There was a woman who advertised in VTV magazine that repaired Hickock tube testors. You could email Charlie Kittleson the publisher; I'm sure he could put you in touch with her. I think her name was Sue Chaffee or something like that.
John_tracy, I believe your thinking of Kara Chaffee, she repaired my Hickok 752 a couple of years ago but is no longer doing repairs.
The only repair I know of presently is here:
Hickok Repair
Here's a Great page for general info on tubes and testers by Harry Straub
Harry Straub/Hickok