Tempco measurements on ASM-1

[Joachim Verghese]

Juergen wrote:

> my own design goal often was to make the tuning
> procedure as easy as possible, so my idea was setting the
> 0V point of the transistor pair to the lowest note on the keyboard
> (CV = 0V); then the frequency offset and scale trimmers would
> work independently. Thank God that I never actually built it that
> way, 'cause with this method I would have chosed the worst possible
> point in terms of drift !

As I mentioned before (I think), the Chroma VCO was built like
this, with the "zero point" at one end of the scale (24 kHz),
and that this was (apparently) necessary for the SW-controlled
calibration to function properly. In a case like this it is
very important to have good thermal contact between the expo
transistors and the tempco resistor, as the tempco resistor
will have a substantial effect at the "other" end of the scale.
And so it is not surprising that in the Chroma the tempcos
were glued (or fixed with heatsink compound) onto the 3086 arrays.
 
> So if I get you right, we should put the 0V point somewere in the range
> of the highest note we would normally play on the keyboard, as
> the beat rate decreases with lower frequencies, but not much higher than
> that point. (?)

This sounds like a good method -- or place the zero point
approximately half-way through the most usable range, i.e.
1 kHz for a 100 Hz - 10 kHz range.

If you look at the Oberheim SEM VCO, you'll find that the
zero point is at around 1-2 kHz, and this VCO is known to be
quite stable even without the use of a tempco resistor.

All of this made me think -- I've previously claimed that the
transistors of an expo converter need not be Vbe-matched, and
while I still stand by that claim, there is one advantage to
Vbe-matching -- you'll have better control of where the
aforementioned "zero point" falls.

> JH. (still learning)

Yes, me too!

-joachim