I've actually got my d-85 apart for the same problem at the moment. I've uploaded a video on youtube describing how to gain access to the circuit boards.
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My d-85 has the same problem atm plus a few others. I sent it away to a technician who after replacing a few ic's was unable to fix the problem and now the upper flutes have stopped working.
So i'm now trying to see what i can learn and maybe do without damaging it.
So far i've learned that many of the IC's in this era of yamaha organ have build defects and most can be replaced by new versions, except for the yamaha ic's which you can only salvage off wrecked organs. Also alot of the capacitors apparently were not designed to last more then 15 years.
Since i'm am not a technician and only have a very basic knowlage of electronics I am just going to continue researching what i can before I have a go at swapping out all replacable end of life components.
I beleave parts of the organ have to be recalibrated when new components are put in.
I'm actually thinking about doing an electronics course just because of this organ.