I have this old Radio Shack Micronta 22-220 which kind of a Simpson 260 wanna bee. It does have the FET front end to provide 10 MOhms input impedance for the DCV. There are a couple of potentionmeter inside which I believe for calibration. Does anyone know how to do the calibration on this thing? Besides I couldn't even find a user manual for it. I am sure I have the user manual somewhere but I can't find it. I bought it new sometime in the early 80's.