"Bob H" <w3huw@comcast.net> wrote in message
news:EaidnSGGhpHsgh3anZ2dnUVZ_uKpnZ2d@comcast.com. ..
>
> "w_tom" <w_tom1@usa.net> wrote in message
> news:9aa5e4f6-9ff1-4bc9-b318-c3594d68fd52@d4g2000prg.googlegroups.com...
> On Jan 5, 10:42 am, "Bob H" <w3...@comcast.net> wrote:
>> On my not very exotic meter the purple wire reads 4.9 volts. The green
>> wire
>> reads 2.7 volts both before and after presing the power switch. The gray
> wire
>> reads nothing at all either way.
>
> The purple wire is above 4.87 volts (assuming your meter is 3.5
> digits; has two digits after the decimal point with is standard for
> any $20 meter) meaning purple wire voltage is probably good. Power
> supply controller has proper (and clean) power to control the supply.
> Green wire voltage must exceed 2.0 volts to keep power supply off. In
> most cases, this voltage should be higher; closer to the purple wire
> voltage. OK. It's in spec but marginal. For now we just move on.
>
> When power switch is pressed, power supply controller should drop
> green wire to below 0.8 volts. It does not. Only other relevant
> controller input is the power supply switch. Previously switch was
> working according an ohm meter. But better is to measure it without
> being disconnected - where it connects to motherboard. Voltage where
> it connects to motherboard should be a high voltage - something
> approaching purple wire voltage. When switch is pressed, then voltage
> between two switch wires should drop to near zero - well below 0.8
> volts. This better measurement where switch connects to motherboard
> reports what the controller sees.
>
> If switch voltage drops to zero when pressed, then power supply
> controller has proper inputs and should be commanding the power supply
> on. Your green wire number says 'not'. Therefore power supply
> controller is defective.
>
> Now perform some visual inspections concentrating on that controller
> area. Carefully remove the motherboard to locate where PC traces from
> the green wire and from the power switch connection converge. Inspect
> for metal fragments (especially wedged between IC pins), a sliced or
> cracked pc board surface (top or bottom), or a standoff that has
> somehow shorted to an IC pin or printer circuit (pc) board trace.
> Chances are no visual defect will be found. Most all defects have no
> visual indication. Therefore the power supply controller is
> defective; motherboard must be replaced.
>
> As promised, what those signal wires do. Purple wire always powers
> the controller (which is why nothing is added or removed from a
> computer without its AC power cord completely disconnected). Then
> when power switch shorts its wires together (switch is pressed), then
> controller grounds (drops green wire voltage from above 2 volts to
> near zero volts) to order power supply on. Supply would put voltages
> on orange, red, and yellow wires. Power supply controller watches the
> gray wire for maybe two seconds. If that gray wire voltage does not
> rise above 2.4 volts, then power supply controller assumes a failure
> and shuts off power supply.
>
> You never saw the gray wire voltage rise because power supply was
> never told to power on. Had it powered on and provided controller
> with the gray wire 'Power Good' signal, then power supply controller
> would have released the CPU to start working. Another controller
> function is to hold (stop) CPU from running until all voltages are
> sufficient. But again, we never got that far because power supply
> controller never told power supply to power on.
>
> Solution: replace a defective power supply controller that never
> orders power supply on.
>
> One final note. Sometimes (almost never), an internal power supply
> problem causes too much current so that power supply controller cannot
> order power supply on. Measure this green wire current with the
> meter. Disconnect the power supply from motherboard. Then restore AC
> power cord. Put meter in DC Current measurement mode at maybe 2
> amps. On some meters, the red wire probe is moved to a current
> measurement hole. Connect the meter to any power supply black wire
> and (red probe) to the green wire. Meter will short the green wire
> just like the paper clip did. This meter connection should cause
> power supply to power on. Switch the meter setting lower from 2 amps
> until a valid current measurement is read. Current should be well
> less than 20 milliamps or 0.02 amps (probably about 1 milliamp). If
> yes, then any working power supply controller can order that power
> supply on by shorting out that less than 20 milliamp current.
>
> Again, this last test is only for a rare (almost impossible) type
> failure inside power supply. So rare and considering that 2.7 volts,
> then test is probably unnecessary. However, this is a learning
> process. The other tests measured voltage. This test taught (safely)
> how to measure current.
>
>
> So thanks to all for an excellent education regarding the care and feeding
> of
> computer power supplies. My only regret is having immediately gone out and
> purchasing a new supply instead of coming here first. I did upgrade to a
> 300W
> supply and now I have the original as a spare. Being an old ham radio
> operator I will have no problem working with a wire and switch hanging out
> of
> the computer until I get around to upgrading. My old Win 98 SE machine has
> helped me thru this and to think I was going to can it. Thanks again!
> Bob H


Did you do like Kony said and reset the cmos.? Removing the cmos battery
for a few minutes is not good enough. Use the jumper to clear cmos if you
have not tried it and make sure the machine is unplugged when you do it.