Star Tech Journal

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STAR*TECH Journal
August 1992
~4.
Kerry Messana
Betson Advance
Syracuse
New York
Closer
Look
CHECKING AND CHANGING
SWITCHING POWER SUPPLIES
Is THE PowER SUPPLY BAD~'
A power supply can fail in several
different ways. It could lose one
or all of its voltages, become in-
termittent, have its voltage regu-
lation fail and cause the voltage
outputs to go out of tolerance or
even introduce excessive noise
into the printed circuit board
(Pcb).
The most common symptom is
loss of all output voltages. The
indications for this would he no
game board functions (video,
sound, etc ... ) but the monitor
powers up. Following will be
some procedures on checking
your P/S's health.
CHECKING YOUR SUPPLY
First you want to check in the
more traditional ways. Start by
taking an A/C measuremEmt of
the 117 volt input to the supply.
Once you have confirmed that the
voltage is there, you could go onto
the actual checking of the P/S.
Take your meter (preferably digi-
tal) and measure the DIC voltage
outputs at the power supply. Con-
firm that they are there and not
at too high a level. You could
expect to see the voltage at those
points to be higher than what is
called for at the logic board but
this is ok since it will drop some of
it due to resistance in the wiring
and connectors.
If the voltages are somewhat
close to what they should be at
the power supply it is time to
check them on the Pcb itself. The
voltages on the main Pcb should
be as close as possible to the val-
ues that are required by the Pcb.
If you find that the voltages are
much lower than the readings
you took at the power supply you
may be dropping voltage due to a
bad connection somewhere.
Check the connector where power
enters the Pcb. This is usually at
the edge connector or at a dedi-
cated power connector.
Check for burning of the pins/
connectors where power and
ground are supplied, if you find
these to be in poor condition then
replace them as needed. Also
make sure that +5 volt and
ground connections are fed
through more than one wire each.
If they go through one wire you
may want to double, triple or
even quadruple the wires up so
voltage drop is reduced to a mini-
mum. This will also prevent the
connector from burning up
(again?). This is less critical on
the + 12 volt line and even less on
the -5 volt line but the +5 volt and
ground connections should have
as little voltage drop through
them as possible.
ALTERNATE T'SHOOTING
At this time we are going to as-
sume that there are no shorts and
that the wiring is beefy enough to
handle the load. Now for the un-
conventional troubleshooting
techniques.
First thing you want to do is try
tapping on the power supply.
Give it a light tap and see if the