Because the fan is making noises like it won't
work for long. In general,
I'd rather have no moving parts in there, since a fan is much more likely
to fail over time than old logic (I got these boards off Ebay and only had
to replace a total of 8 chips, only one of which was not a 4116, and
that's on a 30-year-old board).
Have you tried to use the AT PSU without the fan? Sometimes the current
used is so low, it can turn the fan off
Also, I only have one power supply between the two
sets.
But an ATX power supply is SO cheap...Do you need any help hooking it up
directly to the board and measuring current?