DC to LIGHT op amp, uPD1678 (actually a MMIC), located on the RF unit (bottom side of radio, rear left, remove shield cover) will shut down ALL tx on ALL bands when it fails. I have replaced three of them so far.
MMIC is the "i1678", actually a uPD1678. Input is pin 1, lower right, and output is pin 5, upper left pin on the SMD chip. Check in FM mode any HF band, using RF probe, should have very small sig on pin 1 and very large (>.6volts DC) on the RF probe when applied to pin 5. When chip fails, pin 5 goes to very near 0 millivolts on the RF probe.
uPD1678, icom number is 1110001890, lists about 10 bucks or less. Changing this chip requires proficiency in surface mount desoldering, and anti-static protection measures.