Zenith Radio Corp. Frequency meter Type BC221AF

S/N 4828

BC221 AF Frequency meter manufactured by Zenith Radio for the American Army Signal Corps. Unlike most frequency meters this incorporates a signal generator with a crystal controlled oscillator and a variable frequency oscillator together with a a detector, mixer, LF oscillator and amplifier. It can therefore be used to calibrate receivers and transmitters. It comes complete with a multi page calibration book housed in the front cover door. Order number CZR/29307 Phila. 43 (the CZR refers to Zenith Radio) Chicago USA 1943

This instrument did not work when I acquired it. As it was intended to use dry batteries my first task was to make a suitable power supply to provide 6 Volts for the heaters at about 0.9 Amps and 130-140 Volts at about 15 mA.

There is a schematic diagram on the inside of the lid to the battery compartment

This is what is inside

To the top left you can see the 1000kHz crystal housed in an octal based glass tube. The on/off switch labelled 29A to the right of this view was failing to make connection on one pole and the 6SJ7 valve (lower right had almost zero emission. Fortunately I had a rather rusty replacement which proved to be satisfactory despite having "bc221 dud?" scratched into its black paint!

The crystal appears to be a British Air Ministry type and is dated August 1957. Apart from this and what appears to be a replacement capacitor on the tag board the interior seems as clean and tidy as the day it was made even though it has been in service until 1973. The replacement of the crystal has necessitated a complete recalibration of the instrument which was carried out by G+E Bradley Ltd in 1960 according to the modification label. The calibration book has been modified with replacement calibration figures produced by some photographic means and supplemented with hand written alterations.

Schematic Diagrams
You can obtain a copy of the manual describing this and many other variants of this instrument here: http://www.royalsignals.org.uk/

How were they calibrated?
Have a look here:http://jproc.ca/ve3fab/bc221.htmlI have five or six old copies of the "Wireless World" and in one of these dated November 1956 is an article describing the equipment developed by G.E.Bradley to automatically calibrate and photographically print the new calibration strips for insertion into the calibration books. calibration.pdf