VAX 11/750 power supply fault

[dwight elvey]

> From: ard at p850ug1.demon.co.uk
>
>> People have been talked through some tricky procedures on this
>> list ... For that matter this is maybe the 3rd VAX-11/750 with
>> the "REG FAIL" fault and I don't recall ever hearing of one
>> getting fixed. Someone has to be the first :-)
>
> Unforutnately (?) I've always kept well clear of the 11/750. I really
> wouldn't want to maitain that CPU....
>
> But I can be eaily persuaded to look at the PSU schematics sometime and
> see if I spot anything obvious. Or at least tlak you through the design
> (which might be somewhat unconventional if other DEC supplies are
> anything to go by [1])
>
> [1] This might not be a problem for you. If oyu don't have any experience
> of SMPSUs, any one is as good/bad as another to get started with. The
> problems generally come if you have fixed ideas ofg how supplies should
> work and then you get something strange thrown at you.
>
Hi
 One of the trickiest things about switchers is that they often
use the main output to provide the rails for the regulation feedback.
 This causes a chicken/egg problem. We can see that there is
no output so no feedback to tell the high side to deliver more
voltage ( had just such a problem on my Olivetti M20 supply ).
 These circuits often have some way to kick start the supply
by pulsing the high side once on powerup, delivering just enough
current to the regulator feed back to get the hole thing started.
 This type of circuit is unlikely on a 11/750 ( without actually
looking at the supply schematic ). They most likely have a separate
transformer and analog supply to power the regulator feedback.
 I was just showing that these can be tricky compared to
analog supplies. The problem was located in the feedback.
Dwight

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