[sdiy] Not improving the 901A/901B

[Paul Schreiber]

DISCLAIMER

I've never tried myself to  improve these.

+++++++++++++++++++++++++++++++

Looking at the schematic, there is not much one can do
without "cheating". The culprit here is the mostly the 901A
Driver board.


The main problem with the "design" (remember, this is 1967!) is that
the exponential converter is done with individual, discrete transistors.

Unlike the 921 family, that uses transistor arrays in thermal contact
with a tempco, the lowly 901A uses 4 NPNs and a PNP "scattered" over
the pcb. And the temperature compensation consists of 2 CA3019 diode
arrays that again are not in thermal contact with Q6/Q7.

The second problem is that the CV summing amp is a discrete VCA (with fixed
gain)
which is also not temp "compensated" so even if the expo gen was working
fine, the
CV applied to the base of Q6 is jerking all over the map.

Hmmmm.......maybe Synthesis Technology has a new product
opportunity....hmmm..... :)

<Paul S>