Hi , Tom.
Thanks again for the time you spent for me. I was just coming on line to
post some updates. The mesurements I made on the PWS at MoBo level were
actually foulty. I requested some help to see what happened pressing the
front switch during the mearuremets and the result was that keeping the
meter's on the plug, when the front switch was preessed the voltage went
actually down and the Pc started. So it seem to be a "random" problem. I'll
try living with it and maybe buy a new MoBo. One more question, if it is
"random" can we for sure affirm it's a MoBo problem?
When it powers up but computer does not work, we still see green
wire drop to near zero. We then see orange, red, and yellow wires
above minimum voltages. One other wire is the gray wire. It must
rise above 2.4 volts within second of switch pressed. Gray wire tells
motherboard that those three voltages are OK. Well if that power
supply voltage monitor were defective, then gray wire would not rise
well above 2.4 volts (I probably should have mentioned this earlier.)
But when all those voltages are well beyond threshold values, power
supply system is completely OK.
Look at what remains. Motherboard and CPU use few circuits when
first starting: CPU, BIOS, video controller, and some support
functions. All located on motherboard. If these few functions
start, then floppy disk drive makes noise and lights on keyboard will
go through a power-up flashing procedure. If that procedure starts
and detects a failure, then motherboard speaker beeps. Little on
motherboard will hinder that simple response. Well maybe a video
controller has some strange startup problem. It is a remote
possibility. However if video controller fails, then motherboard
speaker will usually beep an error code. (Does motherboard normally
beep once when system does power up?)
Anyway, any or all of these possible failures are found on
motherboard. A timing delay. A signal that is not quite high or low
enough. A signal with too much noise or a double glitch. Whatever.
The point is that a simplest first function performed by motherboard
after voltages are stable is to reset all hardware and to execute the
simplest BIOS program. That simplest program would then beep the
speaker. Everything outside of motherboard that could adversely
affect that startup is working properly and safetly beyond limits.
You might just look at the gray wire; make sure that voltage rises
above 2.4 volts when computer does not start. If so, the motherboard
has everything necessary to perform flawlessly. I would expect with
age (and with increased room temperature) that the intermittent would
become worse. Temperature being an excellent tool to heat selected
components - to make an intermittent hard - to trace the failure to a
particular spot. Still, it would be on motherboard - the only
remaining suspect.
We would trace such failures to a particular IC using oscilloscopes
and schematics. In your case, we have neither. Often a marginal
signal somewhere between two ICs would be identified as reason for the
intermittent. Obviously we cannot do that here. By confirming that
everything else relevant is working OK, then motherboard is only
remaining suspect. Again, maybe something like removing a stray metal
fragment, a bulging capacitor, or a standoff shorting to a circuit
board conductor might cure the problem. But history says that reason
for failure is rare. So little is necessary to display on video. And
those few functions apparently are not even starting. We don't even
have a speaker beep when computer does not start. Everything to
accomplish so little is only on motherboard - the last remaining
suspect. (But check the gray wire that I forgot to mention).
