Boss TW-1 Repair

[AM]

I tried to repair an old Boss TW1 Touch Wah today. The pedal was left in a basement for years. I tried to repair it some months ago, git bussy with work etc and only went back at it today.
There is signal going through in both active and bypass modes.
Now, the bad news: It oscillates. When the toggle switch is turned to the "up" position the oscillation is louder. Sound-wise there is practically no difference between the two toggle positions.
In both positions instead of a wah sound there is a kind of high pitched oscillation occurring when I hit the strings.
I adjusted the trim pot inside the pedal and I get a point (at extreme clockwise position) where the oscillation doesn't occur any more. At this point I get the same sound one can get from a wah "parked" at the pedal down position. Kind of a treble boosted sound. No filter sweep though, or at least not pronounced enough.
One thing I noticed is my pedal has a single op amp and not a pair as I see on the schematic below.

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

I took some measurements. I get a 0 mA measure on all lugs of Sensitivity knob. It seems there is a slight change in sound when I rotate the knob though. Any ideas? I would really like to resurrect the pedal.
Thanks in advance

[thedefog]

Do you have a scope? Because then you'd be able to pinpoint that oscillation exactly. Other than that, from what you said and from looking at the schematic, I'd check your electrolytic caps. These are old pedals, so they may be dried up, especially if it hasn't been used since the 80's.

[AM]

Thanks for the reply. No I don't have a scope. Maybe I should change all polarized caps?

[AM]

No........It's not the electro caps  
I replaced them all. I still get the same whistle kind of noise and not any sweep. One weird thing. On one position of the toggle I get the whistle sound when I play and it's quiet when I stop. On the other position of the toggle the opposite occurs. I get the whistle sound when I don't play and it stops when I hit a note.   
I was really hoping that changing the caps would sort out the problem. any other ideas about what to do next?