26 Sep
2014
26 Sep
'14
12:29 a.m.
-----Original Message-----
From: cctalk-bounces at classiccmp.org [mailto:cctalk-bounces at classiccmp.org]
On Behalf Of Tothwolf
Sent: 26 September 2014 01:42
To: General Discussion: On-Topic and Off-Topic Posts
Subject: RE: Value of a Tantalum Capacitor
On Fri, 26 Sep 2014, Robert Jarratt wrote:
Comparing a known working PSU would be great. The back of the PSU has two connectors. One
has two sets of three power sockets, and one has two sets of four power sockets. It is the
one with two sets of three that seems to have a short (when powered off and the
subassembly carrying the connectors removed from the PSU). Looking at the connector with
the component side uppermost it is the set of sockets on the left that seems to be the
problem. The two leftmost sockets appear to be shorted to the rightmost of the three
sockets. Looking at the backplane, the markings say that +12V is shorted to GND.
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? From what I can see the components that connect the
tracks between these sockets are: three capacitors (two ceramics and one small aluminium
electrolytic), three 2W resistors, and the large Sprague capacitor (although it is hard to
see this for sure). I am not sure why there would be resistors across the 12V output,
unless it was to provide a dummy load to allow the PSU to work when removed?
If you take the board out you I hope you will see that it is very hard to remove the
heatsink to be able to see the tracks.
Regards
Rob