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.
