If you have an original then it should just go in, but if you want to check the calibration, then I would do it like this:-
A VU meter is designed to read O VU when in series with a 3.6k resistor, when 1.228V rms is across them both. You would need something like a signal generator with a 600 ohm output and set at say 400Hz. You can measure the voltage with a half decent DVM, there is no need to get picky about it, 1.23V would be fine. For more details read
https://en.wikipedia.org/wiki/VU_meter
best
DaveP
