Assuming you are confident you were measuring the correct pins, the DC should be about 15V, and the AC should be zero or very very close to zero...like only a couple-three millivolts. With no DC output and over 8V AC I suspect that supply rail has one or more component failures. It's still possible something downstream of the supply is the root cause.
You probably shouldn't put your good power supply in that machine.
The next thing is to power the supply unplugged from the motherboard so you can test the outputs with it isolated from everything downstream. To do this you have to make a set of extender cables so you can have the card only connected to the outputs of the main transformer, or have to have a way of powering the card on the workbench completely disconnected from the machine. This gets complicated. This is the next step because it is the only reasonable way to determine if something downstream has failed and is jacking the power supply, or if the power supply has failed (and what failed), or if it is both.
The fuses are all good and it's not blowing any fuses when you power it up, right?