Couldn’t find any info on the net on the pinout for these, so I did a few quick measurements to determine the pins required to power up manually.
There is a block of 8 pins on the left-most side of the connector. Bridge the top 2 pins and the bottom 2 pins and the supply will turn on. Note that the fan runs at full speed and is designed to move enough air to cool the 14 drives in an MD1000 so it’s loud…
The large tabs carry the main high current rails. From left to right: GND GND +5V +5V GND GND GND +12V +12V +12V
+5V standby power is available on the middle two right-most pins of the 8 pin block.
Swap the fans out for something quiet and it could make a nice high-current general purpose power supply.