Hi,
I have a Roland Juno-106s that had two voices not working properly due to faulty filter chips, the wave generating chips are fine.
I removed the epoxy from the 2 chips with acetone. Now one of the chips is working fine and the other one still not.
The curious thing is that the chip that working now doesn’t work in the slot of the other broken chip. So before ordering a replacement chip I wanted to find the other issue. I checked the capacitors and everything. I also calibrated everything with a multimeter and oscilloscope.
I cannot find why the slot is not working.
Any ideas?
You could check the pinouts of a working filter chip in another slot and compare it to a known working chip in the slot that’s not working. It could be something to do with the path of the VCF or VCA control signal (pins 6 and 11). If something has failed in that signal path it could be holding the filter or VCA all the way closed so nothing is getting through. Referring to the service manual, TR20 (for voice 1) is a likely suspect. You can check for a 0.6 V drop from base to emitter of the transistor to confirm if it is working.
The expoxy coating goes conductive over time. The same thing happened to ARPS causing them to die.
Synthchaser did a good youtube video on this subject some years ago, and usually has / sells / re-manufactures replacement parts. Contact him on how to best proceed with testing or repair.