Using the LM-13 Frequency Meter As A VFO
by Greg Latta, AA8V
Main Page

LM-13 VFO System
LM-13 VFO System
Click on an item in the photo for more information. Aluminum Case 1:2 Voltage Step Up Transformer VFO Buffer Amplifier Power Supply

I am currently working on this web page and you may see daily revisions.
Some of the information on this page will eventually be moved to separate pages with more detail.
Grayed out links to those pages below will become active as this is done.

For the moment, a lot of information is contained on this page, so be sure to scroll down.

Updated Wednesday, December 13th, 2017

Using the LM-13 Frequency Meter As A VFO
Grayed out links will become active as I complete those pages.
 Using the LM-13 Frequency Meter As A VFO - Main Page  Replacing the Neon Lamps with a 0A2 Regulator Tube
 Power Supply  Grid Blocking the Oscillator During ReceiveNew!
 2 Transistor Buffer Amplifier and 1 to 2 Voltage Step Up Transformer Page  Schematic Diagram and Circuit Descriptions See Link Below
 How to Read a Vernier Scale  Making an Aluminum Case for the LM-13 New!
 Exterior Photos  Interior Photos
 Resources and Manuals See Link Below

Select A Link Below:
 Schematic Diagram for the LM-13
 Modified Schematic Diagram for the LM-13
 Calibration Crystal
 Resources and Manuals

Schematic Diagram for the LM-13:
Below is the schematic diagram for the LM-13 frequency meter. This was taken from the Handbook of Maintenance Instructions. Click on the image or the link for a larger version suitable for printing.

Schematic Diagram for the LM-13:
Small LM-13 Schematic Diagram
Click here for a larger schematic diagram suitable for printing.

Modified Schematic Diagram for the LM-13:
Below is the schematic diagram of my LM-13 where the following modifications have been implemented:
1. The filament circuit has been modified for 6 V operation. I don't recommend this but this is how I received my LM-13, so I didn't change it.
2. The power connector has been changed to a 6 pin Cinch-Jones connector.
3. The neon bulbs have been changed to a single 0A2. Resistor R-103 has been changed to 17.5 kohm for proper operation with my 245 V B+ power supply. My resistor is a 7W unit but anything over about 2W should be OK with a 245 V B+ supply.
4. A 10 uf , 450 V capacitor has been placed across the B+ line in the LM-13 unit itself to filter out the small ripple introduced by the filament current voltage drop in the ground lead of the power supply connecting cable.
5. Grid block control of the oscillator has been implemented. Placing -16V or greater on the blocking bias line will cutoff and disable the oscillator.

Modified Schematic Diagram for the LM-13
LM-13 Modified Schematic Diagram
Click here for a larger schematic diagram suitable for printing.

Calibration Crystal:
The calibration crystal in the LM-13 is an amazing unit, even today. The crystal is in a hermetically sealed case with an octal socket. The crystal specifications are as follows:

Type: Cut "AT" Crystal
Frequency: 1000 KHz ± 10 Hz=1,000,000 Hz ± 10 Hz
Temperature Coefficient: 0.0001% / °C=1 Hz /°C
Calibration Temperature: 20°C

This means that, even without calibration against a frequency reference such as WWV, when used in the LM-13, the frequency of the crystal at room temperature would be within 10 Hz of 1 MHz! Between 0°C and 40°C, which is the probable range over which the LM-13 would be used, the crystal would be within 30 Hz of 1 MHz. This is amazing accuracy, especially for 1941!

Resources and Manuals:
Handbook of Maintenance Instructions - Be sure to download and save this! This is "the book" on the LM-13.
Calibrating Equipment - L Series
Surplus Radio Conversion Manual
QST Article from September 1965

©Copyright Statement:
All images, designs, and materials on these web pages are the property of Gregory P. Latta and are ©2017 by Gregory P. Latta. You may use them for personal purposes and for educational purposes, but any commercial or other use is strictly prohibited unless written permission is obtained from the author.

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