One important issue before we get futher: is the pitch bend also not working propertly? important to know, because the pitch bend and the sense slider exist nearly in the same circuit.
If the pitch bend is working well, then the following things can be cause the sense slider problem:
- Broken slider (but that is not the issue in your case)
- Loose contacts on the PCB around the slider, or the PCB paths of the power supply are broken. Measure the voltage on the slider, it must be 5 Volt.
- Defect resistor 100k, but this is really uncommon, because resistors do not fail very often. If you want to know this, measure the voltage on pin 2 of IC4.
- A defect multiplexer IC4 (HD14051B in the servicemanual, but it is just a ordinary 4051 cmos-device). Btw, it is better to also place a IC-socket when you replace a defect IC.
- Loose contacts or PCB paths to the selection input of the 4051, but since the pitch bend works in this case, this can not be the problem.
If the pitch bend is also not working, then, then the following things can be cause the problem:
- A defect multiplexer IC4 (HD14051B in the servicemanual, but it is just a ordinary 4051 cmos-device).
- Loose contacts or PCB paths to the selection input of the 4051.
- A defect part around the A/D comperator (the hybrid ADC) located on the mainboard. This A/D comperator is build around IC20 and IC4. Check pin 7 of IC4 and pin 7 of IC20 (with a oscilloscope, because this signal changes to fast for a ordinary multimeter), to determine where the fault could be.
