FT241 and FT243 Crystals History

DETAILS ABOUT the old FT243 AND FT241 Crystals

During the World War Two era, the U.S. Military operated a number of different radio sets using these crystals:
AN/TRC-20 (RT-111/TRC-20)
SCR-609/610 (BC-659)
SCR-619 (BC-1335)

There were jeep radios developed by the Signal Corps Laboratory in conjunction with Galvin Corp. (Motorola).
They used the so-called crystal-saver design. The crystal frequency is multiplied by 4 to provide the local oscillator for a receiver with a 4.3 MHz I.F.
In transmit, the DC output of the receiver's FM discriminator is used to frequency lock the TX oscillator to the crystal controlled RX frequency.
e.g. - (8000 x 4) + 4300 = 36,300 KHz. Normal operating frequencies would have been 36.3 through 37.4 MHz.

There were three frequency bands for FM in the WWII era. In addition to pioneering this mode, the Signal Corps chose to crystal control most of the radios. The total US production of crystals before the war was about 100K units, and millions more were calculated to be needed. Most of the raw quartz came from Brazil past the U-boats. There were literally millions of these crystals manufactured, a great many of them during 1943 - 1944.

Armor 20.0 - 27.9 Mhz Channel Numbers 0 - 79 Ch. No. = (Freq (MHz) - 20) x 10
Artillery 27.0 - 38.9 Mhz Channel Numbers 270 - 389 Ch. No. = Freq (MHz) x 10
Infantry 40.0 - 48.0 MHz Channel Numbers 400 - 480 Ch. No. = Freq (MHz) x 10
Tank Sets: SCR-508/528 - 20.0 to27.9 Mc (Armor) SCR-608/628 27.0 to 38.9 MC (Artillery)
These used the Armstrong FM system. The signal from a crystal oscillator at about 400 KHz is modulated by a phase shifter, then multiplied up to the operating frequency. Receivers are not crystal controlled, but have car-radio-style push-button presets.
Crystals are the FT-241 type. Resonant frequency is Ch. Freq. divided by 54 for Armor, and Ch. Freq. divided by 72 for Artillery.

Jeep Radios: SCR 509/510 : 20 to 27.9 MHz (Armor) SCR 609/610: 27 to 39.9 MHz (Artillery)

Crystal-saver Design:. The crystal frequency is multiplied by 4 to provide the local oscillator for a receiver with a 4.3 MHz IF. In transmit, the DC output of the receiver's FM discriminator is used to frequency lock the TX oscillator to the crystal controlled RX frequency.
Crystals are FT-243's. E.g., (8000 x 4) + 4300 = 36,300 KHz.

SCR-300 Walkie Talkie (Infantry) Similar design to the jeep radios. However, the receiver local oscillator is free running with a single crystal in a separate calibration oscillator providing manual calibration points.

The navy used a different crystal scheme, with FT243 crystals for the RCK Receiver. For example, using a crystal frequency of 14.233 MHz and operating frequency of 116.10 MHz. The Navy used FT243 crystals in the TDQ Transmitter and others, for example, Crystal Frequency = 14.300 MHz, operating frequency 128.70 MHz.

Many other crystal CHANNEL NUMBERING and even CHANNEL LETTER schemes were used throughout this era.
For example: Channel 323 = 7000 kHz. System with crystal every 25 kHz. Channel X = Notes:
BC611 used a lot of crystals in the 3500 - 6000 kHz range.
BC745 used a lot of crystals in the 5000 - 6000 kHz range.

The common US crystal controlled WW-II vintage FM sets operated between 20.0 and 38.9 MC and used either FT-241 or FT-243 holder crystals that were multiplied several times to get to either the operating frequencies (sets using FT-241) or receiver mixer injection frequency (FT-243 crystals). There were two bands defined, Armor and Artillery. Infantry didn't get an assigned band until the late 40's. Except for the tunable BC-1000, they mostly used either HF or the Artillery band sets.

Channel numbers were assigned, from which you can read the operating frequency. Armor band covered 20.0 - 27.9 MHz and the channel numbers ran from 0 to 79. Artillery band ran from 27.0 - 38.9 MC and the channel numbers ran from 270 to 389. Channel numbers on the FT-241 and FT-243 match but the FT-241 crystals frequencies were in the low MF range whereas the FT-243 crystals were higher, roughly 5750 to 8700 kHz. The FT-243 crystal frequency was multiplied several times to get to a frequency offset from the operating freq. by the IF freq.

Some other sets had channel numbers assigned. The Pogo Stick radio (SCR-511, BC-745) had channel numbers from 1 to about 69 but they were arbitrary assignments and the channel numbers were not always on the crystals, but on the BC-746 tuning units. The BC-733 Localizer receiver had channel numbers in the 700 range and used FT-243 holders with the channel numbers on them. Crystal freqs are in the same general range as for the FM sets.

The AN/TRC-1 operates from 70-100 MHz and has corresponding channel numbers. For example, Channel 705 is 70.5 MHz. Channel 948 is 94.8 MHz, etc.
It uses FT-243 for receivers, and FT-241 for transmitters.

An additional CHANNELIZED FT243 Crystal Frequency scheme was used for crystals like 7006.667 kHz, 7106.667 kHz, 7206.667 etc. Example: CHANNEL 60 = 7706.666 kHz.

(thanks to AF4K, WA5CAB, Jack Antonio, Al Klase and to a few others for this information)


OR: to inquire or buy crystals:

CLICK HERE to E-mail me

73 from Brian 'Bry' Carling, AF4K CRYSTALS

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