Regarding my earlier request for help repairing an H786 and H7861, it
occurred to me that I could instead use a desktop PC power supply, having
several in the junkbox.
It's even small enough to fit in there once the H786 is removed.
So I patched the wiring harness from the PC supply to the screw terminals on
the backplane... 5 and 12 volts at the terminals, Fault lights on the RL's
went out, but no green light on the KDF-11B.
After a look at the schematics, the simple solution was just to pull the
power monitor daughter-card from the H786, so the power OK lines (8640
buffer inputs, DC OK and Power OK) are not being pulled down. Green light
on, memory test ran, and boots RT-11 from RL02 :) A better fix will be to
drive the DCOK and POK lines from the PC supply, which appears to have a
"P.G." orange wire (I'm betting "Power Good" but will check it out first).
TSX-Plus won't run though, because there's no real-time clock. Looks like a
small transformer and a bit of buffer circuitry is needed to generate a 60
Hz TTL pulse. Or a dongle with a 555 or crystal oscillator. I may have a try
at fixing the original supply before going to the trouble (including
mounting the PC supply in the chassis and jumpering the two ribbon cables
together).
Speaking of trouble, today I learned that the "Restraining Cable Stud"
mentioned in the 11/23 user manual is not just there for decoration. For the
first time (ever), the BA-11S chassis overbalanced and fell out of the front
of the cabinet. It hit the floor with a crash while yanking all the ribbon
cables off (without breaking anything, incredibly, including a near miss to
my feet in socks!)
Perhaps installing that cable is a good idea ;)
-Charles
