A good first diagnostics step is to power the console from bench and measure the current draws.

Check your Initial Power Up circuit in the previous article is all ok, and your BAT+ and BAT- are making their way to the system ground and power switch, or just solder wires directly to the power switch and system ground.

Importantly before injecting 9V into the power input, similar to the Atari Lynx, inject instead 6V, turn on the power switch, then measure the collector (large middle tab) of Q502 is outputting regulated 5V.

If not, do not inject any higher voltage, fix the 5V circuit first.

If ok, you can increase the voltage to 9V and observe the collector outputting 5V.

Start by powering up only the motherboard, then progressively add the other components observing that the power draws are as expected.

The following are tests powering the PC Engine GT from bench power supply injecting 9V into the battery spring terminals, on a known working console.

Power draw is no different with or without a game inserted.