DIY parts

[terry michaels]

Hi Ian

I've got a few more thoughts on the KE4859.  Your suggestion of using a
MOSFET is interesting.  Actually, I hadn't even thought of it.  You're
right in that it typically has a low on resistance.  However, most MOSFET's
have a much greater gate capacitance.  When pulsed to discharge the timing
cap, this will dump a significant amount of charge into the integrator. 
The KE4859 has around 10 pf, which dumps a noticable amount of charge. 
That's one reason why the reset time on this VCO is longer than it should
be, and there are brief glitches on the ends of the sawtooth.

When I redo this circuit, I'll look at adding a small cap to dump an
opposite amount of charge to counter the gate capacitance.  I think it can
be done very simply,  I'll keep you posted.

Terry.