LM-13 Front View
INTRODUCTION
The LM-13 Heterodyne Frequency Meter is a device used for setting the receive or transmit
frequency of any radio set. The frequency range covered is 125 Kcs to 20 Mcs, in 2 ranges.
It is a small device measuring 8-5/8 inches by 8-1/8 inches by 8-1/2 inches. It is in a metal
case, with an external power supply. The LM-13 was made in the USA in 1943, and used by the US Navy,
and called the CRR-74028. This is similar to the BC-221 frequency Meter.
There are several versions, starting from LM and going to LM-19. Models LM to LM-4 were 12
volt filament only. After that, they had a 28 volt internal strap available. Some models used
batteries for the high tension (HT). Other models used the CRR-20104 mains powered rectifier unit.
They could also be powered from the radio set that they were testing, for example, an ARB receiver.
LM-13 Front View
MECHANICAL DESIGN
The frequency meter is in a metal box, a cube shape with a built in shock mount.
The calibration book is clipped underneath it. The case is aluminium, and weighs 11.5 pounds,
and is painted in black wrinkle. All the controls are on the front panel.
LM-13 Top View
The frequency meter electronics are mounted on a 2 level metal chassis, with the components on tag boards. The main chassis has 2 valves and 2 voltage regulator NEON globes. Above this is the second boxed section, containing the coils, tuning capacitor, and the oscillator valve. Underneath the main chassis, are most of the components, including 3 tag boards, 2 cylindrical transformers, and a crystal in a metal valve holder.
The power input socket is on the right hand side.
The knobs are labeled A, B, C, D, and E, as well as having their function engraved next to them. The controls consist of a main frequency tuning dial (D), and the indicator window. The window shows a scale going from 0 to 50, but if you keep on turning the dial after 50, you get back to the beginning. The main knob goes from 0 to 100 and has a vernier scale on the side. There is a corrector knob (E) that allows minor frequency adjustment. The frequency band knob (B) selects the LOW or HIGH band. There are 2 small plates that can be pushed aside, to allow trimming of the L or H bands.
Next to the dial is the coupling terminal (RF CPLG), for output and input.
Along the bottom, at the right are 2 toggle switches, one for the filaments (FIL) and one for the high tension (PLATE). By just using the FIL switch, you can leave the frequency meter warm, and it will be more stable in frequency. Next to this is the MODULATION switch (A) which can add a tone to the output signal, to enable the frequency to be found, and also for radios that do not have a BFO. Next is a PHONES jack that allows headphones to be used. Then there is the RF COUPLING control (C) that sets the input and output level. At the extreme left is the CRYSTAL ON and OFF toggle switch that controls and internal 1 Mhz crystal oscillator.
LM-13 Bottom View
The frequency meter can be mounted on a shock mount, as may be required in an aircraft. It may also be carried in a canvas bag and strap type CMQ-10110 weighing 4 pounds. There is also a carrying case type CRR-10111 weighing 13.5 pounds, and is 9-5/8 by 9-3/4 by 15-7/16 inches. This case also contains some spare valves, and batteries.
Carrying Case
ELECTRICAL DESIGN
The frequency meter uses 3 valves. The Type 77 hexode is set up as the heterodyne oscillator.
There are 2 resonant circuits for the 2 frequency bands. The LOW band is 125 Kcs to 2000 Kcs,
but uses the oscillator in the range 125 to 250 Kcs and higher order harmonics. The HIGH band
is 2 Mcs to 20 Mcs, but uses the oscillator in the range 2 to 4 Mcs and higher order harmonics.
It uses a variable capacitor, and a fixed inductor, with a coil tap for the valve cathode.
When setting the frequency of a transmitter, energy enters through the antenna terminal and C-106, and is mixed in the Type 6A7, with the heterodyne oscillator signal, through C-105. The audio frequency beat note, is coupled through C-108 to a triode amplifier (Type 76), and can be heard on headphones, through T-101. Tuning for zero beat indicates the transmitter frequency.
When setting the frequency of a receiver, a signal from the heterodyne oscillator is radiated through the antenna terminal, through the level control R-106. This signal can be modulated, by switching the triode into an oscillator, and coupling it to the suppressor grid of the heterodyne oscillator by C-107.
HIGH band Frequency Oscillator
HIGH band Frequency Oscillator Harmonics (right)
LOW band Frequency Oscillator (with Harmonics)
Alternatively, the crystal calibrator can be switched ON, by switching power to the second grid of the 6A7. This has a 1 Mhz crystal attached to the first grid. Note that there is no connection to the antenna terminal. The RF level control is shorted out, and coupling is through the wiring only. An external device can be tuned to the 1 Mhz fundamental or one of the harmonics.
Crystal Calibrator
The manual also recommends using the 1 Mhz signal to calibrate the frequency meter itself. When the crystal is switched ON, there is reduced input from the RF terminal, so there will be little interference. The heterodyne oscillator can be tuned to zero beat, and if off frequency, can be adjusted using the CORRECTOR knob. If beyond correction, the L and H trimmers can be adjusted.
The dial reading is used with the calibration book, to determine the actual frequency. The serial number on the calibration book, MUST match the serial number of the frequency meter.
There are some taps that can be changed for 12 or 24 volt operation, for the filaments. The HT can be strapped for the normal input voltage of 200 to 260 volts. In this case, the pin 36 should be joined to pin 27 on the input plug. If the meter is to be used on a transmitter with a higher voltage (for example the GO-9), then connect the low voltage to pin 36, or switch the plate link to the 260 to 475 voltage position, or do both. There is a voltage regulator, consisting of 2 series connected NEON tubes, type T-4-1/2.
Note that there is an error on the circuit diagram in the manual. It shows C-110A and C-110C connected in parallel. Actually C-110C is connected to R-113. The circuit presented here, has been corrected.
LM-13 Circuit (C-110C error corrected)
RESTORATION
I looked at the LM-13 that I had, and decided that I needed to have it working.
So when I moved it, I found another behind it, then another, until I had three wavemeters
in a pile. They were in various states of preservation, one very good, the other two fine also.
I removed them all from the cases, and piled the cases in a corner. Two had not been disturbed,
but the other had been modified. The insides were clean and in good condition.
The modified one had the input socket removed, the AC and HT straps removed, the crystal changed, and the modulation switch had 3 positions, not two. There was plastic wire all through it, not the normal cloth covered wire.
I selected the two in the best condition. I found a 5 pin plug in the junk box, removed the valves, and connected up the 12 volts DC. I checked that there was 6 volts across each of the filament pins. As this was correct, I replaced the valves, and checked that each valve was lighting up.
I tuned each dial, and found all the knobs and switches moved normally. With the mechanical parts all functioning correctly, the bench HT supply was connected, and slowly increased up to 100 volts. All the capacitors appeared to be good, and all the resistors were also good! A receiver was tuned to 7 mHz and the BFO turned ON. Each frequency meter could produce a heterodyne very close to the dial marking. The modulation worked on both. The crystal calibrator worked on both.
The cases were all lightly painted with satin black, not too much to drown the wrinkle finish.
The modified meter, had the 2 position wafer switch removed and a 3 position fitted and wired with cloth type wire, for the modulation. It was rewired from 6 volts to 12 volts for the filaments. The filament dropping resistor R-113 for the 76 was added. The PL-259 connector was removed, and the RF COUPLING control was added, using the same hole. The crystal was added. It was powered up and performed as per the manual.
POWER SUPPLY
Normally the frequency meter would be run from batteries, or from the radio power supply.
There was a mains operated supply available, called the CRR-20104.
Mains Power Supply Front
The power supply is the same size as the frequency meter, in an aluminium box with a shock mount. It is on a single chassis, with a valve rectifier and a choke and capacitor filter. There are fuses behind the front panel. The front panel carries the input and output connectors. There are 3 toggle switches, the central one being POWER ON. To the left and right are another 2 switches, protected by a guard. The switch can be operated from the side, but not bumped on or off. They are labeled COMP 1 and COMP 2. The switches increase the voltage in 2 steps.
Mains Power Supply Top
RESTORATION
The supply was checked and found to be 120 volts AC only. A connector was found in the junk box,
as was a 5 pin connecting cable. The valve was removed, and 120 volts AC was applied. There was
12 volts AC at the output socket. The valve was plugged back in, the supply powered up, and 300
volts DC was measured at the output socket. The 3 electrolytics, did not get hot, and they were
filtering as well. The supply was connected to a frequency meter and it performed properly.
The COMP 1 switch increased the voltages to 13.3 volts and 325 volts. The COMP 2 switch increased
the voltages to 14.5 volts and 350 volts. It was used at the lower settings.
Mains Power Supply Underneath
Cable
MODIFICATION
This frequency meter can be used as a low power transmitter, to produce a short distance
modulated output for local radio receivers. The Type 77 oscillator is unchanged. It can cover
the broadcast band for domestic receivers, and up to 20 Mhz for short wave receivers. The mixer
(Type 6A7) is unchanged. The triode (Type 77) is changed from an audio oscillator into a modulator.
The capacitor C-114 is disconnected from pin 9 of the MODULATION switch and connected to the PHONES jack, with a short piece of wire. The PHONES jack is disconnected from pin 3 of the switch. This allows a program source to be plugged into the PHONES jack and then applied to the grid.
The capacitor C-113 needs one end disconnected, otherwise it lowers the audio high frequency response. These simple modifications are all that is required. It provides a modulated signal. Approximately +10 dBm of audio plugged into the jack, provides about 30% modulation. These capacitors are located on a small tag board behind the front panel, near the power connector.
LM-13 Modulation
LM-13 Modification
CONCLUSION
The LM-13 heterodyne frequency meter is easy to repair and operate. It has a simple circuit
and components. It is not very useful today, as accurate receivers and transmitters are available.
It can be used as a BFO if the receiver has none. It can be slightly modified to become a program
source for radio receivers.
REFERENCES
NAVAER 08-5Q-38 Navy Model LM-13 Crystal Calibrated Frequency Indicating Equipment 1943
Regulators Glowing
Copyright
Ray Robinson VK2NO