Can you receive anything in the other bands-- other than that HF net you reported? ... like the AM broadcast frequencies ? I don't know about your antenna, but I'd stick a simple 10 foot piece of wire in the antenna jack and use that for your tests- you should hear at least the AM broadcasts if the 720 is working at all.
A quick glance at the schematic looks like the signal is then injected into the RF Unit at the 1st Mixer stage thus bypassing some of the front end
circuitry including the TR relay. Also, be aware that the Low Band Antenna input jack is dual functional and if someone has made the
appropriate jumper changes inside the set it can also serve as a relay output to key up a linear amp for example.
So, to sum it up:
1. Sticking a wire in the PL-259 main antenna connector will not be useful for AM broadcast reception
(although the strongest local stations may bleed through).
2. If an antenna were plugged into the Low Band jack and AM broadcast stations were heard the front end is still suspect
as it appears to have been bypassed.
3. If the Low Band Antenna jack was internally jumpered out you would hear no AM stations (except maybe strong locals bleeding