[RESOLVED] [XP] Dell 4600 won't power up

My Dell 4600 began giving me problems a couple of weeks ago. When the power button is pushed, absolutely nothing happens for a period of time, sometimes hours. Finally the fan and hard drives begin to surge as they try to start. Pushing the power button at this time allows the computer to power up. Then every thing runs normally.
I have checked the power up button and it shows ok. I removed the cards and memory, and disconnected the drives. I measured the voltage across the two pins that the power-up button is connected and measured slightly over 3 v dc there. When I had the cards and memory removed, and the drives disconnected, there was a slight improvement, in that the lag before beginning to surge was shorter, and it would even start all the way on its own (again, after the time lag).
Yesterday, I had it all put back together, and I changed a setting in the shutdown instructions, and when it shut down, I left the power strip that it is plugged into on instead of switching it off. When I pushed the power on button, it started immediately. I did this a couple of times, and it worked perfectly. Then, I shut it down again, planning to leave if off all night, but noticed that it was trying to power up on its own.
Any ideas?

Hi Bobrobert, welcome to Virtual Dr :)

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I measured the voltage across the two pins that the power-up button is connected and measured slightly over 3 v dc there.

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That voltage should be 5 volts DC, and I suspect that might be the problem, the the Standby voltage isn't enough to fire the PSU up. Here's how to test a power supply, to make sure that it is that rather than a problem on the mobo:

Testing PSU voltages

In depth look at the power supply

The 4600 is known to have a poor power supply. (tons of posts on Dells forums) I have a three year old 4600 and haven't had psu issues yet.

Thanks SuperSparks for the wealth of information. I am still reading and haven't checked the power supply voltages yet. However, I have been monitoring the behaviour of the voltage across the aforementioned pins at the start-up button. When the computer is first plugged in, the voltage is about 2.5 volts, then rises slowly. When the voltage reaches about 3.1 volts, there is a single surge of motors. Then, pressing the power button starts the computer normally. Yesterday, reaching 3 volts took about 5 minutes. This morning, after the computer was unplugged all night, it is stable at 2.8 v, and has been for an hour.
Thanks to you also Steve.

Well in that case it is a power supply problem for certain. Some Dell's used to use proprietary PSUs - I'm fairly certain that yours just uses a standard ATX, but it would be a very good idea to check the colour coding of the motherboard connector to be absolutely sure. You'll fry the mobo othewise :eek:

http://www.xbitlabs.com/images/other/atx-psu/p1.gif

Many thanks Nick. I just checked two voltages. The PS_ON voltage was 3.2 volts, the VSB was 3.2 v and at this time the voltage to the 'power on switch' was 2.2 v.
So, I guess I begin shopping for a power supply.
One thing though, when the power on switch is pressed, the PS_ON voltage didn't drop. I'm guessing that this is because the voltage at the power switch was insufficient to trigger the switch needed to get to the PS_ON. Another thing that I was wondering about is shorting the PS_ON, in order to make the PS_ON voltage drop below the .8 v needed to make it go low. I won't do this though, for fear of shorting the whole mobo.

one thing before I pull the power supply though. All voltages are correct when the computer finally does take a notion to start up. So, if it is a power supply problem, it is only before start up, and is intermittent. Does that make sense?

Yes it does, the problem here is with the Standby voltage, which is supplied by a totally different part of the PSU to the other rails. Once the PSU has actually fired up then the Standby voltage takes no further part in the running of the PC, which is why oit is all fine when it has actually booted up.

OK, success. You were exactally right Doctor! I put in a new power supply, and the computer has been starting properly since I did. I'm thinking that the 'power ok' was sensing bad voltages, and prevented start up until the voltages did become ok, which sometimes took hours. I checked with Dell and Best Buy. Dell had a replacement (I guess 250w since that is what the original was) for $50 counting shipping, and Best Buy had a 350w for $55. I bought the one at Best Buy and installed it. Thanks Nick,I am forever grateful for your help.

You're very welcome, I'm pleased that you got it all sorted out :)