To service Hickok tube testers usually requires little beyond a few spares and a good DVM.
If you are so inclined, it is a good idea to have a spare 83 mercury rectifier tube (do not use an 80 since regulation is not as good though replacement with silicon is possible) and a 5Y3. In addition a few #81 6 volt bulbs are handy when the "fuse" blows as the tester will be unresponsive until replaced.
Lately I have seen several stores offer the #63 bulb as a replacement for the #81. True, both are 6v but the #81 has a resistance cold of 1.0-1.2 ohms while the #63 is in the 1.6 ohm range. According to the specifications, the 81 draws 1.02A at 6.3v or 6.2 ohms hot. In a 533, the 81 drops about .33v at 360 ma or about the same resistance as when cold. Low current tubes will not show much difference with a 63 once the line voltage is set (will be effectively lower). One easy way to tell if a Hickok has a #63 bulb in it is that the bulb will glow when the tester is turned on. A #81 will not unless the tester has a problem.
A type 97 (12v) bulb will act about the same as a type 63 in a Hickok. I have even seen a type 1156 somehow placed in a TV-7 that worked about the same but the correct thing is to use the type 81.
However, this should not be a problem as General Electric still stocks the 81 bulb at least as of January, 1998.
AES also carries the type 81 at a markup over a card (10 bulbs) from GE however if you only want one...
Note: it is difficult to test an 83 or 5Y3 if the tester is not working.
Other than that, it is rare for a Hickok to develop problems. When I receive one, I generally start by letting the unit warm up for a minute or so, set the filament to 1.5v and the selectors to DX. Pins 1 and 7 of the 7 pin minature socket will then have filament voltage - I check the DVM readout after adjusting the line voltage - should be 1.4vac +/- .05v. If that is good, I reset the tester for a 1R5 and check against a "gold" tube - one with a measured output. If that is reasonably close, then the tester is generally good.
As I get time, I will try to add the alignment specifications for various models.