mrmox, if you were to put the ultimate transistor tester on the market, here are some cool features that it could include:
-- precision 5 volt regulator
-- high precision resistors
-- header pins for programming
-- a hardwired capacitor for convenient self adjustment
-- a relay to short connections whenever a measurement is not being made, preventing accidental tester destruction
-- an ATMega in a DIP socket so that it can be easily replaced if it ever gets damaged by misuse
-- clear markings to show the connections for frequency measurement, voltage measurement, and function generator
-- a 16MHz crystal
-- the latest M or K firmware
A typical user does not really need all of the above, but there is no reason that a kit or completed device could not have all of the above features. I would like to buy one.
Since an ideal tester is not available now, and since I don't quite know how to design my own board, I ordered an AY-AT type of kit from eBay, and precision parts from mouser.com. Earlier posts from "blurpy" describe what to do, and I expect that it will all turn out quite well. My tester will not have the relay or the self adjustment capacitor.
For that matter, if you know how to design a board with all of those features, there must be lots of people on this forum who would like to buy one and populate it themselves, or who would like to get your Gerber file to have their own boards manufactured.