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Troubleshooting power supply and on/off switch

by leojohns / October 13, 2005 3:04 AM PDT

Computer won't turn on. Tried the obvious, different power cord, different recepticle. Is there an easy way to determine if it's the power supply or the on/off switch? On/off switch feels the same when pushed in, not mushy or loose or anything like that.
Can these be checked without taking the whole thing apart?

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easy things first
by linkit / October 13, 2005 4:15 AM PDT

On the back of the computer where you insert the power cord into the power supply, there usually is a rocker switch to turn on/off the computer. Is it in the on ("|") position (not "0")?

Is your power strip/surge suppressor turned on?

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No rocker switch
by leojohns / October 13, 2005 5:48 AM PDT
In reply to: easy things first

This is an old store bought unit with only on/off switch on front panel. Sometimes I think I see the green LED for the HD blink when I hit the switch , but not always. Everything else on power block works and I tried this unit in the same hole I run my new unit out of.

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Yes, no, maybe...
by Dick White / October 13, 2005 7:28 AM PDT
In reply to: No rocker switch

and all of the above, but as we are now two steps farther in on the thread structure I don't remember the order in which you asked the questions Wink

There is no easy way to ''test'' which item is failed without taking it apart. Generally speaking, power supplies are frequent culprits, so you can often get ahead of the curve by getting a new one from your local shop/ BustBuy/ ChumpUSA/ or whatever. If, a few steps farther down the troubleshooting path, you find you don't need it because the problem is found to be something else, you can probably return it for your money back. But having it on hand when you open up your system will save you running out in the middle of the job to get the part that we could have told you in advance is 90% likely to be needed. So for the rest of my reply let's just assume you now have a new one.

The power switch on the front of the case is just a momentary contact switch. The motherboard always has a little bit of power, and pushing the contact switch simply completes a circuit between two little prongs on the motherboard that signals the power supply to start supplying full juice to the system. When you open the case and trace the wires from the backside of the on/off switch down to the motherboard, you can pull the those two wires off the motherboard and touch the two prongs with a small screw driver (be careful poking around - touch only the two prongs that were underneath the wires from the switch). The system should come to life. If so, then the switch is bad, it's not making the momentary contact to complete the circuit as it should. Replacing it will be a chore as there are tons of styles and sizes to choose from. But I doubt the switch went bad, so reconnect the wires to the prongs on the motherboard.

Step 2 - I suspect the system will not come on even from the screw driver ''jumper.'' That means either the motherboard is bad or the power supply is bad. If the motherboard is gone, that is really bad news, but less likely than the power supply. As noted at the beginning, power supplies crap out all the time. Since it is both easier and cheaper to replace the power supply than pull it and send it out for testing, that would be the next step. The supply itself is held into its rack with 4 screws on the back. The tangle of power leads will need to each be disconnected at its termination (one or two will be directly on the motherboard, several will go to the backends of drives, fans, and other peripherals). The connectors are sometimes tricky to wiggle apart, but they are all standard size/types so for everything you take out for the old supply there will be one just like it on the new one. You just mate them all back in the manner you took them apart. When you get all the connectors back together, plug it in a try again. A dollar to a donut, it will start up. Satisfy yourself that it works, and then turn it off. While you have the case open, clean out the dust bunnies and the heat sinks and fans. That will improve cooling efficiency. Then put the case back together and get on with your life.

Possible step 3. If the power supply replacement doesn't fix it, you've got a motherboard problem. That could be as simple as a broken fan (some boards have a sensor to determine if the cooling fan is operating, and if not it shuts itself off to protect from overheating) or more complex, but easily repaired, loose connections, or a deeper failure of the motherboard. If you sense some activity upon clicking the switch but an immediate stop, then watch the fans to see if they spin up or not. Make sure all memory and peripheral cards are fully seated. If it still won't start, you'll have to consider failure of the motherboard.


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Power supply problem still......
by mercury43 / January 11, 2009 5:23 PM PST
In reply to: Yes, no, maybe...

I've read several responses and tried a few things that were not listed. My PC would not turn on. Replaced power supply, power light on motherboard now showed power, but after hitting pwr switch, it only ran for 1 sec before shutting down. Took all periphials apart and put back together. That got the pc to run for 10 secs before shutting down(by the way, only way to restart was to unplug power supply and plug back in). Next step, I figured may be a heat sink problem(even though fan was running and cleaned), so i cleaned the heat sink and reapplied new silver heat sink thermal paste, and put back together. Same problem, which tends to make me think the motherboard is bad. If anyone has any other suggustions before I just get a new pc, I'm all ears.....Mercury

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Power switch followup
by zaarenoc / January 13, 2009 11:56 AM PST
In reply to: Yes, no, maybe...

What about when you plug the powersupply cable into your tester and the power supply comes on. Not when you plug it into the mobo though.


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Quick checks...
by Willy / October 13, 2005 4:18 AM PDT

You need a meter to check it out directly. I suggest though, that you remove the side cover. Find the frt. power wire harness as all it is a momentarily contact sw. and then at the mtrbd. short the pins it connects to. -OR- reset the harness afew times to break the contact surface abit and try again. If none of that starts the system, you probably have a bad "start ckt" on mtrbd. or power supply(psu). If you notice a lit LED on the mtrbd. it shows one voltage suggests the start ckt is gone, remove the AC cord for 5min., re-attach and then try again. If it starts then, bad start ckt.. If not, thgen replace psu with ahigher wattage unit and hopefully that does it. If not, then replace mtrbd. as last resort. These are areas I would look into for now and pretty much covers it. Though can reduce power demands and unplugg the CD drives and note that on the bios if it starts to boot. if it boots then, bad psu, replace.

tada -----Willy Happy

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Friend had the same problem
by techtype / October 14, 2005 1:57 AM PDT

A friend of mine had the same problem. He checked out his everything you did. Still the problem persisted. So he took his self built computer to the local computer store and repair shop. His symptom was, as long as he held the power switch "ON" it would boot up. Come to find out his power supply was faulty. To correct it he had them put in a new power supply. He has never had a problem with his computer since.

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The correct tool for the job...
by johnbiest / October 14, 2005 4:09 AM PDT

Antec makes a power supply checker - about $15 at CompUSA, hanging where the fans and case lights are. You only need to open up your computer and unplug the main power from the motherboard to check your power supply. It simply accepts that plug and puts a load on the power supply - if the light goes on, your power supply is not dead. This bypasses checking your case switch, motherboard, etc. And for $15, it's not too bad. I find myself using it quite a bit more than I ever thought I would.

It only tells you that there is some juice, though. You will have to use a voltmeter along with the p.s. checker to see if you are getting acceptable voltages (it comes with simple instructions on what to do here).

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Computer won't turn On, . .The On/Off Switch Test
by Evoj / November 8, 2007 9:29 AM PST

I am wondering because my on off switch is six wires to each to the led's and two to the switch itself, . . . are these on a circuit and if they are will the screw driver contact work ? Cause if it does then in this case the contacts would be pins three and four, but if there on a circuit, I'm not sure how to make the screw driver contact work, . .

Also I found heat sink full of dust if the comp. turned off because of this and did not warn me of temp. is there some kind of switch like a circuit breaker that I do know about ?

I have power coming out of the PSU, but that it for a computer that wont turn on.

Compaq Presario 5300US
Operating System
Microsoft Windows XP Home Edition
Version: 5.1.2600
Service Pack: 2.0
System Model: DSDT
BIOS Version: Compaq 686P9 v1.01
Memory (RAM)
Capacity: 256.50 MB
Intel Celeron processor
Version: x86 Family 6 Model 8 Stepping 10
Speed: 1096 MHz

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Computer won't turn On, . .The On/Off Switch Test
by Evoj / November 8, 2007 12:27 PM PST

I fooled around with it a bit removed the switch from the pins replaced it unplugged and plugged , . .I got one of the leds to blink and I saw the PSU fan spin, 1 or two sec. at the most,it would not stay on, attempts to do the same thing, it wont blink unless I unplug and then plug in, but I only get a blink.

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Short the two pins quickly
by Pluto035 / January 3, 2008 7:54 AM PST

I've been having this same problem recently. In my case I have to short the wires extremely quickly or else it'll just turn the system off again which is what you're probably experiencing. I made my own switch out of some old pwm cables and used them to make the shorting easier and I found that it worked best after I cycled the power supply with the switch on the back... not really sure why. I'm going to try to replace the switch soon although I'm not sure where to get one from.

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same problem
by vandervere / January 5, 2009 3:22 AM PST

chip fan spins once-on light flicks on- then goes dead.
This happens before i press on switch !system stays dead until
i turn power off then on and the same happens.
When i checked the on/off switch the terminals make a circuit
with a meter when switch pushed in or out.i now suspect switch
and am looking for a replacement.

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Just so you know.
by R. Proffitt Forum moderator / January 5, 2009 3:28 AM PST
In reply to: same problem

If you plug in an IDE drive connector upside down or off by a pin or row (or the drive is dead) the common PC will do exactly as you wrote!

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Replacing power switch with a line volatage push button
by polites11 / January 10, 2009 1:50 AM PST
In reply to: Just so you know.

The mounting plastic supports for my power switch recently broke. The switch itself is still ok. My options are to either find an adhesive
which will reconstruct the broken supports or to disconnect the power
switch from the wiring and replace it with a line voltage push button
switch. Before I do any of the above, I'd like to obtain input as to
the reasonableness of any of the above.

Does anyone have first hand experiece with substituting the swithces?

C.N. Polites

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For your common ATX PSU and today's machines.
by R. Proffitt Forum moderator / January 10, 2009 1:53 AM PST

Just about any pushbutton marked "Single Pole, Momentary On" will do fine.

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For your common ATX PSU and today's machines.
by polites11 / January 10, 2009 3:12 AM PST

Thanks for your advice.

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Power On/Off button stopped working
by DWL428 / December 24, 2010 2:38 AM PST

I have a Acer H233H Monitor and not long after I purchased it, the button in question seemed to simply stop responding when pushed. Your reply to "Just so you know" sounded so much like my problem it moved me to join the forum. Rather than search the forum I thought I would reply directly to your post, so maybe I will find out a little more about your advice. I pried open the bezel and of course that revealed the broken plastic that is the reason the button would not work. Did you have any luck with your monitor switch ? The "line voltage push button switch" sounds like just what I need. (?) I separate the bundle of four wires simply by pulling the connection a part. Do you think its too much to ask that the part I buy, if there is a place to get it, that it will fit the wire connection or will I have to strip the ends and twist them together ? Sorry to drone on I'm working off of very little sleep. I sure hope You can give me some help with this, thanks either way. Here's my e-mail address feel free to communicate with me that way and maybe , if you want I will send pictures instead of babbling. ( lol )

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re ide cable placed incorrectly
by vandervere / January 10, 2009 3:59 AM PST
In reply to: Just so you know.

appreciate the advice but problem occurred without touching cables
or interior. have replaced power supply with new and retested on
switch but computer is now dead-no sign of life at all.-- vandervere

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On/Off Switch Compaq Presario R3000/windows Professional XP
by LENBOY1971 / March 9, 2009 5:16 PM PDT

My computer comes on most times but now it will not responce to my pushing the on/off switch. It lights up the front light as you push the switch on but it goes off the second I let off it,the light goes off. I feel it has to be the switch.I just got a new battery.
Next step is to take it apart and replace the switch.Sounds like fun.

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