-----Original Message-----
From: cctalk-bounces at
classiccmp.org [mailto:cctalk-bounces at
classiccmp.org]
On Behalf Of Robert Jarratt
Sent: 28 September 2014 09:11
To: 'General Discussion: On-Topic and Off-Topic Posts'
Subject: RE: Value of a Tantalum Capacitor
-----Original Message-----
From: cctalk-bounces at
classiccmp.org
[mailto:cctalk-bounces at
classiccmp.org]
On Behalf Of Tothwolf
Sent: 26 September 2014 21:08
To: General Discussion: On-Topic and Off-Topic Posts
Subject: RE: Value of a Tantalum Capacitor
On Fri, 26 Sep 2014, Robert Jarratt wrote:
> On Fri, 26 Sep 2014, Tothwolf wrote:
>
>> 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.
> >
> > 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.
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.
I had a couple of these PSUs within easy reach. The machine this one
came
out
of hasn't been powered in a number of years,
but was working fine last
time it
was powered up. I have another identical PSU in a
companion expansion
chassis
which I can also pull if needed.
The board in question can be removed from the main PSU chassis without
desoldering any parts. There are 2 screws in opposite outer corners
plus 5 more accessible through holes in the board (only the 5 with
black plastic bushings, the others don't attach to the main heatsink
but just hold the semiconductors to the two aluminum plates), along
with the 4 shoulder
screws
holding the self-aligning connector board to the
rear of the PSU's
chassis. Once
those are removed / loosened, the board can be
lifted out, but be
careful
not to
tear the huge silpad (use a nylon spudger to
carefully separate it
from
the two
aluminum plates). You don't want to know what
a sheet of silpad
material
that
large would cost to replace...
Looking at the connector head-on from the mating side, the two
left-most
pins
are connected to '+', and the next pin to
the right, along with the
next
two pins
> towards the right on the other side of the connector are connected to
'-'.
The
pin furthest towards the right seems to be a
different supply or sense
line of
some sort. (Anyone have a pinout for a
H7874?)
Compensating for lead resistance, with a Fluke 177, I'm measuring 12.1
ohms
between the '+' and '-' contacts
on my PSU's board.
I have just replaced the broken capacitor and reassembled the PSU. I
attached
a couple of bulbs to the outputs of the PSU. One on
the 5V side, and one
on the
12V side. The 5V one lit up, but the 12V one did not,
and I tested the
bulb to
make sure it is working. So it definitely looks like
the 12V board is the
problem.
Any chance you could check whether the two PSUs you have are still
working?
I forgot to test the 3.3V output, so I just tested it now. I only have a 6V
bulb, but that did not appear to work either. Interestingly, when I use an
ohmmeter to measure the resistance across the outputs, the ones that don't
work measure about 12ohm, the one that does work measures about 770ohm
Regard
Rob