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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


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