PWR ON control on motherboard? IC?

[BigAL.NZ]

Hi Guys,

I have got a mobo here that is dead. I have done all the usual stuff like trying new PSU, bench testing etc. Its dead - as in no fans, no nothing when you hit PWR button - PSU doesnt even come on.

I connected the ATX molex from the PSU and shorted Green (PWR ON) with GND on the molex and the PC came on. All seemed fine.

I think there is a IC on the motherboard between the PWR BTN headers from the power on switch and the ATX header on the mobo that tells PSU to start up.

I am trying to find this IC.

I have attached a image of the mobo with some of the IC's that seem possible labelled.

Interested to know if anyone knows which IC controls the PSU and tells it to turn on?

Thanks


-Al

[studiot]

Have you checked the switch itself, before messing with the motherboard?

[BigAL.NZ]

Have you checked the switch itself, before messing with the motherboard?

Sure did. Switch is fine.

[Tweak]

I'll probably sound like a jerk for this but that looks like a 20-30 buck ECS mb which I'd sooner replace than repair most likely.

[ithinkyou5.0]

im with tweak on that one, espesh if the customer is paying per hour..

[LunchBox]

Hey Big All, I am glad you made it here.

Like I said may know and other may not. Hopefully the ones that have that knowledge can give you advise.

[NYJimbo]

Such an old board. Old agp, no SATA, Why are you trying to save it?.

[BigAL.NZ]

All valid comments about biffing it and replacing it, but there no learning in that.

I am trying to fix for my own satisfaction....

[NYJimbo]

All valid comments about biffing it and replacing it, but there no learning in that.

I am trying to fix for my own satisfaction....

The problem with these boards is that there are no standards, no schematics and even if you can find the chip you might not be able to buy it. For instance, the old I/O Controller chips are now almost always part of the southbridge chipset. You CANNOT buy those, they are made for the companies making the boards so if an i/o controller goes you usually are dead. In the old days you could easily replace them.

[Tweak]

I am trying to fix for my own satisfaction....

Makes sense and fair enough. :)

[studiot]

If the switch is fine, I don't understand how shorting the green to earth fires up the pc. Surely this is all the switch does, unless it is debounced by a an IC.

[BigAL.NZ]

If the switch is fine, I don't understand how shorting the green to earth fires up the pc. Surely this is all the switch does, unless it is debounced by a an IC.

When I first started looking into this problem I thought the same thing, its not that simple. The switch shorts off to an IC which helps to deal with various things like Wake on Lan etc.

I wouldnt be surprised if maybe it even goes to soutbridge????

For the other person that asked I do have +5v on the pwr header pin.

-Al

[studiot]

unless it is debounced by a an IC.


I would look for a small SMD IC containing a handful of gates. Debouncing is normally achieved by creating a monostable (one shot) from a pair of cross coupled gates, with perhaps a third gate used as a buffer on the output. I would check this is functioning by signal tracing as it will be directly connected to the switch and you can check the output at DC or on a scope.

[BigAL.NZ]

I would look for a small SMD IC containing a handful of gates. Debouncing is normally achieved by creating a monostable (one shot) from a pair of cross coupled gates, with perhaps a third gate used as a buffer on the output. I would check this is functioning by signal tracing as it will be directly connected to the switch and you can check the output at DC or on a scope.

Now you talking serious ;-) I dont know much about debouncing or how to signal trace on a multilayer PCB, but would be pretty keen to learn if you care to elabourate, else its off to google.

Thanks in advance,

-Al

[studiot]

Any mechanical switch is a crude device, in electronic terms. The actual pulse from closing (or opening ) the contacts looks like a whole series of switchings on and off to a faster circuit device. A bit as though you had pressed say the e key lots of times on the keyboard.

Prevent the circuitry interpreting the a mechanical input this way is called debouncing.

Basically the first pulse seen flips an electronic switch into an unresponsive state for a predefined period so the rest of the bounces will have no effect.
Then the monostable resets and can receive and respond to another switching action.

Keyboard switches are also debounced.

[BigAL.NZ]

Any mechanical switch is a crude device, in electronic terms. The actual pulse from closing (or opening ) the contacts looks like a whole series of switchings on and off to a faster circuit device. A bit as though you had pressed say the e key lots of times on the keyboard.

Prevent the circuitry interpreting the a mechanical input this way is called debouncing.

Basically the first pulse seen flips an electronic switch into an unresponsive state for a predefined period so the rest of the bounces will have no effect.
Then the monostable resets and can receive and respond to another switching action.

Keyboard switches are also debounced.

Any words of wisdom re signal tracing? I assume this is how I would identify the IC?

[studiot]

Can you not follow the tracks on the board from the switch pins to the first block of plastic?

[BigAL.NZ]

Can you not follow the tracks on the board from the switch pins to the first block of plastic?

I am pretty sure its multi layer pcb, so I dont know how you could trace not being able to see the middle layers?

-Al