Curiosity about power supplies (BA11)
ard at p850ug1.demon.co.uk
Mon Mar 27 16:35:27 CST 2006
> Okay, so I was thinking, would it be possible to replace the regulators and
> whatnot in a BA11 with several standard PC power supplies? Obviously there
Firslty 'BA11' covers dozens of mounting boxes for PDP11 stuff, both
Unibus and Q-bus. And there were many different PSUs used with them
> would be adjustments that need to be made, but considering the DEC units are
> now 20+ years old and the out-of-service modules may have dried up caps,
> would this not be a possible option?
I fail to see why you want to do this. The DEC PSUs are conservatively
designed, well built, and properly documented. PSU PSUs are none of these
in my expserience. Most, if not all, compoents for the DEC supplies,
certainly those components that are likely to fail, are still available.
Now, I said earlier that there were many different PSUs. Most of the
older ones used the DEC 'Bricks' or similar circuitry. While these are
switch-mode units, there are 'friendly' switch-mode units. The input is
about 25V AC from a step-down isolating transformer. There is no mains or
rectified manis around. Yes theres enough power to burn the tip off your
screwdriver if you short the wrong thing out, or to make some of the
components explode (the latter is direct experience), but it's a lot
nicer than the rectified mains in a lot of later supplies.
There are exceptions. The supply in the 11/44 is a mains voltage
(on-line) swithcer. In fact it's a particularly complicated one. This is
not a supply that I like working on, but I've done it, and lived to tell
the tale. So it is _possible_. Just be aware that there are a couple of
coke-can size capacitors at the left side that charge up to 200V each and
are in sereis. Touch the output of that (and it's on barrier strip
terminals on top of some of the PSU boards) and you won't feel anything
ever again. Seriously!
But getting back to your suggestion. It probably is possible to
substitute PC power supplies, but it would be a lot less work to simply
repair the existing supply. Once you realise it's a swticher, and once
you find out what the 723 chip really is (read the data sheet, it must be
out there somewhere) and understand op-amps, then the design of the
'brick' is not hard to follow.
> Secondly, if I have less than half the BA11 box full of cards, do I need
> both of the fans plugged in?
Possibly not, but I've yet to find a computer that objects to
over-cooling... WHat's the reason for wanting to disconnect one of the fans?
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