H720 PSU?

Ethan Dicks ethan.dicks at gmail.com
Wed Apr 15 15:28:11 CDT 2009

On Wed, Apr 15, 2009 at 12:25 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
>> I've found pictures on the 'net of the fans - looking at the wiring,
>> they appear to me to be pretty standard 110VAC boxer fans.  ISTR
>> hearing that the fans in the 11/40 or 11/45 are bizarre, but I'm
> The fans in an 11/45 are plain 110V ones (even in the UK, they're run off
> the primary of the mains transformer acting as an autotransformer). I
> suspect the 11/40 is the same.


> 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.

That's the one.  I guess I should have paid more attention - I have an
11/44, but I've never fiddled with the fans.  I think I'll put "check
and clean 11/44 fans" on the list of summer classiccmp tasks.

>>  ... If I were going to go with MOS memory, I wouldn't worry about
>> what to use - the Unibus itself only needs to provide +5V, and +/-15V.
>>  Because the 11/20 routinely held core, it's expecting +8V (unreg)
>> plus -22V on top of the usual voltages.
> You may find that +8V is used eleewhere. I remember a +8V supply for the
> lamps on the KM11, and it may be used by the front panel lamps.

Good point.  I don't know the 11/20 inside and out, but I do know my
way around the PDP-8/e, and the lamps on the front panel don't run off
the +5V on the Omnibus (there's a PSU lead right to spade connectors
on the front panel).

OK.  So... that makes finding at least one H720 more important.

Thanks for the info.


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