Adam’s Yaesu G-5500 stopped working 🙁

Meter lamps came on, but non responsive otherwise.

An open circuit diode was found, D3, so no 0V GND rail existed.
Easy to find using an ordinary DMM.

A better method of eliminating the voltage drop problem across R10 20 ohm 5W resistor (that feeds +12V to the External Control connector) was suggested by Mike, WB8CXO, and implemented. 

This involves removing R10 (20 ohm 5W WW resistor) and replacing it with a resettable fuse.

No voltage drop issues, and still with short circuit protection on the 12V feed to the External Controller connector.