VAX 4000-500 PSU Overload?

[Holm Tiffe]

Robert Jarratt wrote:

> 
> 
> > -----Original Message-----
> > From: cctalk [mailto:cctalk-bounces at classiccmp.org] 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 ntlworld.com>
> > 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:
> > http://1drv.ms/1micVN7.
> > > 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
> find.
> > 
> > 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
> being.
> 
> 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
> here: http://1drv.ms/1QT7641.
> 
> 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.
> 
> Regards
> 
> Rob

Just resolder the SMD Caps, they sometimes get a loose pad.
You see if that happened while resoldering.

Regards,

Holm

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