Another thing that may not be evident (unless you own a D5500) is that this power supply board lives up in the lamphouse. In fact the thermal overload thermistor is on this board.

That big orange electrolytic capacitor has a date code of 1983 on it and this unit came from a busy lab. So, I suspect every component on the board has had a rough life.

I was looking over the components I removed and I did find a fried 27 ohm resistor leading to the power pin of the IC U14, which contains a bank of general purpose transistors. It still reads 27 ohms and its not likely the cause of the light not working, but just an example of how every component on the board has likely been stressed.