I'd check the pot in the Tx itself.

I've only owned a few HLs and have experienced bad solders on the RX-18, in the Tx, broken wires in the harness as well as non matching wire colors so they can be pretty frustrating. You can't rule out anything. I ripped apart one spending hours to finally find a broken wire still in the insulation.

You are getting some good help, but it's hard to diagnose this way. I'd start from scratch, isolate the elevation motor itself to make sure it works via a simple battery hookup. Then use the RX-18 schematic and just plug it the elevation (plus battery and Rx). If still no go, then open the Tx and look for loose wires.

One thing that I've found you absolutely need if you're going to stick with HL is a good multimeter and an alternative battery source and some wires with alligator clips that you can use to jump connections.