[sdiy] MOTM300 is an interesting design

[Paul Schreiber]

a) The core uses a *descending* sawtooth, so it is sort of 'wired backwards' 
from normal. Instead of sourcing current into an integrator, we are pulling 
current out (as an old prof once said, "this design sucks, not blows!")

b) the BJT discharge transistor is used as a 'trick'. I'm not going to divulge 
just how, but again you have to look closely on how it's wired (the fact it's a 
PNP is a clue).

c) the zero-offset FET buffer was indeed used in the Rossum 1200 VCO module from 
the 1970s. This isolates the high input bias current of the LM319 from the 
expopnential converter. One can also use a *very good* op amp follower here. 
This design is almost 6 years old: the dual FET was cheaper than op amps at that 
time. Today, there are several that are comparable.

d) Yes, it is necessary to *really clean* the flux off the VCO. Even though the 
instructions say to run water over *both sides* of the board, I have seen cases 
where the water was only on the solder side, and the organic flux was left all 
over the top (component) side. People still think it's incorrect to run water 
over electronic parts. Heck, my daughter spilled a full glass of orange jucie 
into my new Toshiba TV. I opened the case outside and used a garden hose under 
high pressure and washed it all out. Let it dry 4 days and it worked for 11 
years after that.

Paul S.