Replacing failed powersupplies on qbus PDP-11s

[Tony Duell]

> 
> The part of this that really amazes me is where it says something like "long 
> power supply input lines" can also cause problems or oscillation.  I suppose 

Well, it's farily common for long (or otherwise high-inductance) power 
lines to cuase problems in some circumstances. That's why we use 
decoupling capacitors on logic boards :-)

This could be a problem with a DC-DC converter or similar (in other 
words, if you're using one of those, make sure you decouple the input 
side well). 

But it shouldn't be a problem with mains-input SMPSUs, at least not 
normal ones./ In the normal small SMPSUs, the chopper drive is not 
synchronised ot the mains frequency [1], so there's no way of the supply 
knowing waht the mains input will actually be when the chopper turns on. 
And thus the enrgy from the mains is stored in large (and low impedance) 
capactiros, the chopper effecrtively runs off that. So the HF component 
of the mains input current whould be prtty small, and thus the adition of 
mains filters or input wiring shouldn't be a problem

[1[ There heve been supplies using SCRs triggered at the right point in 
the mains cycle. They're not common, they put a lot of 'hash' back on the 
mains and are geenrally frowned upon.

-tony