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.