H720 PSU?

Tony Duell ard at p850ug1.demon.co.uk
Thu Apr 16 13:44:59 CDT 2009

> > It's the 11/44 (a much later machine) that has odd fans -- 35V 70Hz or
> > something. They run off a transistorised full-H driver circuit in the
> > PSU, so they can be run off the backup battery.
> I have some power supplies -- out of an IBM storage setup or similar -- that 
> use fans that operate on an odd voltage like that.  I get the impression that 
> they chose that voltage as being the maximim input for some 3-terminal 
> regulator part on the other end.

The 11/44 PSU is complicated, and not pleasant to work on. It's actually 
3 SMPSUs running off one set of mains smoothing capacitors (a couple of 
coke-can sized things at one end of the chassis). 

One SMPSU is small and simple, and just powers the control circuitry for 
some of the other PSUs.

The second one provides the +5V (at 125A or something) and +/-15V for the 
CPU and Unibus boards. This one is only enabled when the machine is 
turned out.

The third one provides 36V. This is then regualted down (more switching 
regulators) to provide +/-5V and +/-12V for the RAM PCBs. It also powers 
the fan full-H driver. The resaon for this is that this 36V supply can be 
battery-backed (18 cells of lead-acid battery IIRC) to keep the memory 
contents if there's a power failure. And of course you want ot keep the 
fans running...

Some people would prefer normal 12V fans for this, but actually the 
special fans inthe 11/44 rarely fail, and the drier circuitry is a lot 
easier to repair than that built into most DC fans.


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