LM21 HETERODYNE FREQUENCY METER

The LM 21 Heterodyne Frequency Meter is vintage test equipment that is still sufficient for all but the most stringent requirements even though it has been around for more than 70 years.  It has a frequency range from 125 to 20,000 kHz with an accuracy of about 0.01%. 

Plenty good for most Ham radio purposes!

This was a much sought-after, and highly-prized piece of military electronic gear immediately following WW II, and for many years thereafter.  I enjoyed using an LM 21 as a VFO for a few years, until I obtained a commercially built Ham radio rig.

For several years, I have had LM 21, serial # 745, in storage, along with a  CQC-20104A power supply, cables, and connectors.  Sadly, when I recently took the LM-21 from the shelf in the garage and attempted to power-up, there was no signal coming from the device.  I am hopeful that the trouble is simply a corroded or loose connection somewhere.  The tubes are rare and difficult to find, and if (when) they need replacing, I will probably replace them with easier to find equivalent tubes, or solid state devices.

The photo, below, shows a top-front view of the LM21 with the box in which it is normally housed is removed.

For this top view photo, I have removed the plate that covers the variable capacitor and dial mechanism that determines the frequency.  The gearing and precision of this mechanism is that of a fine mechanical clock, which provides accuracy limited only by the skill of the operator.

The LM 21 uses three vacuum tubes: a type 76 triode (upper right-hand corner in photo, above), a type 77 sharp cutoff pentode (upper left-hand corner of the photo), and a 6A7 pentagrid converter (horizontal tube in the middle of the photo), with associated circuitry.  The power supply, housed in a separate box, uses a 6Z4 rectifier tube. 

The photo, below, shows the bottom view of the chassis.

The photo below shows a rear view . . .

The LM 21 is an excellent example of the best packaging and workmanship in the electronics industry of the late 1930’s and early 1940’s.  These instruments were assembled by hand using point-to-point wiring.