Hi. I have no solution for you but I am here to say that I had a quick look at the JUNO-106 service notes here (http://www.synfo.nl/servicemanuals/Roland/ROLAND_JUNO-106_SERVICE_NOTES_1st.pdf - great resource!). I am having a very similar problem with my JX-3P (see the latest JX-3P issue in this forum). Looking at the 106 service notes, the architecture is quite similar and the same chips are used in several places (eg. the demultiplexer). You can read in the issue about what I have done so far, but my limited electronics (repair) knowledge is holding me back; I have all the tools to test, but am not sure where and what to test.
As I mention in the issue, I have not found any broken tracks on any part of the circuit, so I am wondering if there is a problem with one of the ICs. And it certainly seems possible that this could be the same issue as mine…
The JX-3P issue: Roland JX3P - Cannot write sequences or new patches