Here are some Infos about Troubleshooting
Schematic JP8 - didn't match for all functions
https://manuals.fdiskc.com/flat/Roland%20Jupiter-8%20Service%20Maunal.pdf
VCF
https://www.ericasynths.lv/media/AS662D.pdf
https://alfarzpp.lv/eng/sc/AS3109.pdf
Resonance failures:
works the resonance or only NOT the Self Oscillation ?
when the resonance is completely not working - the Controlboard can be involved in Troubleshooting - (connect scope on pin16 of as662 as described few lines on bottom)
at first check the 68K and 560R in the VCF Core around the AS3109, only when all resistors are correct works the Self Oscilallation.
involved are too: q4, R79 (12K next to the Reso Trimmer), as662D close to the q4, Trimmer 10K
you can connect a scope on pin16 of as662 (which is also connected to q4) - when you move the resonance slider, the voltage change between -14v range, the difference is 100mV by moving the Reso slider.
swap the AS3109 from a working card is an option too for testing.