Ongoing Prophet VS aftertouch repair story follow. Would appreciate any further advice on issues I raise below please...

Finally started fixing up the VS keys.

Investigations disclosed that there were breakages in the pressure sensor ribbon cables. Continuity was restored by exposing metallic conductor with scalpel blade and using silver paint.

Keyboard now generates aftertouch. Am getting somewhere...

However, I noticed there were sticky backed foam pads in the keybed installation groove, so someone has attempted to solve the infamous 'no aftertouch due to flexing case' issue.

At the moment, the keybed responds erratically with AT. Some areas generate signals, others don't without huge pressue, and some keys generate the equally infamous 'aftertouch always on' issue.

I have removed the old foam pads under keybed and replaced with sticky backed felt pad 'floor savers' that are usually found under furniture legs to stop scratches. That has worked very well, but still AT is intermittent.

I've tried using a piece of timber underneath to keep the case flat and reduce flex, to some benefit.

So, my VS aftertouch is suffering from combined issues of both keyboard case flex and sensor pressure issues.

1. Where is the AT trimmer to be found please? I can not locate on schematics. There are four potential trimmers; Offset / Scale on left hand pcb, and two other on far right
pcb (which have glue on top, to deter use)

Sheet D of the schematics shows the left pressure sensor circuit (and the two associated trimmers);

sheet E shows the right pressure sensor circuit.

The two trimmers involved are on the lower left corner of the main pcb.

The service manual should also contain ECR 738 and 739, which detail problems with the initial pressure circuit (12V across the sensors was too much; caused early burnout, separate from the cracking-plastic issue).  The fix involves adding a resistor divider to both sensor inputs to drop the 12v down to about 1.5V; also, the 2K feedback resistor on both opamps is changed to 15K.

In your case, it's possible you got one sensor working, but the other isn't -- this might cause the problems you describe.  Measure the voltage change under pressure on both.

I found a force-sensing resistor replacement from Jameco (http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2128260_-1) which will work, with some fiddling of the sensor placement and resistor values.