The symptom is a low voltage on the "13.8V" line of the ACC port when an external keypad or FR or IFR is connected to it (or another ACC powered device, of course). The cause has been a short between the "13.8V" line and ground, that has damaged the tiny R1235 10 ohm 1/8W resistor. In both reported cases the resistor has increased its resistance, effectively protecting the remaining circuitry.
On the circuit diagram it is obvious that R1235 is the only protection on the unfused Vcc line (internal battery or external supply) for the 13.8V ACC output. There is a resettable fuse, but that one goes towards the FINALS board and has nothing to do with the ACC port.
How to fix it? I can think of three ways:
- replace the resistor with an identical one (it is tiny, you need proper equipment, SMD reworking ability and a firm hand);
- replace the resistor with a physically larger one, and a fuse if there's room for that [but be aware that a 1/4W resistor won't blow up the same way!]
- parallel the existing resistor with another 10-15 ohm one, but still consider adding a 500mA fuse
Summing it up. Be very careful with the Vcc line on the ACC port. If you are reading this because the damage is already there, refer to Yaesu assistance or a skilled electronic technician (yourself?) to trace and fix the damage.