Accurate Instruments Model 257 Tube Tester

[Tubejunke]

This is a tester that I used for a number of years due to it's capability to test old and newer tubes as well as vintage color and other CRTs. Like my other post on the Sylvania tester, I am wondering if anyone has owned one of these, but for a more important reason than curiosity.

I noticed a long time ago that I virtually never came up with a bad tube. A member here helped me out with some needed tubes recently that he had tested on a basic emissions tester. I got them and tested them on some high grade units such as a Hickok 6000A and a B&K 747B and nearly all the tubes were bad. Recently I dug this tester out and they all tested good, but don't perform well in circuit.

I have had testers with glitches of various types, but not one that tests most tubes good when either bad or good. I assume that there is some electronic failure going on which I am unfamiliar with. I really don't know the ins' and outs' of tube tester repair. Has anyone seen this condition before?