TUNING SYSTEM RADIO SET AN/ARC-131.

TM 11-5820-670-30

1-61. TUNING SYSTEM RADIO SET AN/ARC-131.

Receiver-Transmitter Radio RT-823/ARC-131 operates on any of 920 channels within range from

30 to 75.95 MHz, in 0.05 MHz increments. The operating frequency is selected at Control, Radio Set

C-7088/ARC-131 and the actual tuning takes place in receiver-transmitter, Radio RT-823/ARC-131

and the antenna coupler. The control unit provides whole and decimal MHz control information to the

gearbox in the receiver-transmitter.

The gearbox is a motor-driven electromechanical device which simultaneously tunes the tank circuits

in the rf amplifier, oscillator buffer, vhf tuner, and inserts proper crystals in the reference and inter-

polation oscillators of the crs as the operator selects the frequencies at the control unit.

The gearbox consists of two autopositioner circuits and their drive motors, a differential, several gear

trains, and a positioning cam. A third motor within the gearbox drives the A band and B band selection

linkage. The autopositioner circuit is a motor-driven rotary positioning mechanism which provides

automatic and precise selection of any one of a number of fixed positions, each of which is determined

by the switch positions at the control unit. One autopositioner circuit tunes the receiver-transmitter in

1 MHz increments; the other autopositioner circuit tunes the receiver-transmitter in 0.05 MHz

increments.

FREQUENCY SELECTION CODE

Frequency selector switches in the control unit develop the frequency selection code which controls

tuning circuit operation in the gearbox of the receiver-transmitter and the antenna coupler tuning

circuits.

The selection code for each of 920 available channels is developed by grounding specific combina

tions of 19 frequency control lines. Multiwafer rotary switches S9101, S9102, S9103, and S9104

determine combinations of frequency control lines which are grounded. The frequency selection

codes applied to switch S9705 in the gearbox are determined by the front and rear wafers of S9101A,

S9102E, S9102H; the rear wafer of S9101 B and front wafer of S9102G.

The selection codes for determining the proper band selection in the receiver-transmitter are devel-

oped by the front wafer of S9101 B and rear wafer of S9102G. The band selection codes are connected

through pins G and J of control unit connector J9101 and are applied to motor 69701 in the gearbox

through either switch S9701 or S9702.

Tenths selector switch S9103 and hundredths MHz selector S9104 determine last two digits of the

selected frequency. The front and rear wafers of S9103 and S9104 determine the combination of fre-

quency control lines grounded. The grounds are applied at pins P, R, S, T, and U of control unit

connector J9101 and fed through interconnecting wiring to switch S9703 to the Iedex assembly (auto-

positioner circuit) in the gearbox.

The Iedex assembly is mechanically connected to the crystal switch of the rf oscillator assembly. The

crystal switch selects the correct combination of crystals to determine the last two digits of the selected

frequency.

Tens MHz selector switch S9101 and units MHz selector S9102 also develop frequency selection codes

used to tune the antenna coupler. The frequency selection codes applied to the coupler are deter-

mined by the front and rear wafers of S9101C, S9101D, S9102A, S9102B, S9102C, and S9102D.

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