VAX 4000-500 PSU Overload?

Robert Jarratt robert.jarratt at
Sun Dec 20 17:31:02 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Paul
> Koning
> Sent: 20 December 2015 22:32
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> > On Dec 19, 2015, at 4:05 PM, Robert Jarratt
<robert.jarratt at>
> wrote:
> >
> > ...
> > I can't tell if this is a Kelvin connected one or not, although I
> > suspect it probably is. You can see a picture with two here:
> > These are from a second board in the PSU, but they look identical to
> > the one on the problem board. It would appear to have 4 connections,
> > but doesn't look anything like the only Kelvin connected one I could
> It does look like a Kelvin hookup.  Yes, usually they are two fat and two
> skinny connections, but so long as two connections carry the current while
> the other two do not, it's valid.

Thanks Paul, I have been assuming that it is Kelvin connected for the time

I have no idea if the resistor is faulty or if the fault is in the detection
logic. I am not entirely sure how to test it, and if it is faulty then
finding a replacement may be hard. If it is the detection logic, then I am
really in trouble because it is all SMD stuff, the board it is on appears to
have multiple layers and I can barely make out the markings on the
components, so I can't identify their type (resistor or capacitor) or value,
and I have never desoldered/resoldered SMD before. There is an op amp and a
comparator, which are easily identifiable, but there also appear to be some
transistors, and although I can see the markings "A4H" I have been unable to
identify what type they are, what the pinout is or if they are even
transistors at all (but I think they must be). Pictures of the logic board

I also noticed another very low value resistor, 0.01R, which looks far more
conventional (not kelvin connected for sure, it looks just like any ordinary
resistor, only a bit chunkier), but sadly, it seems to measure fine using my
ESR/Low Ohms meter, when measured in circuit.



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