September 16th 06, 11:06 PM
Phil Weldon wrote:
> 'Dave' wrote:
>> Holy crap, this is the longest thread I've seen where nobody has the right
>> answer for the OP. Simply, replace the bad power supply. The mainboard
>> is not receiving a power_OK signal, and so it never starts. That's because
>> the power is NOT OK.
> There is no possible way to reach your conclusion from the information given
> by the original poster. Your diagnosis is wrong. If a 'power good' level
> is not provided, the power supply shuts down. Also, the MOTHERBOARD
> supplies the 'power good' level, unlike your belief that "The mainboard is
> not receiving a power_OK signal." The original poster reports "The lights
> come on, fans, drives spin up, usb devices get power, everything is normal,
> except that my monitor stays black."
I used to modify the RC time constants in the Power_Good circuitry of
cheap AT and even XT power supplies to make them compatible with
certain motherboards. The Power_Good (Power_OK) signal (grey wire on
ATX supplies) is definitely provided by the power supply, not by the
motherboard, unless you're referring to any pull-up resistor on the
motherboard that raises the signal from about 3.3V to 5.0V (I don't
remember if P_G is a normal TTL signal or open collector). On the
other hand, the motherboard does provide the Power_On signal (green
wire) to the supply.
I'm going to say the 12V is kind of OK (fans spin, drives spin), and so
is the 5V (drives spin). But I don't know if the Power_On circuitry is
handled by chips that are powered by the 5V or 3.3V. I have a feeling
the power supply is bad, but I'd try to borrow another powe supply,
just in case.
> 'Dave' wrote:
>> Holy crap, this is the longest thread I've seen where nobody has the right
>> answer for the OP. Simply, replace the bad power supply. The mainboard
>> is not receiving a power_OK signal, and so it never starts. That's because
>> the power is NOT OK.
> There is no possible way to reach your conclusion from the information given
> by the original poster. Your diagnosis is wrong. If a 'power good' level
> is not provided, the power supply shuts down. Also, the MOTHERBOARD
> supplies the 'power good' level, unlike your belief that "The mainboard is
> not receiving a power_OK signal." The original poster reports "The lights
> come on, fans, drives spin up, usb devices get power, everything is normal,
> except that my monitor stays black."
I used to modify the RC time constants in the Power_Good circuitry of
cheap AT and even XT power supplies to make them compatible with
certain motherboards. The Power_Good (Power_OK) signal (grey wire on
ATX supplies) is definitely provided by the power supply, not by the
motherboard, unless you're referring to any pull-up resistor on the
motherboard that raises the signal from about 3.3V to 5.0V (I don't
remember if P_G is a normal TTL signal or open collector). On the
other hand, the motherboard does provide the Power_On signal (green
wire) to the supply.
I'm going to say the 12V is kind of OK (fans spin, drives spin), and so
is the 5V (drives spin). But I don't know if the Power_On circuitry is
handled by chips that are powered by the 5V or 3.3V. I have a feeling
the power supply is bad, but I'd try to borrow another powe supply,
just in case.