Also probably would apply to IC 735 and IC 745 as well.
The symptoms were distortion on transmit, a very unstable display,
often tuning dial would not move frequency, display would give
all kinds of random numbers, sometimes no numbers at all on sections
of display readout. This appeared to be a mechanical problem, as a
sharp "rap" on the case would change things, sometimes curing it
temporarily. We pulled off the covers of the set, and gently tapped
each board. The PLL board appeared to be the culprit. This is the
small covered board to the rear left on the underside of the 740.
We popped off the little cover over the board, loosened the 4 mounting
screws and took out the board. Everything looked good, but the points
on the board that contacted the metal standoffs were corroded and dirty.
We flowed fresh solder around the screw holes on the board (underside),
cleaned the standoffs and cleaned the chassis where they connected
to it. This cured the problem. Interestingly enough, we did the same
fix to an Icom 22S 2 mtr rig several years ago that would keep going out
of "lock". It cured that problem as well. The bottom line is, make sure
all your board to ground/chassis connections are real clean. Just a
thin film of crud can cause all kinds of problems on a unit that uses
the mounting screws as the ground return. Sometimes (but not always!)
just loosening and retightening the mounting screws will help the