The Frequencymeter CBLC-74028 is a part of the American Navy testequipment LM-20
Above a picture of the heart of a LM * installation, the Heterodyne Frequntie Meter, the appearance is similar to the BC-221 but is essentially different. The tuning and frequency-choice look identical, the rest not.
This equipment provides a simple and reliable frequency indicating source between 125 kc and 20 mc for frequency setting and measurement of receivers, transmitters, signal generators, or any electronic devices where the frequency must be known. It is a heterodyne type of unit having a variable oscillator which may be point-checked against an internal crystal oscillator and its harmonics at frequent settings. An external signal may be mixed with the variable oscillator output by a wire or antenna connection to a binding post. A jack provided on the front panel permits the use of phones for listening to the heterodyne. The variable oscillator signal may be internally modulated.
An image of the complete LM-20, under the power unit and above the Frequency Meter. There were many LM's. Here below a list of these devices with the typical properties:
Model |
Freq Meas Unit Type |
Het Osc Freq Rnge --KCs |
Operating Voltage |
Mechanical Design |
Note |
LM |
None |
195-400 |
12-14 |
|
|
LM-1 |
None |
195-400 |
External battery supply |
Same as LM |
|
LM-2 |
None |
195-400 |
12-14 |
Minor improvements throughout |
1 |
LM-3 |
None |
195-400 |
External battery supply |
Same as LM-2 |
|
LM-4 |
None |
195-400 |
12-14 |
Same as LM-2 |
2 |
LM-4A |
None |
195-400 |
24-28 |
Same as LM-4 |
|
LM-5 |
None |
195-400 |
12-14 |
Same as LM-4 |
|
LM-6 |
CRR-74023 |
195-400 |
Rect Power Unit Type CRR-20104 |
Same as LM-4 |
|
LM-7 |
CRR-74024 |
195-400 |
12-14/24-28 |
Redesigned shockmount base and capacitor and drive assembly |
|
LM-8 |
CRR-74024 |
195-400 |
Rect Power Unit Type CRR-20104 |
Same as LM-7 |
|
LM-9 |
CRR-74024 |
195-400 |
12-14/24-28 |
Same as LM-7 |
3 |
LM-10 |
CRR-74028 |
125-250 |
12-14/24-28 |
Minor mechanical revisions |
3, 4 |
LM-11 |
CRR-74028 |
125-250 |
Rect Power Unit Type CRR-20104 or CRR-20104A |
Same as LM-10 |
|
LM-12 |
CRR-74028 |
125-250 |
12-14/24-28 |
Same as LM-10 |
3 |
LM-13 |
CRR-74028 |
125-250 |
Self-contained batteries |
Same as LM-10, less shock mount base |
5 |
LM-14 |
CKB-74028 |
125-250 |
12-14/24-28 |
Same as LM-10 |
|
LM-15 |
CKB-74028 |
125-250 |
Rect Power Unit Type CRR-20104A |
Same as LM-10 |
|
LM-16 |
CKB-74028 |
125-250 |
12-14/24-28 |
Same as LM-10 |
3 |
LM-17 |
CKB-74028 |
125-250 |
Self-contained batteries |
Same as LM-10, less shock mount base |
5 |
LM-18 |
CRR-74028 |
125-250 |
Rect Power Unit Type CRR-20104A |
Same as LM-10 |
|
LM-19 |
-74028 |
125-250 |
12-14/24-28 |
Same as LM-10 |
|
LM-20 |
CBLC-74028 |
125-250 |
12-14/24-28 |
Same as LM-10 |
|
LM-21 |
|
125-250 |
Rect Power Unit Type -20104 or -20104A |
Same as LM-10 |
6 |
Notes:
1. Voltage regulator circuit added.
2. Symbol designations reassigned.
3. Included in Type -10086 waterproof carrying case.
4. Low frequency oscillator fundamental frequency changed.
5. Mounted in Type -10111 carrying case with batteries.
6. Recalibrated and new calibration pages inserted in book where necessary. Aluminum cover and beaded metal chain added to Calibration Book. Spare crystal installed in Meter Case. Set screw in Corrector Knob E replaced with a threaded extension to facilitate calibration to the meter's crystal oscillator.
You can see that there are lots of versions. For the collector a shock, when he was just completing a LM14 installation after much research. So do not start collecting LM's . But yes, it is a beautiful frequencymeter.
The front of the meter. At the bottom ypu see the tray with the missing calibration booklet. Incidentally, this copy is calibrated in 1976. These instruments, the BC221's are so good and stable that they now (2012) still perform perfectly and are verysuitable for receiver or transmitter frequuency calibrating.
ELECTROMECHANICAL DESCRIPTION
FREQUENCY COVERAGE.-
Range: 125 to 20,000 kc.
Fundamental: 125 to 250 kc and 2000 to 4000 kc.
Crystal Oscillator: 1000 kc.
Modulation: 500 cps.
Accuracy: +-.02%, 125 to 20,000 kc; +-.01%, 2 to 20 mc between -32 degrees and +65 degrees C ambient temp.
PRESENTATION AND CONTROLS.-The drum and disc dials are precision ganged for accurate frequency readings. A panel trimmer is provided for setting the variable oscillator at any check point by heterodyne with the crystal oscillator or its harmonics. A head phone jack for a 600 ohm audio output is mounted on the front panel.
FITTINGS AND ACCESSORIES.-The rf coupling terminal is a single binding post. A Navy Type 49025 Telephone Jack is used.
CONSTRUCTION.-The waterproof carrying case provided is made of aluminum with an olive drab wrinkle finish. The frequency meter case is made of aluminum with a black wrinkle finish.
| Symbol | Navy Type Nr. | Description |
| V-101 | CRC-77 | Heterodyne Oscillator, Triple Grid |
| V-102 | CRC-6A7 | Crystal Oscillator and Detector, Pentagrid Converter. |
| V-103 | CRC-76 | Audio Amplifier and Modulator, Super Triode. |
| V-104 V-105 | None | Voltage Regulator, 2 element, 1/4 W, Neon Bayonet Base, T4 1/2 Mod. |
The alignment of the meter and RFconnection.
The filament voltage and anode voltage can be switched separately, further, this meter can modulate the signal..
The right side with the connector plug.
Detail of the 6-pin plug.
The underside of the mounting plate.
The top with a whole series nameplates. At the top a picture for modification, calibration and maintenance of the Dutch Navy, where the instrument has served.
The pictures of the LM-20, the entire contract was executed by the Bendix company.
This frequency meter is a product of Loral Electronics Corp. in New York. However, there were many suppliers of this instrument.
The opened right.
A look at the left.
The back with the missing V104 and V105, which are JAN 991 types, neon lights to stabilize the supply voltage. How do you get them??
There are plenty of decent parts, solid porcelain tube sockets are examples of this.
The missing lights.
These are the stabilizers, the only problem is, they have just one connection on the bottum, I need two connections ........
Top view of the meter, under the cover plate is the tuningmechanism.
The tuning mechanism with the variable capacitor. A very nice part, this costs a lot of money.
The open bottom, solid work here.
The crystal oscillator, Right under a small capacitor for calibrating. The black tube is the 1000 kHz crystal.
For enthusiasts, De maintenance instructions van de LM-13
The LM-13 also contains the power batteries, the range and performance of the meter are the same as the LM-20.