Old analog VOM reading half AC voltage

Thread Starter


Joined Jul 30, 2021
I have a SP 142 Sperry analog vom from the 70’s that for some reason now only reads half voltage on AC. 240 vac now reads 120 on all ac settings.
I don’t have a schematic for it but I did notice unlike my analog Simpson Vom with Germanium diodes the Sperry has two zener diodes. The zener diodes are good.
Any ideas what would cause this old analog meter to suddenly read half AC voltage?

schmitt trigger

Joined Jul 12, 2010
Adding to What the previous posters have mentioned, there must be a rectifier diode BRIDGE somewhere.
The failure signature is identical to one of the diodes in a bridge developing an open circuit.


Joined Apr 2, 2020
The last selenium diode I had about 8v forward voltage (25kV). Depending on the voltage the OP is measuring, 8v is way less than 50% voltage.:eek: