Vintage Synth Repair

Check if the slider it's working, use a ohm meter to check, also you
can check the slider loading any patch and move the NOISE slider a
little and watch the display, if you see the DOT on the display the
slider works, the DOT means that the patch was edited, try it in
diffenrent positions.

Use the same procedure to check the HPF slider.

Hope this works for you.

Regards,