VAX 11/750 power supply fault

[dwight elvey]

> From: ard at p850ug1.demon.co.uk
>>
> You might be confusing this issue with another one. That is, the power
> supply control circuitry needs to be powered. Once the supply is
> running, it; can be powered from one of the outputs of said supply, but
> at swich-on, the power supply outputs are all 0, to the power control
> circuitry is not powered, so the supply can't do anything, so the outputs
> remain at 0.
>
Hi Tony
 No, I'm not confused. If the feedback circuit isn't running, there is
no feedback. The feedback is such that it is positive at the interface
between the low and high side. This way a failure in the feedback
would bring the supply output to zero.
 To Make this work, there needs to be some way of getting things
started. There is a oneshot on the high side that gives the supply
a single kick on powering on. This also gets enough voltage to
start the overall feedback that is negative from the output to the high
side. It is just enough to blink some LEDs.
 A clever circuit but hard to trouble shoot. The first kick is
so short, it is hard to catch on a meter and distinguish between
lack of feedback and the over voltage protection( which also kills the
feedback at the interface ).
 I spent time analyzing the overvoltage parts before realizing
that it was a complete lack of feedback that was the problem
and not the over voltage protection. The problem traced to
an open ( poor solder ) inside the pulse transformer that
connected the low side to the high side and provided the
pulse width modulation for the supply.
 I'd call it a tricky circuit because as you seem to consider,
it goes against common sense in that no feedback is no
output. No feedback just means no pulse modulation and
hence no output.
 As I also stated, the DEC supply most likely used a second
powersource for the regulation feedback.
Dwight


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