The protection circuit path traverses every board in there, so you'll not have any luck with any board out. IIRC, it's the protection circuit that provides the relay coil V. Simply been too long to offer much from memory, but I can dig out the schem for a refresh if need be. A couple of things come to mind offhand, the push-on connector on the driver boards can cause problems, a small transistor and diode on the upper half of the driver boards are repeat offenders, and the foil paths on the control board are prone to lifting as well as the board itself is easily cracked by unprepared disassembly ( hold the front securely, it's heavy, and if dropped jerking the wires it's a problem).

Tha's all I got from memory. Trace the relay coil circuit back all the way and you'll probably find it.