Boss TW1 Auto Wah Oscillation Problem

[AM]

A friend gave me an old Boss TW1 Auto Wah. The pedal was left in a basement for years. I started fooling around with it and:
It oscillates even when bypassed. When the toggle switch is turned to the "up" position the oscillation is louder. I assume that the toggle positions imitate the down-up or up-down drive of a wah rocker pedal.
In both positions instead of wah sound there is a kind of high pitched (bird-like sounding) oscillation occurring when I hit the strings.
As I said, the oscillation is there even when the pedal is bypassed.
I would like to try to fix it but I have no idea what to do. Any idea where to start from?
Thanks in advance

Schematic link:
http://www.experimentalistsanonymous.com/diy/Schematics/Filters%20Wahs%20and%20VCFs/BOSS%20TW-1.jpg