E-Machines T2898 desktop

  • Thread starter Thread starter RMW
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RMW

Hello.. I'm trouble shooting my roommate's friend's computer. Her
computer is an E-Machinese T2898 desktop. The motherboard looks like
this:

http://dexplor.com/phpBB2/viewtopic.php?t=7747553 (the one on the
first post ....OEM Intel D845GVSR Motherboard).

I have tested the power supply with a power supply tester, and it
seems to check out. The problem is that the computer does not turn on.
I suspect it is a motherboard issue because I have already stripped it
down to the bare system (just motherboard and CPU).

I am curious on how I should go about trying to fix this. I do not
have the schematic to the motherboard. However, I have access to an
oscilloscope and multimeter for component level troubleshooting. I
just want to know where to start.

Thanks!
 
Hello.. I'm trouble shooting my roommate's friend's computer. Her
computer is an E-Machinese T2898 desktop. The motherboard looks like
this:

http://dexplor.com/phpBB2/viewtopic.php?t=7747553 (the one on the
first post ....OEM Intel D845GVSR Motherboard).

I have tested the power supply with a power supply tester, and it
seems to check out. The problem is that the computer does not turn on.
I suspect it is a motherboard issue because I have already stripped it
down to the bare system (just motherboard and CPU).

I am curious on how I should go about trying to fix this. I do not
have the schematic to the motherboard. However, I have access to an
oscilloscope and multimeter for component level troubleshooting. I
just want to know where to start.

Thanks!
Click to expand...

Just because the power supply tester green lights a power supply does
not mean that the power supply is good.
The cheepie PSU testers will prove athat a PSU is bad, but aren't good
enough to 100% confirm a good unit. Especially for emachines Bestec
power supplies.

Emachines have a nasty failure mode where the power supply dies and
takes the motherboard with it. Even sometimes the processor. However,
you have an Intel motherboard, which is not as vunerable to the issue
as some of the other boards with a Via or SiS chipset.

Start by replacing the power supply. It's a POS that should be
replaced on general principles anyway.
 
RMW said:
Hello.. I'm trouble shooting my roommate's friend's computer. Her
computer is an E-Machinese T2898 desktop. The motherboard looks like
this:

http://dexplor.com/phpBB2/viewtopic.php?t=7747553 (the one on the
first post ....OEM Intel D845GVSR Motherboard).

I have tested the power supply with a power supply tester, and it
seems to check out. The problem is that the computer does not turn on.
I suspect it is a motherboard issue because I have already stripped it
down to the bare system (just motherboard and CPU).

I am curious on how I should go about trying to fix this. I do not
have the schematic to the motherboard. However, I have access to an
oscilloscope and multimeter for component level troubleshooting. I
just want to know where to start.

Thanks!
Click to expand...

These are several of the ATX power supply specs. They'll show you
the pinout. Remember to make the right rotation in your mind, as
the view of the pinout may not be the one you need for the following
procedures.

http://web.archive.org/web/20030424...org/developer/specs/atx/ATX_ATX12V_PS_1_1.pdf
http://www.formfactors.org/developer/specs/atx/ATX12V_1_3dg.pdf
http://www.formfactors.org/developer/specs/ATX12V_PSDG_2_2_public_br2.pdf

To turn on the supply, connect PS_ON# to COM. COM is an adjacent pin
to PS_ON#. Try stuffing a bare metal paper clip, into the back of the
main ATX power supply connector. I believe PS_ON# is a green wire.
while adjacent COM wires on either side are black in color.

Theory is, the motherboard IC driving the PS_ON# signal, is open
collector. Open collector is safe to ground, since only
weak pullups are used to make a logic one. The open collector can
pull strongly to ground, at the same time as the paper clip, without
damage.

You;ve already proved the supply will turn on, by using your "tester".
The tester connects PS_ON# to COM for you, plus puts a small load on
only one of the rails. If, by connecting the PSU to the motherboard, and
connecting PS_ON# to COM with a paper clip, all you get is a slight
"twitch" from the fans, that means the motherboard has shorted the
PSU. The PSU turned off after 35 to 50 milliseconds or so, if a short
is detected at powerup. Sometimes a short may only be a partial one,
in which case, the PSU may run, and you end up smelling something burning.
If that happens, switch off and figure out why. Use the ohms setting
on the multimeter, to check for any rail on the motherboard, which is
close to 0 ohms with respect to GND or COM.

HTH,
Paul
 
Just because the power supply tester green lights a power supply does
not mean that the power supply is good.
The cheepie PSU testers will prove athat a PSU is bad, but aren't good
enough to 100% confirm a good unit. Especially for emachines Bestec
power supplies.

Emachines have a nasty failure mode where the power supply dies and
takes the motherboard with it. Even sometimes the processor. However,
you have an Intel motherboard, which is not as vunerable to the issue
as some of the other boards with a Via or SiS chipset.

Start by replacing the power supply. It's a POS that should be
replaced on general principles anyway.
Click to expand...

Hi there! I actually replaced the E-Machines power supply with another
one because it showed as having a fault. I was referring to the new
power supply in my original post. Thanks!
 
These are several of the ATX power supply specs. They'll show you
the pinout. Remember to make the right rotation in your mind, as
the view of the pinout may not be the one you need for the following
procedures.

http://web.archive.org/web/20030424...rg/developer/specs/ATX12V_PSDG_2_2_public_br2...

To turn on the supply, connect PS_ON# to COM. COM is an adjacent pin
to PS_ON#. Try stuffing a bare metal paper clip, into the back of the
main ATX power supply connector. I believe PS_ON# is a green wire.
while adjacent COM wires on either side are black in color.

Theory is, the motherboard IC driving the PS_ON# signal, is open
collector. Open collector is safe to ground, since only
weak pullups are used to make a logic one. The open collector can
pull strongly to ground, at the same time as the paper clip, without
damage.

You;ve already proved the supply will turn on, by using your "tester".
The tester connects PS_ON# to COM for you, plus puts a small load on
only one of the rails. If, by connecting the PSU to the motherboard, and
connecting PS_ON# to COM with a paper clip, all you get is a slight
"twitch" from the fans, that means the motherboard has shorted the
PSU. The PSU turned off after 35 to 50 milliseconds or so, if a short
is detected at powerup. Sometimes a short may only be a partial one,
in which case, the PSU may run, and you end up smelling something burning.
If that happens, switch off and figure out why. Use the ohms setting
on the multimeter, to check for any rail on the motherboard, which is
close to 0 ohms with respect to GND or COM.

HTH,
Paul
Click to expand...

Thanks, Paul! When I check for "any rail on the motherboard", does
that mean I have to probe every single voltage regulator or voltage
source terminal with respect to ground? I do not know where these
points are located.
 
Thanks, Paul! When I check for "any rail on the motherboard", does
that mean I have to probe every single voltage regulator or voltage
source terminal with respect to ground? I do not know where these
points are located.
Click to expand...


At the motherboard power supply connectors, you'd look at a
pinout diagram for ATX supplies (Google will find some,
though they'd be reversed logically if looking at a diagram
of the contact end of the PSU connector instead of the back
side where the wire enters) and check between each voltage
"rail" pins and one of the ground pins. So you put negative
meter probe against the ground pin and just move the
positive meter probe to each- 3.3V, 5V, 12V, -5V, -12V,
5VSB.
 
RMW said:
Thanks, Paul! When I check for "any rail on the motherboard", does
that mean I have to probe every single voltage regulator or voltage
source terminal with respect to ground? I do not know where these
points are located.
Click to expand...

Since we were talking about the PSU and whether it is working, you
would probe a minimum of one pin for each rail. If the +5V has four
pins, measuring at one of them gives a representation of +5V.

If you poke in the holes where the wires go into the back of the main
connector, you can contact the metal of the pin. That is where you probe,
to measure the supply voltages, while the system is running. The measurements
are easiest to do, if the components are out of the case and sitting on
your work table. Placing a telephone book, with cardboard cover, underneath
the motherboard, insulates the bottom of the motherboard, and leaves room for
the faceplates of things like video cards, to hang below the plane of the
motherboard.

When working with components sitting loose on a table, you have
to be careful not to tug on the video card cable. Since nothing
holds the AGP or PCI cards in place, they can be pulled from the
socket, which can damage them. From that perspective, leaving the
components inside the case, reduces the odds of damaging them, but
does make it harder to see what you are doing.

HTH,
Paul
 
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