Duodisk analog board troubleshooting

I'm trying to troubleshoot the analog board on a DuoDisk unit.  It's a 676-[]102.  Specifically, of of the two main jumpers (J3 and J4), J4 appears to be dead.  That is both disk drives spin when connected to J3, but neither do on J4, additionally, the LED is constant on for the J3 side, but not at all for the J4 side.

I've found some basic schematics that I'm slowly working through, so maybe I'll come across something eventually.  However, I wondered if maybe there was some better information out there than what I've found, which, at the moment, is a schematic for a 050-5014-D board, which appears largely similar at first glance...