I purchased a secondhand AVL58. This is fitted to an f550.

The range is very limited. At 100m without obstruction things get fuzzy. At 200m it's a complete snow storm. Using the standard TX antenna. I am on the right channel. Things I've tried,

Used both AVL58 RX and goggles with the same result so it's the TX.

Switched every channel. Some a slightly better but overall it's not good

The antenna is pointing horizontal to the front of the drone (seems to be the standard way on the f550)

The next thing I'm thinking is to hook in a power meter to have a look at what sort of watts it's pulling. This might indicate damaged hardware if it's low wattage? I've got no other ideas.