146 - Rescuing an old HP3400A RMS voltmeter - the Frankenstein way 

Thanks. Yes, you are right. Sometimes I do things because its a challenge or using an interesting principle or looks cool. In this case, I think I will use this meter mainly for noise measurements (like from power supplies).

Yes, I was seriously contemplating this and using an Arduino or Raspberry Pi with a suitable display. The microcontroller could do smart auto-ranging giving always the best resolution and even show the units, eg "mV" or "dB". I decided against it because it would be way more effort than this thing is worth. Let's face it, its pretty much amazing my 3400A still works anyway. The thermocouples have a limited lifetime and so does the nuvistor. Secondly, while its relatively easy to rig two micro switches or two reflective light sensors to detect the right and left movements of the range knob, you have to know the starting position to determine the current range. If I leave the range on the 1V setting, turn everything off, and then back on, how is the microcontroller supposed to know what range is selected? Remembering the last position would be very error prone. I considered a method to sync it by turning the range all the way to the right (300V) and pressing reset, to be done at every power-on. Clumsy. The other way, to encode the decimal point directly into something that turns with the range switch is very tricky mechanically. If one has a 3D printer and good mechanical design skills, maybe, but I know my limits...

@@TheHWcaveI must have misunderstood the problem. I got the impression that there was a simple one-to-one mapping of decimal point position to switch position. But it sounds as if the problem is more complex than that. So as you say, this unit is probably not worth the effort...