How to force power on?

[Mike Faithfull]

A year or two ago I bought a case and PSU an assembled a "bitza" from a
donated mother board and processor left over from an upgrade, together with
odds and ends of disc drives, NICs etc. from the junk box. The unit has
worked without missing a beat until recently when it refused to power up.
The "power switch" is a momentary action push-button that connects to the
mother board, so I don't know whether it's the m/b or the psu that's faulty.
Is it safe to power up the psu with no load? Does the PON connection have
to be grounded to force power on, or what? It's been a good PC and I would
hate to put it in the trash if I don't have to ...

 

[ric]

A year or two ago I bought a case and PSU an assembled a "bitza" from a
donated mother board and processor left over from an upgrade, together with
odds and ends of disc drives, NICs etc. from the junk box. The unit has
worked without missing a beat until recently when it refused to power up.
The "power switch" is a momentary action push-button that connects to the
mother board, so I don't know whether it's the m/b or the psu that's faulty.
Is it safe to power up the psu with no load? Does the PON connection have
to be grounded to force power on, or what? It's been a good PC and I would
hate to put it in the trash if I don't have to ...

To test the ATX PSU:

First, disconnect everything from the power supply (including MB.) Plug
AC cord into the rear of the PS. IF the PS has its own rocker on/off
switch, turn it ON. There should be +5v on pin 9 of the 20 pin connector
(+5vsb, usually a violet wire.) There should be >0.8v on pin 14 (PS-ON,
usually a green wire.) This is from an internal pull-up to the +5vsb.

Connect PS power leads to 1 or 2 IDE HDs (for a load). Jumper pin 14
(PS-ON) to pin 13 (ground, black wire.) At this point, fan in PS should
start spinning, drives should spin, and + 5/12v, -5/12v, +3.3v, and
+5vsb should be present at 20 pin connector.

Disconnecting pin 14-13 jumper should turn supply back off.

Further:

When your system fails to start, what is the voltage on pin 9? If it is
less than 4.75v, either a) the logic on your motherboard is shorting
b) your +5vsb is under powered, or c) your PS is bad. It is
most likely a combination of b) and c).

If your +5vsb is OK, measure pin 14 of the PS/MB connector. If it is <0.8v
(and the +5vsb is OK), the power supply should be on. If pin 14 is <0.8v,
the +5vsb is OK, and the PS is OFF, the PS is bad. If pin 14 is >2.0v, the
power supply is being told to stay OFF by the MB. If depressing the front
panel ON/OFF switch does NOT lower pin 14 to <0.8v (and the +5vsb is OK),
then the problem is NOT the power supply, but rather MB related.

Good luck.

 

[Zotin Khuma]

A year or two ago I bought a case and PSU an assembled a "bitza" from a
donated mother board and processor left over from an upgrade, together with
odds and ends of disc drives, NICs etc. from the junk box. The unit has
worked without missing a beat until recently when it refused to power up.
The "power switch" is a momentary action push-button that connects to the
mother board, so I don't know whether it's the m/b or the psu that's faulty.
Is it safe to power up the psu with no load? Does the PON connection have
to be grounded to force power on, or what? It's been a good PC and I would
hate to put it in the trash if I don't have to ...

From your description ("momentary action"), your PSU must be an ATX
type with a 20-pin connector going to the motherboard. If it's a
standard type as against some proprietary model, you will have a green
wire going to pin #14, flanked on both sides by black wires.

To test the PSU, disconnect all loads and short the green wire to a
black wire on either side, using a short length of wire or whatever is
handy. This will manually turn on the PSU. You can check this by
looking at the cooling fan. Some people have pointed out that you have
to provide a minimum load to test an ATX PSU, but it's never made a
difference in my experience. Some people recommend connecting a floppy
drive as the minimal load.

- Zotin

 

[Paul Murphy]

snip>
Some people recommend connecting a floppy
drive as the minimal load.

- Zotin

As you refer to Zotin, it's much better to either use a floppy drive or
CDROM drive as the minimal load (rather than a good hard drive) as there
have been cases of HDDs having SMART flags tripped by just applying power
with no ribbon cable connected.

To the OP, there are also cheap ATX PSU testers out there which can be
plugged into the 20 way ATX mobo connector from the PSU.

PSUs are right up there in the least reliable PC component list - often
giving intermittent "red herring" symptoms before they finally die. If you
have a known good PSU in another machine that you can beg or borrow for
testing purposes, that would also confirm whether its the PSU - especially
if you rule out the on/off switch by using the temporary bridging wire
technique (make sure any reset switch is unplugged at this time as that
could also be a cause). The motherboard would be the last part I'd suspect.

Paul
(PS Zotin my spell checker suggested "Satin" as the correct spelling of your
name...... I bet I know one of the first additions you'd make to MSFT Words
custom dictionary

 

[Zotin Khuma]

(PS Zotin my spell checker suggested "Satin" as the correct spelling of your
name...... I bet I know one of the first additions you'd make to MSFT Words
custom dictionary

Well Paul, I usually turn off the spell checker in any word processor
I use, and not just because of my name. I'm sending you an email about
a strange experience I had in Usenet in connection with my name - I
don't want to risk annoying other readers by posting a long OT message
here.

- Zotin

 

[Mike Faithfull]

My first post to this group and I got a superb response. Thanks to all. I
shall blow the dust off my DVM and dive inside the case as indicated. I
have been known to component repair power supplies in the past, but that was
on "big" (proper!)computers! These "modern" personal jobbies are outside my
realm of experience, so the guidance you have provided is much appreciated.