A number of these have come into the shop and for most of them it is a 80017a voice module issue. Roland put 3 chips (including the 3109) and supporting components onto a module, then covered it with a moisture sealing epoxy coating. There is a belief that the coating was required in order to protect surface mounted resistors from humidity change in order to stabilize their value, and/or protect their IP. This coating is thought to become conductive after 20 years or so. The only source of replacement parts is through chip pulls from scrap machines, and they are rare so costly. A typical symptom is a hanging voice, and/or crackling. I recently invested in a de-soldering station in order to get these chips out cleanly. The keyboard here was purchased on Craigslist by the customer for a good price but when he got it home he found it crackled badly, and a voice was hanging (you can isolate which of the 6 voices it is by putting the keyboard in test mode). For this unit it was not obvious which chip it was but careful testing showed it, and it could be verified with an oscilloscope. A replacement was obtained and fitted; all seemed well. After 3 hours of just sitting warming on the bench another chip started to crackle and had to be replaced. Speaking with the chip vendor this scenario is not unusual. The keyboard was not such a deal now that the cost of two replacement chips is factored in but the customer loves it and will get good use from it.
I have been using the acetone strip method to try and re-use a bad module before scouting a replacement. It takes 3 days of soaking and careful work to get residue out from between the chip pins. I silicone seal the module after washing it in order to seal the above mentioned resistors from humidity. This method has been pretty successful with crackly modules, but has not been good for completely dead ones, or where the VCF is drifting as we are looking at chip failures in those cases. It is a sad fact that all of these 800017a modules will eventually fail, and one unit eventually did have all 6 replaced. I have not tried the aftermarket replacements for the 800017a, but would advise folks to consider that. Also there are places that you can send the card and they will acetone strip all 6 modules; I’m not sure what the deal is when the strip does not work on a module though.
The bottom line is that if you are thinking of buying a Juno-106 then be aware of its issues and that they are not cheap to repair.