e machine power switch

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i have got an emachine desktop, and recently it has taken a couple of
of pressing the power button for the machine to start up, i
think the fault is
with the button rather than anything more serious
(well i hope so) now it will
not power up at all, would it be possible
to change the power switch to just a
normal push to contact switch, and
does anybody know what rating switch i would
need, first time post so
hope i have got it right.
many thanks

Re: e machine power switch

bradsheff wrote:
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First, step away from the power button... :-)

Take the side panel off the Emachine. What brand of power
supply is in there ? Is it a "Bestec 250W" ? If so, I would
not torture the computer. According to this advert, the Bestec
250W is used in Gateway and Emachine computers.


The thing is, there is one model of Bestec, which, when
it blows, it ruins not only the power supply, but also
the motherboard and perhaps the hard drive. So, if you
are experiencing a problem, where the computer won't
start, it could be the power supply itself is defective.

This is how the power stuff works.

      Front   -----> Motherboard -------------- PS_ON# --> Power_Supply ---AC SW
      Button  -----> Logic Chip  -------------- COM ----->            
    (Momentary       (Latches Pulse)
     Open cct)

First, you switch on the power supply at the back of the computer. That
is the "AC SW" on the right of the diagram.

Now, to activate "soft power", you press the momentary switch
on the front of the computer. That is plugged into a motherboard
PANEL header. The switch is a momentary contact type, and the contacts
are normally open. When the switch is closed, it sends a pulse to
the motherboard. The motherboard latches the pulse, and sends a
steady logic level signal via PS_ON# to the power supply.

One way for that to go "flaky", would be for the
motherboard output on PS_ON# to be weak. Otherwise, the
symptoms are more likely to be a power supply problem.

If the supply is a Bestec and it is "sick", then every time
you try to get it to start, you're taking a chance. If it
does start, and it is "sick" like the other Bestec supplies,
it could ruin your motherboard. Ideally, you'd try to find
a replacement supply in that case, to preserve the rest of
your computer.

If it was some other brand of supply, one with a better
reputation, then perhaps playing roulette with it, wouldn't
be quite so dangerous.

Yes, you can use a momentary switch to replace the front
power switch. You'll need some way to wire it up.

One trick, is to swap the reset and power buttons, if the case
has two buttons. Swap the reset cable and the power button cable,
and change their position on the PANEL header. Both switches
are momentary contact and normally open. By swapping them,
your power switch becomes the reset, and the reset becomes the
power. And then you can press "reset", to turn on the computer.

But if it is a Bestec, you should Google the part number and
read some of the horror stories.


Re: e machine power switch

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I've got an old E-Tower 700 that has a sticky power button. It not only
sticks, but the button will slightly twist to the side and jam if you
try to press it with one finger, which causes it to turn on/off quickly,
or remain depressed so that It doesn't react when you press it again, at
least until it gets unjammed.

I got used to using two fingers to
evenly depress the button, and that works pretty well.

Re: e machine power switch

bradsheff wrote:
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Very unlikely it's the switch...
but you can just touch the two pins together with a screw drive to confirm.

If it is a bad switch, the current is negligible and a one amp switch
would be fine.

I do a lot of computer repair work and thus far all e machines I've
worked on had either a bad power supply, bad motherboard or both

Re: e machine power switch

The most recent emachines I've worked on had the front panel switch
operating part as a movable plastic plate that when pushed warped back
into the front panel cavity and pressed against the switch itself. The
plate had ears on either side that were the return spring portion and
the ears tend to break.

On one machine I broke the other ear, removed the operating part, and
gave the user a broken pencil with an eraser and had her use the eraser
end to reach through the hole in the front panel to operate the switch.

On another, with a defective switch, I paralleled the contacts with a
normally open pushbutton with wires going through the hole in the front
panel to reach the board on which the bad switch was mounted.

Neither were elegant fixes but got the systems back up. The chances of
finding replacement parts to fix the problem, if yours is like either of
mine, is slim to none.

Re: e machine power switch

On Sat, 22 Aug 2009 22:27:34 -0500, edfair

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So it is, and has been since the 80486 era, on most OEM
systems, though fortunately some and many aftermarket cases
used the more durable common design with a metal spring
behind a plastic plunger for the button.

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I don't know if I'd call any conventional fix elegant but
does a pushbutton switch need to be elegant?

When it formerly relied on the springiness of plastic
bending one fix is to just make a bracket.  Take a flat
piece of metal, bend it so it has a channel in the middle
and two flaps on the ends which can be screwed to the casing
frame metal wall or at least the interior plastic on the
front bezel, drill a hole in the back to accomodate the
plunger for the switch and/or the button's rear stud, and
slip a common light tension spring from a hardware store in
before screwing it down.  Not exactly elegant but it can
restore normal operation of the switch... just a little
change in the actuation pressure.

A second alternative is to buy a regular pushbutton switch
that extends a bit further forward of it's mounting  hole
and use that to replace the existing switch so the new
switch with it's typically higher resistance spring, has
enough length and force to keep the front switch botton in
place and cause return travel to it's non-actuated position.

First option requires cutting and screwing metal, 2nd one
sourcing the right switch and soldering.  Either takes about
10 minutes if one is accustomed to doing such work and
happens to have the switch metal and a spring (I like to
keep generic materials and parts like these around for many

Re: e machine power switch

Both solutions are interesting and have been filed away for future use.
Unfortunately neither would have worked with the e-machines I was
involved with,
primarily because the fixes were "in the field" repairs
and miles away from the
junk cabinets where my stash of parts are

Re: e machine power switch

edfair wrote:
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even if you would have filled a pickup truck full of parts...
it's pretty well guaranteed that you still would not have had the right
stuff with you! <G>

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