Fan problem with DEC H7822 power supply in MicroVAX 3100

[Peter Coghlan]

I have a MicroVAX 3100 which has a H7822 power supply.  The power supply
and the machine itself mostly work (there is a problem with the SCSI
interfaces but that's another story) except that the two fans in the
power supply don't run.  If left on for a long time, the machine gets
too hot and a thermal trip operates, shutting it down.

The fans are DC 12V 0.2A and if I connect them to +5V or +12V, they
work fine and don't draw excessive current so there would seem to be
a problem with the section of the power supply which drives the fans.
Unfortunately, it's operation is not obvious and the power supply is
a pig to work on.  It consists of two double sided PCBs connected by
short leads and having live parts on both boards making it difficult
to get access to both sides of the board where the fan circuit is when
the power is on.

I don't have an identical working power supply to compare the faulty
one to but the fan circuit looks superficially similar to the one in
the H7821 which I do have working examples of so that may be a way
to proceed.

Does anyone have a service manual for the H7822 or H7821 or know how
the fan circuit is supposed to work in these power supplies?

Regards,
Peter Coghlan.