5V Circuit (PC Engine GT)
The 9V from the battery comes in, through the F500 fuse, into the Q502 transistor (BJT PNP).
Note this isn't a regulator, it is a PNP BJT transistor, that uses current and feedback to generate a 5V rail from the 9V.
Q503, R507, C507 and other components in that area are responsible for the regulation of the circuit.
Test for shorts on the 5V rail by measuring in resistance mode from the Collector pin to ground.
Test the transistor is ok by putting it into a component tester or measuring in diode mode between the pins.
Replacing Q502 with 5V LDO Regulator
If you want to replace a faulty 5V circuit with a standard regulator, you can simply remove the Q502, grab yourself a 5V LDO regulator, solder the Input to the Emitter, the Output to the Collector, and the Ground to a Ground Pad (not the Base).