Fan problem with DEC H7822 power supply in MicroVAX 3100

Jon Elson elson at
Mon Jul 20 11:45:42 CDT 2020

On 07/20/2020 10:55 AM, Peter Coghlan via cctalk wrote:
> 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 :-)
Wow, lucky it didn't smoke anything.  there must be a series 
resistor somewhere that probably got pretty hot.


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