Rad Alt systemsreally are very simple and reliable as mentioned above, the T/R (the box) is the heart of the system and nine times out of ten is always the cause of the problem.
This has not been my experience. Perhaps this is an aircraft type/manufacturer thing? I haven't changed a Rad Alt box for 20 years, but have had lots of bonding problems with antennae.
Some aircraft rely on bonding through the screws (and countersunk holes on the antenna). The mating surface of the antenna and the fuselage don't always provide a good bonding surface, especially if the corrosion inhibitor is forming a barrier.
We started to find some of the countersunk, metal surfaced holes on the antenna painted over with white paint (not sure if this is ex-manufacturer or ex-workshop). This was creating lots of problems with bonding.
Rgds.
NSEU