Yep, I've got the BC-20.
I have a manual for it to....um..here...somewhere.
I did try using a Potentiometer in place of the resistor to make an "adjustable" for it. But that didn't work at all!
Please forward me any info you can on making your own resistor setup for it.
For anyone without the manual, Mr Sharkey has a copy on his web page.
http://www.mrsharkey.com/SCT/bc20.pdf