This little PCB is creating the -5V power rail from the -12V rail.

Some older ISA cards require the -5V as a reference voltage. Unfortunately, with the introduction of ATX Power Supplies, this voltage became less important and was eventually removed.

Modern ATX power supplies do no longer deliver -5V.

Nevertheless, with adapters, it is possible to use ATX power supplies to run retro hardware.

With the -5V missing, some hardware may exhibit unpredictable behavior.

An example is the Creative Sound Blaster 2.0 which does not create sound output when -5V is missing.

Since ATX power supplies are way more reliable compared to old AT power supplies,

a solution to re-introduce the -5V rail is necessary.

Fortunately, -5V is only used as a reference voltage and does not need to deliver much power.

Therefore, a simple linear voltage regulator mod is enough.

In this video, you can see one use case for this project: