You don't care what could be reason for failure. That is
just wild speculation. First identify the defective part, and
only then fix it. Start with three possible reasons for
failure - power supply, motherboard, and switch.
First, motherboard control circuits may lock out to protect
from external power line events. The lockout is cleared by
removing power cord from wall - not just power off.
Identify the problem means an inexpensive and essential tool
is required - 3.5 digit multimeter. With power cord restored,
those motherboard control circuits require uninterrupted
power. Measure voltage on purple wire (other meter probe on
black ground wire) that connects power supply to motherboard.
Voltage should be within limits and not in lowest quarter of
those limits, as defined by chart in:
http://www.hardwaresite.net/faqpowersupply.html
If motherboard control circuits have power: motherboard
control circuit orders power supply on and off via green
wire. Power supply should be off when voltage is above 2.4
volts. Power supply on when voltage drops to below 0.7 volts
(when power switch is pressed). If motherboard control
circuit does not responde, then multimeter can verify that
power switch functions as a pushbutton (is not getting stuck
in the on position).
Next are the output voltages from power supply. Red (+5 V),
orange (+3.3 V), and yellow (+12 V) wires must see a rising
voltage. If any of these voltages is not available, then
power supply cuts off other voltages. IOW with multimeter,
monitor each voltage as power switch is pressed. Do they rise
up and meet limits in that above table?
Failure to maintain those voltages could be due to a
motherboard or other component short circuit, or because a
'bean counter' supply cannot provide sufficient power on one
voltage. Latter problem discussed in:
http://firingsquad.gamers.com/guides/power/default.asp
and so often demonstrated by:
http://www6.tomshardware.com/howto/02q4/021021/powersupplies-15.html
No simple away around using this 3.5 digit multimeter
especially when meter is so inexpensive and identifies
failures so quickly. Unfortunately many cannot be bothered to
learn why failures happen. They speculate. They shotgun:
keep replacing components until something works. They may
also recommend 400-500 watt supplies rather than first learn
why that $40 supply could not provide only 200+ watts that
most computers really need.
Select a power supply using a 'bean counter' mentality or
using value. Power supplies typically don't fail when value
is the criteria. But too many buy power supplies based only
upon one specification - price. Minimally acceptable power
supplies start at $80 retail (although some $80 supplies can
be found at discount).
Overseas suppliers have learned how to increase profits by
selling to bean counters those $40 supplies. Supplies that
are routinely missing essential functions such as overvoltage
protection and EMI/RFI filtering. IOW a power supply that
meets Intel requirements cannot damage other computer
components. Not true of too many $40 supplies that don't meet
those Intel specs. Furthermore, many 300 watt $40 supplies
cannot even meet that power which is why so many bean counters
recommend 400 and 500 watt supplies.
A bean counter's supply does not provide a long paper list
of specifications because bean counters don't demand important
specifications such as:
Specification compliance: ATX 2.03 & ATX12V v1.1
Acoustics noise 25.8dBA typical at 70w, 30cm
Short circuit protection on all outputs
Over voltage protection
Over power protection
100% hi-pot test
100% burn in, high temperature cycled on/off
PFC harmonics compliance: EN61000-3-2 + A1 + A2
EMI/RFI compliance: CE, CISPR22 & FCC part 15 class B
Safety compliance: VDE, TUV, D, N, S, Fi, UL, C-UL & CB
Hold up time, full load: 16ms. typical
Efficiency; 100-120VAC and full range: >65%
Dielectric withstand, input to frame/ground: 1800VAC, 1sec.
Dielectric withstand, input to output: 1800VAC, 1sec.
Ripple/noise: 1%
MTBF, full load @ 25°C amb.: >100k hrs
No specs means a supply probably is missing essential
functions - which is why it can sell for less than $80 - at
higher profit.
Procedure defined above using 3.5 digit multimeter
identifies a problem in minutes. Shotgun solutions take far
longer. And then a criteria to select replacement parts: on
price or on value. Minimally acceptable supplies provide a
long list of specifications - including those listed above.
