28 Sep
2014
28 Sep
'14
1:11 a.m.
-----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:
out
On Fri, 26 Sep 2014, Tothwolf wrote:
I had a couple of these PSUs within easy reach. The machine this one came 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. 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?
Thanks
Rob