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Power Supplies
Last updated: 5/8/02

Q.  I replaced the power supply in my Baby AT computer.  Now it trips the power line circuit breaker...

A.  The older models PS/2 form-factor, AT power supplies have clips on the end of the cable going to the power switch, which clip onto four lugs on the switch. Newer ones come with the switch soldered to the cable and they are shrink-wrapped. If you installed one of the older supplies and attached the cable to the switch, the most likely cause of the problem is that the wires were not plugged onto the correct lugs on the power switch. Most power supplies have wiring diagram on a label stuck to the top of the supply. If the power supply was one of the newer models, the most likely cause of the problem is that the power cables going from the new power supply to the motherboard were plugged-in backwards. The plugs plug-in so the black wires are in the middle… “Black you’re OK, red you’re dead.” Unfortunately, if that is the case, the motherboard may now have been damaged. It may have been dead to start with, but other things could have caused the same symptoms.  If the switch and motherboard connectors appear correct, try to isolate the problem.  See if the circuit breaker still trips if all power supply connectors are disconnected. Look carefully at the motherboard power connectors again to see if one (or both) of them is plugged-in so it is shifted one pin on the connector. If you are using 220 volt power, be sure the 110/220 volt power switch, on the back of most power supplies, is in the correct position. Try another power cord. Check the power supply drive connectors for pushed-in pins that are shorting. They can usually be worked back out with needle nose pliers. Pick-up and shake the computer and listen for loose screws. Check for cock-eyed expansion boards. Check expansion slot covers are installed correctly and not shorting the motherboard, etc. See if one of the speaker wires broke off the speaker and is shorted to the case (real long shot and you will usually see sparks instead of blowing the breaker). The new power supply could be bad or have something loose inside of it.  Remember, if you open the supply there are dangerous voltages inside, so unplug the power cord first. Also, you can be zapped by a disconnected power supply if you don’t wait for things inside to fully discharge.  Do this at your own risk.

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