Replacing failed powersupplies on qbus PDP-11s

Roy J. Tellason rtellason at
Fri Jan 30 15:41:03 CST 2009

On Friday 30 January 2009 02:47:56 pm Chuck Guzis wrote:
> On 30 Jan 2009 at 18:52, Tony Duell wrote:
> > The design _is_ something of a black art (in particular the design of the
> > transformer -- the exact characteristics of that are critical to the
> > design). But repairing an SMPSU that has worked (and in th case of a DEC
> > Q-bus box, has been made many time,s and has worked well for years in
> > general) is a lot easier. You know the basic deisgn ins sound, you know
> > that particular transformer can work in that circuit, and so on.
> While repair is interesting in its own right, it's the design that
> really shows the mind (or lack thereof) of the engineer.  For
> example, it never occurred to me that putting an EMI line filter in
> front of a SMPSU, while it may not disable, may compromise its
> operation.

Eh?  Why would it do that?

> How many engineers have simply spec-ed an off-the-shelf PSU and stuck
> it after one of those dandy little fuse-receptacle-power-switch-EMI
> filter boxes without thinking to ask the PSU manufacturer if it was
> advisable to do so?  It certainly wouldn't have occurred to me.

Me neither...

Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
Information is more dangerous than cannon to a society ruled by lies. --James 
M Dakin

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