[sdiy] Boss PC-2 CT pot replacement

[Ben Barwise]

Hello,
For a project I have been working through and re-drawing old simple
drum synthesizers and making Strip-board layouts / PCB's for them and
I am going to be publishing it here
http://clacktronics.co.uk/information/drumlab

Because the PC-2 uses op-amps the envelope doesn't go to the negative
rail - it floats at about 1 volt- schematic is here

http://img821.imageshack.us/img821/4396/amdekpck100schematic.gif

what I have been trying to do is make a version of it so people can
build it with more widely available parts (replacing the BA662 with a
LM13700, I had to use a couple of LED's from its -Ve pin to 0v to make
sure it properly closes )

My problem is that I have been trying to find an alternative to the CT
potentiometer for sweep, the best solution I came up with was to use a
switch here is my progress ... bottom part is were I am at

http://img15.imageshack.us/img15/3672/pc2scem.jpg

but the problem is that when the sweep is off by putting the pot CCW
the pitch changes when you switch because of this offset on the
envelope.
I was wondering if anyone could give me tips on how I could null this
offset (Is it not possible??)

I have tried to put resistors everywhere but I cant seem to get it to work!

Regards
-Ben