Power leads going into A1200?

[AmmoJammo]

sounds broken.

if the plug that goes into the 1200 only has the + and - 12volt lines still connected, it would mean that the motherboard would be getting its ground from the floppy connector, via the ribbon cable, and wont be getting the +5volts at all...

I wouldn't even be using it.

are you sure the old PSU was even faulty?

[AmmoJammo]

I wanted to replace the PSU but no current PSU out there has the  odd-ball aux power connector that the Amiga adapter cable (just a cable,  no hardware on it) plugs into, anymore. That connector is here:

http://www.playtool.com/pages/psuconnectors/connectors.html#aux

and that isn't the connector your power supply used, this was already discussed in the other thread.

If it used that connector, you'd be giving the A1200 3.3, and 5volts.

As I already said in the other thread, your old power supply had an AT power connector:

http://www.playtool.com/pages/psuconnectors/connectors.html#oldpc

with +5volts and +-12volts.

edit: heres the other thread for those wanting the background info:
http://www.amiga.org/forums/showthread.php?t=62317

[AmmoJammo]

Solder wires directly to the bottom of the A1200 board.

Leave the original connector intact on the motherboard.

You could even use a standard 4 pin hard drive connector for the +12 and +5volts, and just a spade terminal for the -12volts.

[AmmoJammo]

I'm beginning to think you should have gotten someone else to do it for you.

If you measure between the 5 and -12 with the leads around the other way, does it still show a short circuit?

[AmmoJammo]

could you please take a clear photo of the bottom of the board?

[AmmoJammo]

your +5 volts is connected to the ground plane.

I'd suggest using a multimeter to figure out where each pin goes, and rather than solder to the underside of the connector, solder to the row of 4 pins just down from the connector, as they're a larger pad to try and solder to.

[AmmoJammo]

your +12volts also appears to be connected to ground.

[AmmoJammo]

I went and checked my spare, dead A500 motherboard.

-12volts is actually in the right place! that's a start... :/

and your ground is connected where +12volts should go...

It appears you've swapped every connection from left to right.

I have no idea if the motherboard will still work after this abuse.

[AmmoJammo]

I powered up the board to make 100%, with out a doubt sure, that those voltages were correct, and this is what I measured.

[AmmoJammo]

Do it now!

[AmmoJammo]

I would.

Although I have no experience with an A1200, or A1260.

The power connector on the A1200 is a long way from the expansion slot. So any high current device on the expansion slot is probably going to suffer from voltage drop...

Adding voltage to the floppy connector can only help with this