Problems with ATX power supply

Hi all, I'm a noob based in Wellington, New Zealand.
I have nearly completed a Prusa Mendel and have modified an old 500W ATX power supply that I have had sitting around for ever.
Mods done according to http://www.reprap.org/wiki/PCPowerSupply#Building_an_affordable_power_supply
At power on, nothing happened. Each time I cycled the DC switch, the fans would give a twitch and then stop.
After a few switch cycles the fans start up, the LED comes on and all output voltages appear at the terminals.
The number of switch cycles that it takes to kick start this thing is variable and when running, the PS emits a very high frequency squeal.
It is right at the limit of my hearing but it irritates the hell out of my wife.
Is there likely to be a simple solution to this (the switch is OK) or should I just dump it and buy a PS off eBay?
Any advice is appreciated.

Check for swelled or leaking capacitors, swelling can be minimal. I had a bad squeal coming from mine, but not the power up problem you have.

I replaced 3 capacitors and that fixed the squealing. I've had it on almost 24/7 for a couple months now without a problem. Probably only cost effective if you have spare caps, or something to salvage them from. I pulled my from a dead PSU.

Not sure what's wrong with your ATX power supply, but you don't have to go through all that trouble to use it on your Prusa. You mainly need the +12V leg. Just take all the yellow wires in the same leg and combine them into the +12V terminal of your printer controller board and take corresponding ground (black) wires into the Gnd terminal of your controller. Then put a 10 ohm/ 10 watt load across one red wire (+5V) and corresponding black wire.

Is it possible that your PS has multiple legs for the +12V? I wonder if connecting the output from the two legs together would cause a problem like this. Depending on what type of controller you have, you might want to use one leg on the main power of the controller and the other leg to power your heated bed.

Edited 1 time(s). Last edit at 10/28/2012 05:50AM by brnrd.

It's important to connect the PS_ON signal to ground as well.

I'm sure that the instructions he followed included that. It won't come on if PS_ON is not grounded.

Thanks for the replys guys.
I've shelved this PS for a while. I could spend a whole lot of time and money trying to sort it out and new PSs are so cheap.
I've bought a 12V 30A PS off ebay for $US28. Should have done this in the first place.

Sounds like you don't have the power on wire grounded. Normally it's green on the large connector. This is a soft on / off that normally would be controlled by the computer main board.

It wouldn't even start if that wire (the green one) wasn't grounded. High frequency noise sounds like a buggered capacitor, one of the big ones.
Buy a new one here like this one I use: Ascent
Make sure the one you buy off ebay is 230/240 volt

Edited 1 time(s). Last edit at 11/21/2012 03:20AM by Wired1.

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