| Electronics > Repair |
| How to Repair PC Motherboard Switch and Its Circuit |
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| BravoV:
--- Quote from: Mechatrommer on December 08, 2011, 01:45:16 pm ---even if the psu is still working ok? --- End quote --- 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. |
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