One very well reviewed shop was unable to diagnose it, and recommended buying a different turntable instead. I took it to another equally well reviewed repair shop, and he was able to fix it! The motor control board needed re-soldered, and some wires with degrading insulation needed replacing. After that, he found that a rubber part of the bushing/oil situation around the spindle was swollen, so he fixed that and changed the lubricant to an apparently better kind.
I’m listening to it now with the mystery cartridge from our Pioneer PL-150, and while it’s an improvement, this thing is holding it back.
It came with a beautiful headshell and a Pickering XV15/625E that I will be getting a stylus for tomorrow, so once that’s on, balanced and broken in, we’ll have a way better idea of how much of an improvement this big beast is.
Now the 9090DB is reunited with the turntable that it came over with from Hong Kong in 1976.