w_tomi am not trying to do shotgun repair but those 2 parts were tested
and are bad. I read the articles in the link and the power supply does
have the specs mentioned. any other ideas???
The power switch starts the process. One of the chips on the motherboard,
conditions the signal from the power switch. The power switch
produces a momentary pulse, and the chip on the motherboard makes
that into a continuous "ON" signal.
The signal may pass through two or more chips on the motherboard.
The Southbridge and the Super I/O chips want to play with the
signal, because of the wake-up functions that the computer has
got. To wake the computer, any chip with a wake-up function
has to be able to turn on the computer power.
Eventually, the signal gets to the power supply. In the "old
days", you may have found an open collector driver IC as the
last device in the chain. Now, it might be the last of the
two chips mentioned above that drives the power supply. The
signal is called PS_ON#. When it is driven to GND (ground),
that turns on the power supply. (PS_ON# is one of the wires
on the 20 pin ATX power connector.)
For all of this logic to work on the motherboard, it needs
a source of power. The +5VSB supply on the power supply,
will light a LED on the motherboard (if Emachines spends the
pennies to put a LED on there). Seeing a LED like that lit,
tells you there is some +5VSB to work with. Otherwise, a
multimeter can be used to verify that the +5VSB from the
power supply is present.
If there is +5VSB present, and a momentary contact closure on
the two power supply switch pins (a screwdriver tip can
be used to simulate that), then the motherboard should be
driving PS_ON# to ground and the supply should come on.
Any fans that don't have fancy speed controls and the like,
should come on immediately.
When the Southbridge is put under stress, you'll get different
symptoms - that being, the power supply will come on without
pushing the button on the front of the case. A typical way
to stress the Southbridge, is put an IDE cable on crooked so
that only some of the pins touch. The Southbridge doesn't
like that.
In the above description, anything that breaks that chain of
operations can stop the board from starting. No +5VSB, bad
switch, bad chips on the board, inability to drive PS_ON#
signal properly and so on. Many of the root causes will
require replacing the motherboard, as DaveW explained.
There really isn't too much debugging you can do (at
least without a schematic, and I doubt Emachines gives
those out).
Paul
