It's an AT motherboard. What kind of post diag card did you use to test for 3.3V?

Picture of mobo and card attached.

I would guess you would have to try to insert the post card in the pci bus slot to test for 3.3V. I have not looked at one of those close enough to know if this card works well enough in a pci slot. I only have done work with that card in a isa slot, but then you wouldn't look for the 3.3V then off of an isa slot. One of those voltage regulators next to the pentium socket should have 3.3V on the output. You can test for it with a voltage meter. I'd check for voltage levels from the power supply, and the power good too.

Did you try jumpering the CPU to 1.5x66 MHz instead of 2x50? I'm not sure that it would make any difference, though.
The 3.3 V LED being off is normal on the POST card, there is no 3.3 V in these PCI slots.

However, your main problem is the RESET light being constantly on. In this state, the CPU will obviously not execute any code.
I agree that you should check the power good signal and CPU core voltage.

I've taken out the two power regulators (D45H2A) and one of them is faulty.
These transistors aren't in use anymore. What could I use as a substitute?

Is the one that was attached to the bigger heatsink faulty? That one generates 3.3 V. The one with the smaller heatsink generates 2.8 V for MMX CPUs.

After a quick look, D45H11 is widely available and it looks close enough according to the specs.

Yes, it was the one on the bigger heatsink.
Have ordered D45H11.

The new transistor brought the board back to life. Yay!
Thanks for the help!

Excellent, I'm glad the board is fixed!