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This has to be the oldest bit of kit i have, this used to belong to my dad, and has been up in the loft for many years (after his passing), i think it's from the 1940's, used in the 2nd world war, for the U.S Army, it would of been powered by a 135 volt battery (but at some point had been converted to run off AC), from the info i have found, this was used for calibrating radio equipment, by using a method called Zero-Beating, the Frequency Meter still works, but the dial reading's are a bit off from the setting in the Calibration Book, i did replace some of the components in the power supply to make it a bit safer to use, but for it's age it still works really well.
Here's a YouTube video of a newer Frequency Meter, plus the same model as mine (not my video) - (https://www.youtube.com/watch?v=9iZy41OfGHM)
and some more info and the User Manuel - (http://radionerds.com/index.php/BC-221) - (http://www.pa3esy.nl/military/us/meet/BC-221/BC221AE/html/bc221ae_set.html)
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Here's a YouTube video of a newer Frequency Meter, plus the same model as mine (not my video) - (https://www.youtube.com/watch?v=9iZy41OfGHM)
and some more info and the User Manuel - (http://radionerds.com/index.php/BC-221) - (http://www.pa3esy.nl/military/us/meet/BC-221/BC221AE/html/bc221ae_set.html)