[ARC5] ATA, SCR-274-N, AN/ARC-5 Transmitter Calibration Crystals

[Mike Morrow]

A few days ago an inquiry was made about the basis for the calibration crystal Y-50
frequencies in these transmitters.

The 1940 manual NA-08-5G-4 for the ARA/ATA lists the following frequencies for Y-50.
These agree with the units in my collection, with one exception:

CBY-52232   2.1 to 3.0 MHz    2.5 MHz
CBY-52208   3.0 to 4.0 MHz    3.5 MHz*
CBY-52209   4.0 to 5.3 MHz    4.6 MHz
CBY-52210   5.3 to 7.0 MHz    6.2 MHz
CBY-52211   7.0 to 9.1 MHz    8.0 MHz

*I have a CCT-52208 whose Y-50 is 3.265 MHz.

The 1943 manual TO 12R2-3SCR274-2 for the SCR-274-N lists the following frequencies
for Y-50.  These agree with the units in my collection:

BC-696-A    3.0 to 4.0 MHz    3.5 MHz
BC-457-A    4.0 to 5.3 MHz    4.6 MHz
BC-458-A    5.3 to 7.0 MHz    6.2 MHz
BC-459-A    7.0 to 9.1 MHz    8.0 MHz

These manuals contain this, or a similar note:
"Other crystal frequencies may be specified at a later date for these transmitters,
 hence it is essential to note the nominal frequency of the crystal before checking
 the calibration accuracy of the dial of the transmitter."

The 1944 manual AN 16-30-ARC5-2 for the AN/ARC-5 lists **no** specific frequencies.
Mike Hanz lists the following frequencies at http://aafradio.org/flightdeck/arc5-2.htm .
These agree with the units in my collection, with one exception:

T-15/ARC-5  0.5 to 0.8 MHz    0.8 MHz
T-16/ARC-5  0.8 to 1.3 MHz    1.3 MHz
T-17/ARC-5  1.3 to 2.1 MHz    2.1 MHz
T-18/ARC-5  2.1 to 3.0 MHz    2.601 MHz**
T-19/ARC-5  3.0 to 4.0 MHz    3.265 MHz
T-20/ARC-5  4.0 to 5.3 MHz    4.435 MHz
T-21/ARC-5  5.3 to 7.0 MHz    5.505 MHz
T-22/ARC-5  7.0 to 9.1 MHz    8.87 MHz

**I have a T-18/ARC-5 whose Y-50 is 2.604 MHz.

The AN/ARC-5 maintenance manual contains the following information in section 2-14
and 2-15:

"The process of factory alignment results in zero error exactly at the high-frequency
 end of the band.  Consequently, if retrimming becomes necessary, it is important, if
 the original accuracy is to be restored, to retrim with reference to an accurately
 known frequency standard exactly at the high end of the band...

"This standard preferably should be a crystal plugged into the transmitter, and the
 resonant frequency of this crystal should correspond exactly to the highest frequency
 line on the dial...

"NOTE  If a crystal for the high-frequency end of the band is not available, any
 crystal within the frequency range of the transmitter may be used.  The resulting
 calibration accuracy, however, will not be as good as when a high-end crystal is
 installed."  

The ARA/ATA and SCR-274-N manuals do not contain these details.  It is curious that
the calibration crystals supplied with only the T-15, -16, and -17/ARC-5 meet the
frequency criterion specified in the maintenance manual.  Most Y-50 crystals are
mid-band and even low-band crystals, except for the T-22/ARC-5.

That makes a person wonder if somewhere there wasn't a "failure to communicate".
Maybe Strother Martin worked for A.R.C during WWII.  :-)

Mike / KK5F