Fan problem with DEC H7822 power supply in MicroVAX 3100

[Peter Coghlan]

Jon Elson wrote:
> 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.
> 

I was thinking about wiring them to 5V because it was only marginally
overheating after running for a long time.  However, I started
poking around with the multimeter and discovered a low resistance
across one of the connectors for the fans.  This led me to a
1N759A 12V 400mW zener diode which read about 20 Ohms in both
directions.  Looking at the H7821, there was a 1N4742 12V 1W zener
diode in a similar position.  It had much more plausable readings
so I borrowed it and fitted it to the H7822 in place of the dud
1N759A.  The fans are spinning nicely now with about 7.5 to 8V
across each one.  This was a lot easier than I was expecting :-)

>
> Jon
>