ATX Power supply question
I'm building an Athlon system (Athlon 900, ECS K7S5A MB, 256 M SDRAM, ATI 32M video) and everything went pretty smoothly 'till I tried to power it up and nothing happened. No fan, no PS running, no nothing. I searched several posts on this and other forums and have tried the following:
- jumpered the power-on header on the MB
- checked all jumper settings and connections
- removed the board and PS from the case and tried to power it up that way
- tested voltages from the PS
- cleared the CMOS jumper.
None of these made any difference. The only possible problem appeared to be the PS. when I jumpered pin 14 to 15 it would run, as would the case fan which I had connected to it. However, when I checked the output voltages (both with the PS plugged into the MB and unplugged) the only pins that registered were
pins 14 (3.3 v) and pin 9 (5 v). Is this normal or not? Will the other pins only register a voltage once the system is powered up?
If the PS is ok, any other ideas on what it could be or other troubleshooting I could try?
Thanks in advance and merry xmas.
Right at the moment, I can't remember exactly what pins 14 to 15 do when jumpered. If it powers up with those two pins shorted, but you only read 3.3 and 5 volts, I would guess the supply is bad. Try the following, take the meter leads and place the black lead in either of the two middle holes in one of the molex leads coming from the supply. You have a yellow, a red and two black wires in the middle. The blacks are common, and are connected together inside the case. The read wire is 5 volts and the yellow is 12 volts...(or the other way around...can't remember right now) anyway, you should be able to read 5 volts on one between the black and red (I think) and 12 volts between the black and yellow (I think). If you do not read anything, plug a fan or a hard drive, cd rom drive or something into the molex connector (one of them...dosn't make any difference which one) to load the supply. Most supplys don't need a load to make them work, but just in case yours does...load it with something. If that dosn't cause it to work, then you probably have a bad supply.
Other possible causes for the supply not coming on is: 1) A bad case switch, 2) not connecting the two wires from the case switch properly to the mainboard headers, 3) a defective motherboard trace between the pin headers and the socket where the psu connects to the mainboard.
ATX Power Supply Question
Thanks for the reply.
If I have the ps unplugged from the Mobo, I get all the voltages as you described, the ps fan runs, as do any drives or case fans connected to it.
Once I plug the ps back into the mobo, everything is dead, no fans no lights , no nothing. I also tried removing the mobo from the case ( in case of a short to the case metal) and it was still completely dead. I'm leaning now more towards a dead motherboard.
What do you think?
Yep, sure sounds like a defective mobo. One more thing to try before shipping it back. Remove everything from the mainboard...cpu included. Plug in the cable to the mainboard, and with a fan or something plugged into a molex connector, and with the meter also monitoring either the red or yellow wires from another molex plug, turn on the supply and short the pins to start up the supply...I know, kinda hard with the cable plugged into the mobo, but you must start the supply somehow. If it then has voltages, something else is shorted...maybe the cpu has a dead short, killing the supply. A supply will shut down if it is overloaded with a hard short.
All ATX power supplies has a seperate 5 volts that runs continusely when the psu has AC connected. This 5 volts is used to turn on (or off) the main power supply...so, it is necessary to get that 5 volts back to the psu in order to turn the main 12 and 5 volt part on.
You can try simply shorting those two pin headers on the mobo where the case switch cable 2 wire hooks up...the case switch is a momentary type switch that provides a short when you press it in...so, using a small screwdriver or something else small and metal to short those two pins is fine. If this dosn't start the psu, you may have to insert 2 very thin wires in the mobo socket molex connector before plugging it into the socket on the mobo, then when you are ready, short the two thin wires...
If this fails to get power out of the supply and the mobo is plugged in...the mobo has a short somewhere.
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