Vintage Synth Repair

Hi Duncan,

Many thanks for your comments on my page; I'm glad you found it useful. The Ace Tone PS-1000 is indeed similar in many ways to the early Roland synths of the SH series, but are also different in fundamental design.

> all three of these early roland SH synths use a high-frequency VCO & divide-down circuitry, an idea that was common practice in electronic organs but (from a moog/arp perspective anyway) rather unusual in synths. this is why the SH synths, & your ace tone, have a single VCO with multiple "taps" in different octaves.
> 
> (I bet you can combine the octaves by simply forcing down two or more of those switches....)

The PS-1000 uses a more traditional VCO design than the SH divide-down circuitry, so unfortunately pressing down two switches doesn't give me two octaves. Basically, the octave switches connect to a parallel array of resistors, which alter the control voltage to the various frequency ranges needed for each octave. Pressing down more than one simply combines resistors in parallel, so the Roland SH-1000/2000 and PS-1000 differ in that aspect.

> I have noticed also that these SH synths track at somewhat more than 1V/octave. it varies... typically 1.2-1.5. I think the problem was in getting the maximum working linear range from a particular selection of components; the tuning procedure for the SH1000 is quite involved, & the carefully chosen transistors are in a metal enclosure for thermal stability. since none of them have cv/gate interfaces as standard, it wasn't an issue at the time. it will be when I hook up my 303 to one of the SH1000s.... :-)

My PS-1000 tracks at 3V/octave!! I was a bit surprised when I measured that, even wondering if something was wrong... But it works, and isn't meant for CV/gate, so I guess it's ok!

> I think the operating principle of these FETs was to present a high-impedance "barrier" to the capacitor, to prevent it discharging once the voltage was stored. so as long as the CV input of the oscillator itself isn't causing the voltage to wander, you might be able to get this part functional again with a simple switch, operated by the keyboard trigger, that allows key volts onto the capacitor as the key is pressed, then disconnects so that the only place for the capacitor to discharge is the CV input of the VCO, which should take a long time...
> if I were you, I'd look at stripping out anything that remains of the circuitry between the keyboard & the VCO, & replacing it with something more "modern", e.g. the design that's used in a rogue or a prodigy. it would work just as well even if the scaling is different, & you would be able to get the components you'd need to build a copy of the moog circuit reasonably easily.
> hth-
> duncan.
>

Thanks for your input on the missing components; with your feedback, and others too, I've got a much better idea now of what the circuit was, and how to restore/improve it. Next time I open up the PS-1000, I'll have a few things to try that could very well stabilize the keyboard hold.

J.F.