I spent the afternoon up to my elbows in grease, stripping down the elevation half of the G5500 rotator, which had seized up.
After much fun removing a stuck casing screw (had to drill the head off the screw in the end), the seizing appeared to be caused by a small hole in the top of the casing (which seems to be a deliberate design thing) which allowed water into the casing and had rusted a small part of the large output gear (top-right of the photo). This corroded on the part of the gear directly below the hole (you can just about see the orange-ness of it in the pic), and also where the large gear met the gearbox, causing it to seize.
After a strip, clean and re-grease everything appears to be good to go again. The casing bolts have been replaced with proper hex bolt head ones, instead of the philips head ones it came with. Before it can go on a roof a new batch of sealant needs to be applied to the case edges (making sure to fill the hole at the top), when I can find somewhere to nick some from.
While I'm at it I'll crack open the casing on the azimuth half tomorrow and check everything looks okay in there...