2n5163----------HELP

[jhaible]

> does anyone know of a source or substitute for the above FET as used in
the
> synthi AKS or VCS3 envelope shaper cct. it apparentley requires a "pinch
> off" voltage (Vp) of 2.5 to 3.0V.
> thanks in advance
> geoff james

I'd say that almost any small signal N channel JFET should work, if you
select it
for the desired threshold voltage.
In my Synthi Clone, I have used a BF245. Don't know if it fits into the
original
pcb layout; if not, there's also the BF244 which is the same FET but
different 
pin order.

While choosing a certain FET type is uncritical,
selecting the right threshold voltage is extremely important. The circuit
is very
sensitive here, because you want the full range of OFF-times, *and* a
region
where the trapezoid doesn't re-cycle, within the turn of one knob. 
It's a funny circuit, an RC filter to get the time constant, the filter
loaded with
another resistor, so when the potentiometer resistance is above a certain
value, the filter output doesn't reach the schmitt trigger threshold
anymore.
The said FET being the buffer stage between the RC filter and the schmitt
trigger.
So, the voltage drop across the FET is critical.
I'd advise you to buy a bunch of BF245's, make a coarse selction for those
between
2.5 and 3V threshold, and then try them one by one in the Synthi. (Use a
socket.)
Then see which one fits best.

JH.

PS: This circuit is *truely* sensitive. I had the trapezoid recycling all
the time, or
       for settings not much longer than a second, depending on room
temperature.
       The reason was the leakage current of an electrolytic capacitor. Put
in an MKT,
       and everything was fine.