All the Sadelcos I had ran on 18 volts. Four 9 volt batteries series parallel. I may still have some info on these meters. I'll try to find it and get back. Even my Blonder Tongue field strength meter runs on 18 volts.
I used the meters to measure digital TV signals. You can peak up your antenna direction using the meter.
Fred
The battery pack, I forgot to mention it is 12 cells, if they are @1.2V then it would be 14.4V. It's 2 wires, so there's no thermistor. I can see it using a bit more since NiCd has a range up to what 1.25V or 1.3V max? It's not marked, it's just a board with the cells, wrapped up in nasty old electrical tape.
There is maybe what looks like a charging circuit with a transistor but I can't really tell. I just want to be careful.
Most likely I will want to use it with external power since I don't have the opportunity to do a lot of tests of this kind, and NiCd tend to go bad when left for months at a time.