This 539C was not working because all three meters were bad. The meters were replaced with digital meter replacements and the tube tester was calibrated. Analog meters in tube testers that are 50 plus years old will have problems. If they are working, they are most likely no longer accurate and readings will be off. The digital meters are very accurate and not prone to the mechanical issues and overloads as with the older analog meters. The micromho reading is read from the Gm meter as a percent of the selected scale.
Digital Meters are no longer available.