In sci.electronics.design piglet wrote:
On 13/12/2020 3:52 pm, Don wrote:
In sci.electronics.design piglet wrote:
snip
My guess is it was some kind of analog synth VCO, inputs EFG are summed
and make the ramp, square and triwave outputs at jacks HDJ.
Dig the 3.6V D3 zener in the path, the 3.9Meg and 680R R19 R20 and the
kooky voltage follower Q3 Q11 Q4 - the unijunction Q2 is probably the
least nasty part of it all.
Thats why I used the word unsavoury!
Someone elsewhere mentioned how late designer John Simonton tried to use
as few parts as possible, presumably to keep the total price down. So my
earlier Woz analogy may be more spot on than first realized.
Woz's Breakout game design reportedly earned a bonus from Atari
because it kept its chip count below 120. Supposedly way down, somewhere
in the neighborhood of 50 chips, it's said. Legend has it although Atari
paid the bonus (to Steve Jobs) Atari didn't understand how Woz's circuit
worked.
Well in this design he didn't optimise enough. I can't see the need for
R14, but if needed for adjustment range R15 could be increased in
value to 200R. And R19 and R20 are providing an offset voltage of 1.5mV
but to a diff amp made from presumably unmatched and non-isothermal
Q6,Q7 and whose other input is derived from a vanilla zener and
follower. Even across minor room temperature changes the drift could be
horrible. I am baffled by that 1.5mV offset, bet you can just tie the
base to ground and save two resistors.
Also the polarity of C3 C4 C5 looks strange?
At some point there was a small ~ 1" x 4" scrap of paper included with
my documentation. It was an addendum, which noted how D4 was now 5.6 V
instead of 6.8 V. (A fact duly recorded on my paper schematic in ink,
but missing from the PDF, which was uploaded then shared with the
group.)
IIRC it also said Q6, Q7 were a matched pair. Of course, the scrap
of paper with irreplaceable information on it got misplaced.
Nonetheless, if you seek to match Q6 and Q7 to make the pair as
isothermal as possible, will this circuit do the job?
https://crcomp.net/paia/matcher.png
The D3 kludge is easy enough to grasp. But R19 and R20's eccentricity is
a little too far out for me. Simonton's next generation VCO removes some
unsavoriness:
https://crcomp.net/paia/4720.png
Danke,
--
Don, KB7RPU
There was a young lady named Bright Whose speed was far faster than light;
She set out one day In a relative way And returned on the previous night.