Charging spektrum dx3s via jack connector?

[Monkeysmate]

I don't seem to be able to get either of my dx3s TX to charge via the jack, rather than continually dragging 4 AA batteries out of each... I know that spektrum run reverse wiring for their jacks, but when I plug a jack into the TX, I get no voltage at all when tested with a multimeter.
I would have expected to get circa 5v from the AAs but not a sausage, which would on the face of it explain why I'm struggling to charge via the jack...
I feel that I'm missing something here?
Can anyone enlighten me please?
Ta