It all depends on what you believe is required for calibration. I've become more and more convinced that most tube testers should be thoroughly gone through if you're going to bother. That means checking every resistor and capacitor (which may requiring some desoldering). This is what the guy who redid my 700 did, it's time consuming and a working knowledge of the device in addition to the tools and equipment portion. My tech's reasoning, which I agree with, is that otherwise you're simply calibrating the tester at a point in time and if you have components that are drifting the calibration that you just performed is essentially worthless. The 747 by virtue of it's SS design my be more immune to this than others but it's at least worth considering and deciding whether or not you (the buyer) feels comfortable with the end result.