How to Repair PC Motherboard Switch and Its Circuit

[Mechatrommer]

my PC power switch now playing up. when mains is turned on and i press the PC switch, it will not turned ON, i have to switch off and on the mains, and press the switch several times until success. i think i will repair it before its totally dead but not sure what to repair. i have 2 or 3 previous motherboard with same problem i was simply tossed off and buy new system. now i dont think i can afford that and dont have time to install new system (new MB means new CPU, new RAM new OS etc). any pointer on how to repair? what sort of components involved in there? i can take a picture of inside the casing but now its a liltte bit hard to dismantle everything (so by request). fyi: the mother board is Intel DG35EC (i just take out the box), its say "3 year limited desktop board warranty", now i bought it just passed 3 years (bought in nov2008), i thought intel name MB will be good, i guess its only designed for the dump now just like the rest. hope some pointers. thanks.

[sonicj]

are you sure it requires cycling of the mains?  have you tried another power switch?  a different power supply? flashed and/or cleared the cmos?
-sj

[joelby]

What sort of power supply do you have, and how much did you pay for it?

Cheap, nasty power supplies usually exhibit these sorts of problems. Swap yours out for a spare, and if that fixes the problem, buy a better power supply.

[alm]

Corrupt CMOS, flaky power supply and dried out electrolytics on the mainboard would probably be my first guesses. You could measure the PS_ON and POWER-GOOD signals on the ATX power connector and ripple on the power lines. Typical troubleshooting strategy would be parts replacement, however. That tends to be much more efficient that messing around with test equipment. The first part I'd replace would be the power supply, after basics like clearing CMOS and shorting the power switch pins with a screwdriver.

[Psi]

Check all the capacitors on the motherboard for any bad ones (curved on top surface)

[Mechatrommer]

@sonicj: cycling the mains is not the right word i think. this thing is pretty unstable, sometime i have to off-on 2-3 times, sometime i dont have to. when it cannot ON, i can see a light from transparent plastic of power switch behind, if its turned ON, then i cannot switch ON the PC, i have to switch OFF the mains until the green light is turned off, and then ON the mains again, if it doesnt lit, it will switch ON the PC if i press. if its lit, i have to OFF the mains and redo again.

@joelby: its gigabyte superb 550. power seems ok when its turned ON, i dont see any short of supply so far. i used to buy noname brand before, but that time (3yrs ago), i choosed this gigabyte brand and thought it will be longlasting. its powering Pentium Quad Core Q9400, the dealer said 450W PSU can do iirc, but to be on the safe side, he recommended 550W. if its the gigabyte psu, then the same with no name... designed for the dump.

@alm: yes parts replacement will be time saving... but only if i know what to replace. i never repair a motherboard b4, so maybe i need to do the testing measurement exercise.
@psi: opening the side cover, there are alot electrolyte caps scattered, maybe around 20, they all look ok, no bulge, still shiny, should i test all?

i will gather info here as preparation, in case of full shutdown i cannot ON anymore (meaning i cannot online anymore, from my "framework" at least, i got the wifey netbook for emergency though), i will do full autopsy based on advices here, for now i will let it connected to all my devices (printers, wires etc) until its last blood drop. about the clearing CMOS and reflashing it, any detail info on how to do that? CMOS is the square looking chip inside that we can take out right? i will look for it. but i have no idea on reflashing, is it the same procedure of upgrading the MB firmware?

my suspect will be the momentary switch on the MB, my previous MB that i tossed (to junkyard dept) got the same problem, but the power supply (noname) is still usable.

[ElektroQuark]

Check PSU electrolitics for high ESR ones.

[Mechatrommer]

Check PSU electrolitics for high ESR ones.

even if the psu is still working ok?

[ElektroQuark]

Yes. Sometimes the PSU instability affects MB behaviour.

[oliver602]

Had a very similar problem recently. Had to cycle the mains several time to get it to switch on. Turned out to be a bad capacator inside the PSU on the 5V rail. It was so bad i could see low voltage with a multimeter, yet the PC still worked perfectly fine when I did get it to start. Easily checked with a scope. Look up the ATX pinout and probe http://www.smpspowersupply.com/connectors-pinouts.html

[BravoV]

even if the psu is still working ok?

Even the main 12 volt and 5 volt rails are working fine, I've fixed few pcs that had similar symptom in the past, apparently the problem was the 5 volt VSB (standby rail) at the psu, that is always on all the time even the pc power is off.

Remember though, the 5 volt VSB rail is different from the main 5 volt rail.

This vsb rail is reponsible in supplying the multi-io ic that is handing the main power on and off at the mobo.

[orbiter]

Do you mean the soft 'On' switch on the PC isn't working?

Perhaps you could check that the fault is not just with the soft switch itself on the computers case?

To test it just disconnect the soft 'On' connector from the motherboard, then use a suitable 2 pin jumper, or even a small screwdriver to jump the power pins directly on the motherboard. 

If the system powers up ok using just those pins, the problem is with the soft 'On' switch rather than the PSU, and in this case you can just attach a new switch or a suitable 2 pin 12v momentary push switch somewhere that's easily accessible.

[Mechatrommer]

i dont think the switch's mechanical is the problem. as the switch comes with a green (power?) led which i believe connected to different pin. when this led is turned on during power startup (hard mains switch, actually from UPS), the PC refused to turn ON, so i think the bios is tricked by seeing MB ON flag (green led lit) and ignore momentary switch. the symptom is described better by others saying the PSU PS_OK problem. now each time power up, i'll check for this green led light. if its lit, i'll simply turn OFF-ON the mains until its not lit (without torturing the soft switch), and then successfull switch ON after that.

[metalphreak]

There is a wire in the main 20/24pin ATX connector that is the turn on wire. It's usually green. Short this out to any ground pin and the power supply turns on. This is essentially all you are doing when you press the power button on the front. The motherboard also gets standby power (5v/3.3v? low current) when the PSU is turned "off". If the power supply isn't turning on at all then it is more than likely a problem with the PSU. Most of them have short circuit protection and won't turn on if there's a dead short somewhere. Try unplugging all unnecessary devices (ie just leave the motherboard power plugged in) and see if it turns on.

[oliver602]

Holding down the soft power switch for 5 seconds forces the  PC to switch off.