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1. How do I test a power supply while it is
out of the case?
Reader: How do I test a power supply
while it is out of the case?
Computer Doctor: That depends on what
you mean by test. No matter what, the power supply
will not run without a load. PC Power and Cooling
http://www.pcpowercooling.com makes an ATX
Power-Tester that supplies a load to one 5V line
and then lights an LED if the power_good signal
trips. This basically tells you whether the supply
is dead or not. To test the power supply thoroughly, you'd want
to load all of the 3.3V, 5V, 5VSB (standby) and
12V lines to the maximum ratings specified by the
power supply manufacturer. If you look back in the
6th edition of my book (you'll find the 6th
edition PDF file on the 12th ed. CDROM) on page
357 you'll find information on how to construct
power supply loads using common automotive bulbs
or heavy duty wirewound resistors.
2. Can the fan on a power supply be disabled
while my computer is running?
Reader: When using Windows 98's Power
Managment, my harddisk, monitor and CPU fan can be
turned off after a while but not the fan of the
power supply. Is there a way to turn off the power
supply's fan?
Computer Doctor: The power supply fan is
wired to run continuously; there is no way to turn
it off, unless you custom modify the unit (not
recommended) by adding a switch, or perhaps wire
up a thermostatic or other type of circuit to
control it. If you're concerned about noise, there
are power supplies that are specially designed
with extra-quiet fans. Check out the "Silencer"
series of supplies at
http://www.pcpowercooling.com.
3. How can I verify the good operation of a
power supply with a multimeter?
Reader: How can I verify the good
operation of a power supply with a multimeter?
Computer Doctor: By using a multimeter,
you can check the output voltage. For this to
work, the power supply must be connected to a
load, meaning a motherboard and at least one hard
drive. More of a load is better so it's best to
test the supply while fully installed in a system.
Use the meter to backprobe the connector terminals
and check voltages between the terminals and the
chassis ground. All voltages should be within plus
or minus 5% of the rated amount. Grounds should
register LESS than plus or minus 10 millivolts
(thousandths of a volt).
To properly verify functionality a power supply
manufacturing company uses a load tester which
could switch in different load amounts, and also a
scope would be used to verify the cleanliness of
the signals.
Lacking this type of equipment, the best test
device a technician can have besides a voltmeter
is a "known good" spare. That means a high quality
replacement supply you know works, and which can
be swapped in place of the suspected bad unit to
verify system operation.
Bad power can cause all kinds of symptoms,
which is why it is good to swap in a known good
spare if you suspect the power supply. Bad power
can make you think your processor, motherboard,
memory, drives, etc. are failing, when in reality
it was the power supply all along.
The power supply is by far the single most
failure prone component in a PC. It has probably
10 times the failure rate of most other
components, maybe more. I replace more power
supplies than all other components combined.
4. Can a bad power supply be the culprit for a
lot of otherwise unexplained problems?
Reader: I was reading in your book that
a cheap power supply can cause alot of problems
and I live in a house that is not wired to today's
wiring standards as far as grounding is concerned.
Could this cause all types of intermittent
problems, too? I also have many power drains due
to the wiring in- efficiencies.
Computer Doctor: It could be the cause of
the problems, but not if the errors you get are
repeated and consistent. I think you are having
other problems. Try removing items from your
system and reconfiguring until it becomes stable.
Try also playing with the timing, reset the CPU
speed to 200MHz for example and try slowing down
the memory timing. If these things help then you
have an unstable or bad motherboard or CPU.
5. If there are no markings on the connectors,
how do I determine how to plug them in?
Reader: How do I determine how to plug
the connections between the power suplly and the
motherboard? I have about the most generic of each
- a 'Slimline' power supply with 2 6-pin
connectors labelled P8 and P9, and 3 motherboards
between 386 and Pentium socket 7. Each board has
the 12 pins in a straight line, with no clue that
I can read as to which goes where. There’s no
keying either, apparently. I've tested each
connector, and they provide the voltages described
in the section on power supplies, but it seems to
me that connecting them the wrong way around would
be a good way to fry the system.
Computer Doctor: Ahhh yes, that keying was
eliminated by cheaper system mfrs. to save money.
The original IBM connectors were all keyed. The
8th edition has better diagrams for this, but I
think they were in the 6th ed. also. If you look
at the power supply pinout table, you will see
that the P8 and P8 connectors each have 6 pins,
each with two grounds. The connectors should be
plugged in such that the grounds are adjacent on
each of the connectors. "Black to Black" is the
rule. If you plug them in backwards, or off by
even one pin, you will destroy the motherboard the
second you turn on the power! This is one reason
the ATX boards are superior, they use a single
keyed power connector that you can't mess up.
6. How do I set the jumpers for my voltage
regulator?
Reader: I have purchased a voltage
regulator. When I started to install it, I noticed
two jumpers beside the CPU called the "CPU
Voltage Regulator Output Selectors" which can be
set to 3. 3 volts or 3. 5 volts (default). Now,
when I install the voltage regulator and CPU, what
do I do with the jumpers?
Computer Doctor: Those jumpers will become
ineffective when the VRM is installed. In other
words, it doesn't matter how they are set. When
the VRM is installed, it will be supplying the
dual plane voltage (2.8/3.3) to the CPU, not the
motherboard.
7. Could a black screen and a long beeping
noise at power up mean problems with the power
supply?
Reader: After reassembling my PC, I got
a black screen and the PC would make long beeping
sounds off and on (Beeeeeep-beeeeeep-beeeeeep-beeeeeep)
at startup. I switched it back off and checked all
the cables and they were all ok, so I turned it
back on and it worked properly. It did this a few
times. Could this have something to do with a bad
power supply?
Computer Doctor: Most standard PC power
supplies are terrible. Startup errors like this
are symptoms of substandard supplies. I recommend
Astec or PC Power and Cooling. You might consider
a high quality replacement from one of those
companies.
8. After installing a new HDD my computer, it
will restart seemingly at will, please help.
Reader: Recently I installed a second
HDD into my computer. After a week of usage, my
computer would randomly reboot, as though I was
pressing the reset button every now and then. What
could be the cause of this?
Computer Doctor: Normally, problems like
this are caused by overloaded or defective power
supplies. If that is indeed the case, then
replacement of the supply with a better (higher
output and more reliable) unit is what I
recommend. See
http://www.pcpowerandcooling.com for some of
the best power supplies in the business.
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