13 Apr
2008
13 Apr
'08
9:51 a.m.
I have two Dell switchers out of P4 based 1U servers.
The first one has a failure in the startup circuit... pressing the power
button will
lead to a flash of the power light... and that's it.
The second one failed spectacularly....
There is a wire wound power resistor (5W15RJ) near the AC input... it
cooked...
Di U deduyce from that marking that it's a 15 Ohm resistor?
Is it possibly part of a soft-start circuit, to be shorted out by a triac
or similar once the supply has got going. If so, then if that fails and
the resistor is left in circuit, it will overheat.
There have been supplies where there's a startup resistor -- to power the
control circuitry until the rest of the supply gets going -- which would
overheat if said supply took too long to get goiog. The resistor simply
couldn't power the circuitry for long on its own. But I would expect such
a resistor to be a lot more than 15 ohms
and I do mean cooked. It came out in pieces (with
almost no effort),
melted the
nearby capacitor and a nearby relay. It also did a pretty good job of
puckering/burning
the PCB (but not bad enough it could not be used).
What is that relay connected to/used for? It doesn't short out the
reistor by any chance as part of a soft-start circuit, does it?
I suspected maybe shorted primary full wave rectifier... but I used the
DVM on
diode test and got what I'd expect.
Oddly, despite there being a fuse on the primary side... it's still
good... it wasn't
taken out.
That resistor got hot hot hot. Could that resistor have failed on it's
own ?
I suspect something further down the circuit must have a problem.
I'd be happy if I could make one power supply out of the two. These are not
simple supplies however. While much of the switching transistors,
regulators,
bridge rectifiers, capacitors and the usual fair in a switcher are
pretty readily
identified, there is a 6" x 1" "brain" board (I'll call it that
as it
has adjustment
pots, the ps fan circuit, quite a # of surface mount components on both
side of
it, and it interfaces with the main PCB with 34 connections.
Any recognisable chips on that board?
FWIW, there have been some very complicated SMPUs in classic machines.
The HP9845B, a complicated machine in all respsects, has a PSU on a
3-dimentional sculpture of PCBs, with 4 chopper transistors, 2 main
chopper transformes, and IIRC 17 ICs on the control PCB.
-tony