[Boatanchors] Hickok 539B problem

[Yong Surk Lee]

Hello;
    I am repairing a Hickok 539B tube tester and it has following problem.

- When turned on it tests OK but if leave it turned on the test value goes up to a few times 
the start up value. 
I left it on for a few hours and tested a tube. Tested value is a few times higher
than cold start up value. 

I have seen another 539B had the same symptom before.
Anybody has experience like this before??