PDP-8/E H724 power supply and serial interface

Pete Turnbull pete at dunnington.plus.com
Wed Nov 19 18:54:39 CST 2008

On 18/11/2008 02:12, Eric Smith wrote:
> Pete Turnbull wrote:
>> Part of the reason for not powering it up is that it's a 110V version, 
>> and here in the UK my mains is 240V.  Looking at the engineering 
>> drawings (on David Gesswein's site, and Highgate) for the H724 (110V) 
>> and H724A (240V) supplies, I see transformer taps shown for both 
>> voltages, and on the face of it, it should be very easy to convert. 

> If the print set shows the same part number for the transformer for both
> the H724 and H724A, it will be fine.  However, DEC sometimes used 
> transformers in 60 Hz gear that was undersized for 50 Hz, despite the 
> presence of appropriate taps for 240V mains.  IIRC, the 120V/60Hz RX01
> had this issue.

The scan of the 240V schematic isn't very clear but it certainly looks 
like the same part numbers.  Anyway, I took it apart this evening and 
after a bit off a tussle I managed to get at the primary terminals on 
the power transformer (the small one for the relay power is easy to 
see).  It did indeed have both taps, labelled as per the schematic. 
I've changed the mains input connections on both transformers (but not 
the 115V fan connection!) so as far as I can see, it should now be ready 
to go.

I've decided to be extra careful, and re-form the main electrolytics. 
So far the ones I've done look fine, but it'll be a while before I get 
them all done, as I have to do one at a time (more or less), and isolate 
each from the circuit.  The terminals are easy to get at but the wiring 
is tightly packed and I can't see the values so I'm not entirely sure 
which ones are which.  It would help to have a component layout drawing, 
which I imagine would be part of the engineering drawings set.  However, 
all I can find online are the schematics and layouts for the two little 
control boards.  Does anyone have the rest of the drawings?

Pete						Peter Turnbull
						Network Manager
						University of York

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