WHOLE AND DECIMAL MHZ TUNING CIRCUITS

TM 11-5820-670-30

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

Correct ground combinations are applied to pins j, K, M, Z, and AA of control unit connector J9101.

The selection code for determining proper band selection in the antenna coupler is developed by the

front wafer of S9101D. The band selection code is connected through pins g and H of connector

J9101. All frequency selection codes applied to the antenna coupler are used to drive the automatic

positioning circuits similar to those used in the gearbox.

Table

on FO-18 lists the receiver-transmitter coding combinations for tens and units MHz selec-

tors; table

lists the receiver-transmitter coding combinations for tenths and hundredths MHz

selectors; and table

lists the antenna coupler coding combinations for tens and units MHz

selectors.

WHOLE AND DECIMAL MHZ TUNING CIRCUITS

See FO-19 while reading this section.

The tuning system automatically tunes the radio set and antenna coupler to the frequency selected at

the control unit. Each of the four decimal digits defining one of 920 channels between 30.00 and

75.95 MHz is independently selected at the control unit. The control converts four decimal digits into a

12 wire code consisting of grounds and opens. This information is decoded in the receiver-transmitter

and is used to operate tuning motors which correctly position the induct tuner shafts of the rf amplifier,

oscillator buffer, vhf tuner, and rf oscillator. The control unit also converts the two decimal digits de-

fining one of 46 channels between 30 and 75 MHz into a seven-wire code for tuning the antenna

coupler.

The 30 to 70-MHz and 0 to 9-MHz frequency selector knobs on the control unit rotate two switches,

S9101 and S9102, which are electrically interconnected. When rotated through their 46 different

combinations (30 to 75 MHz), 14 wire codes representing those frequencies are generated. The 14

wire codes consist of five control lines plug two band lines for the receiver-transmitter and five control

lines plus two band lines for the antenna coupler. Tenths MHz and hundredths MHz frequency selector

knobs rotate two switches, S9103 and S9104, which are electrically interconnected. When rotated

through their 20 combinations (0.05 to 0.95), a five-wire codes, used only for the receiver-

transmitter unit, is generated which represents those frequencies.

Band

and band

information is used primarily to control the band switch motor in the gearbox

which drives the band switch arms on the oscillator buffer, vhf tuner, and rf amplifier. The band

ground signal is also used to energize relays in the rf control assembly which the switch from band

to band

antenna filters. Band

and

information is also provided to the antenna coupler.

The procedures below describe tuning system operation for the operating frequency of 30.00 MHz.

The circuit operation for all remaining frequencies is identical with that for 30.00 MHz except that

different tuning control signals are originated at the control unit. For simplicity, only those control

lines grounded during 30.00 MHz operation are shown on the simplified diagram of FO-19.

For each receiver-transmitter and antenna coupler frequency selected, there are 19 tuning control

lines used as follows:

Five control lines for receiver-transmitter first and second digits

Five control lines for receiver-transmitter third and fourth digits

Two band selection control lines for the receiver-transmitter

Five control lines for antenna coupler first and second digits

Two band selection control lines for the antenna coupler.

1-78