Fan problem with DEC H7822 power supply in MicroVAX 3100

Jon Elson elson at
Mon Jul 20 10:03:09 CDT 2020

On 07/20/2020 07:00 AM, Peter Coghlan via cctalk wrote:
> 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.
Yes, it probably has a temperature sensor and a fan speed 
controller.  If you don't care about noise, you
could probably just rewire the fans to 12 V directly.


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