I was just given an old (from what I can tell) IBM AT power supply. It's pre-ATX and has a physical switch that connects the wall power to the internal circuitry.
12 V 8 A
5 V 20 A
-12 V 0.3 A
-5 V 0.3 A
Now, the 12V rail sits around 10V unloaded, and if any load is connected, the voltage drops to basically 0. However, if the 5V rail is loaded (tested at around an amp), suddenly the 12 V rail maintains around 11.2 V 0.7 amps.
I'm wondering whether this is expected behavior or if something is seriously wrong with my power supply?