[sdiy] saw core VCO tuning/scaling (Roland SH-2)

[Magnus Danielson]

Adam,

On 20/08/11 10:07, Adam Inglis wrote:
> Thanks Magnus, Harry and list
>
> I've spent a few more hours looking at this..
> After a full calibration after 30 min warm up, VCO 1 tracks and tunes
> nicely with VCO 2. After 2 hours, the highest notes are still in tune,
> but the lowest are sharp by a full tone (e.g. from 175 hz to 198 hz).
> After 4 hours the highest notes are just slightly sharp, but the lowest
> are about the same, a full tone sharp.

Sounds pretty stable to me.

> DMM measurements at the output of the CV summing opamp show barely 20mV
> change over this time, at various octaves, and the voltage differences
> remain linear. The off-board trimmer in the feedback loop ("VCO width")
> and it's contacts are sound. The "Range" voltage divider and it's opamp
> scaler that connect to the summer show no variation in voltages hot or
> cold.

What about mechanical stability? Push and shake it around a bit. Both 
connectors and pot.

Sometimes you need to resolder things, since there can be a hidden creep 
so that a pin may be in a tube of solder and depending on temperature 
the contact varies. This is typical of physical stress and temperature 
differences. Not too obvious at times, but can show up for older equipment.

> The "temp adjust" pin, no.6, of the uA726 shows -6.75 V for VCO1, and
> -6.54 V for VCO 2. I haven't yet measured the current here - it's a bit
> of a fiddly old board, looms just soldered straight on, making access a
> bit tricky.

You have a resistor, measure the voltage over the resistor and the 
current comes out by dividing the voltage by the resistor value... no 
need to "cut in".

> Questions, given the above
> 1) Can I assume that the problem is NOT in or around the CV summing
> circuit, given the stability of it's voltages over time/heat?

Until you found your problem, you should not use strong words like that, 
you should just say that it is less likely. I've found both myself and 
fellow friends and colleagues to lock their mind up to early and not 
seeing the actual fault mechanisms. Instead they go into more and more 
obscure things rather than checking easy stuff like contact problems and 
power supply voltages.

> 2) Does the low frequency drift point to a typical problem?

The scale of the problem is fairly low, Could be many different sources.
Solder joints, connectors, bad cap, bad op-amp, bad pot.

Cheers,
Magnus