This was a pretty straightforward repair. The Juno 106 is one of those synths which tend to always have the same issues. One of the main ones is that the VCF/VCA voice chips, sometimes called voice cards, are very prone to failure. Each is a little tiny SMD board, encased in some kind of black plastic resin, mounted perpendicular to the main board. Their notes crackle, hang, and eventually stop working entirely. The black resin capacitively shorts across the little SMD circuit and can make it stop working either permanently or temporarily. This synth had two dead voices and I stripped the resin off each of the dead voice cards by soaking them in acetone. One worked at that point, the other did not and I had to get a replacement.
Service Tips: If three or more voice cards on a Juno have failed, I would insist on stripping and/or replacing all of them, not just the broken ones, because the others likely won’t be far behind. Even if they are working, I would still suggest stripping them because if you wait until they fail, they may not be redeemable at that point, as was the case with one of these.
After voice cards are stripped, I coat them with silicone conformal coating to protect them from moisture and oxidation.
When I dump factory patch presets onto a J106 via the tape interface using a file I downloaded onto my laptop, the only way I can get it to work is by running it through my mixer, turning the level and gain on the mixer WAY UP until it looks like it’s clipping, and turning the high frequency EQ way up and the low frequency way down. I’ve come across two versions of the patch dump audio file, one that works and one that never has for me… the one that works came from RetroSound.de, here.
Work Done: stripped and restored one voice card, replaced one voice card, recapped power supply and chorus/jack board, replaced the two MN3009 BBD (bucket brigade delay line) chips in the chorus section to make it less noisy, replaced patch memory battery, replaced most of the tactile switches under the buttons, cleaned switches and sliders and lubricated sliders with silicone, cleaned and treated buttons to improve color, replaced old crumbly slider shields with hand-cut felt sheets.