure, it’s famous for it’s “borrowed” transistor ladder filter, however it possess lots of wonderful quirks.

The single VCO - how come no-one ever gushes about 555 timer chip based VCOs? Is this one unique? And those choppy approximations of various waveforms delivered by OR-ing a few diodes coming off the divider chip outputs, these crude waveforms sound so good! (OK, it’s probably that filter..)
Not to mention the envelope-shaped PWM, and the super-responsive keyboard with aftertouch… 

I’ve made some brief videos of some interesting presets, as shown on the scope. The upper trace is the audio output, the lower is the output of the waveshaper/divider board before the VCF/VCA.


The first one demonstrates 4 presets: Planet, Singing Voice, Fuzz Guitar 2 and 1. Note the PWM, in the first two it is provided by the dedicated “LFO 3” (not user-adjustable) and in the last two it is controlled by an attack envelope (again not adjustable).

The second video shows another 4 presets: Clarinet, Flute, Oboe and Bassoon. The “wobble” you see here isn’t PWM, it’s the vibrato. The oboe and bassoon, to my ears, are just gorgeous. One thing I’ve found with these and a few other striking presets - there’s quite a variation in the amplitude across the octaves.

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