Replacing failed powersupplies on qbus PDP-11s

Chuck Guzis cclist at
Fri Jan 30 13:47:56 CST 2009

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 

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.


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