Curiosity about power supplies (BA11)

Tony Duell ard at
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. 
Easily availalbe.

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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