I owned two of the 730's which had the same problem. It is common with the 730 to get dirty contacts on the preamp relay, works with it on dead with it off. The fix is to clean the relay. I used DeOxit spray cleaner on mine and it worked perfectly. Try to stay away from the tv tuner cleaner, it will leave an oily film, but just about any other contact cleaner should work.
I used to have aa Icom tech bulletin on this radio for the problem, and they too recommended cleaning the relay.
Hope this helps.
KB5IYB