25 Sep
2014
25 Sep
'14
5:41 p.m.
On Fri, 26 Sep 2014, Robert Jarratt wrote:
I am back home and have had another look at the
problem board. The
construction is such that it may prove really hard to get at all the
components, because some transistors are screwed to a large heatsink
plate that is on the non-component side of the board. I would have to
successfully desolder 4 of these to get the plate off to be able to
access the tracks so I can desolder other components. I suspect I would
actually have to cut the pins on the transistors to remove this plate.
Which outputs are measuring a short? I just pulled one of these PSUs from
one of my systems so let's see if I can follow along. I doubt that large
square Sprague capacitor would fail short. It looks to me like a custom
designed flat-style snap-in electrolytic capacitor. The failure mode for
electrolytics is usually high esr and open. Ceramic and especially
tantalum capacitors are much more likely to fail short.
From what I can see the possible remaining
shortable components appear
to be 3 parallel 150R 2W resistors and a large square Sprague 88D
capacitor (value hard to read, perhaps 6800uF, looks a bit like this
one: http://www.tedss.com/88D682M040BB). It should be possible for me
to lift one end of the 3 parallel resistors to test them for shorts,
but the big capacitor is going to be really hard to remove, for the
reasons mentioned above (ie I can?t reach the tracks on the back of the
board to desolder it).
How likely is it that this capacitor, which I believe is an aluminium
electrolytic, could have failed short?