5150 not booting. Please help?

[modem7]

Didn't the AT- class machines have another pinout for the power connector?

Technically yes. In the move to the AT (5170), IBM used an unused pin (pin 2 of P8 - the pin next to POWER GOOD) to supply an additional +5 volts line.

However, fitting an IBM AT power supply to a IBM PC or IBM XT isn't an issue because on the IBM PC and IBM XT motherboards, pin 2 of the corresponding socket isn't connected to anything.

 

[modem7]

I know a guy who will let me borrow his. Which wires should be positive and negative?

Refer to the diagram at http://members.dodo.com.au/~slappanel555/misc/IBM_5150_PSU_pinout.jpg (from 5150 technical reference).

1. Keep motherboard connected to power supply (so power supply is loaded).
2. Set multimeter to DCV (DC volts).
3. Negative lead of multimeter (black lead) to the metal case of the power supply (good enough reference point in this situation).
4. Move positive lead (red lead) about as appropriate to measure voltage.

As for the POWER GOOD line (wire colour usually orange). That should be sitting at approximately +5V. If that line is dipping periodically, each dip would cause your motherboard to reset, I don't know if a digital multimeter will be fast enough to pick any dips. It will probably depend on how long the dip is.