Fan problem with DEC H7822 power supply in MicroVAX 3100

[Peter Coghlan]

> 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.
> 

It looks like the zener is connected between the adjust terminal
of a heatsink mounted LM337T adjustable negative voltage regulator
and the positive connection to one of the fans.  Also connected
to the adjust terminal is what looks like a thermistor mounted on
the heatsink of a MBR3045, presumably a switching transistor.
Perhaps the zener didn't end up carrying much current, however if
that is the case, it is a bit strange that it failed.

Regards,
Peter Coghlan.

>
> Jon
>