RF OSCILLATOR ASSEMBLY MECHANICAL TUNING

TM 11-5820-670-30

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

The shaft of the 0.05 MHz cam is mechanically coupled to the rotor of crystal selector switch S2001.

The camshaft rotates the switch through 360 degrees in 20 18-degree steps. At each switch position,

the rotor of switch S2001A connects 1 of 10 crystals into the collector circuit of interpolation oscillator

Q2001 within the rf oscillator assembly.

Each crystal remains in the interpolation oscillator collector circuit for two positions of the switch

rotor. The rotor of switch S2001B alternately connects two crystals into the collector circuit of refer-

ence oscillator Q2002 which is also part of the rf oscillator assembly. The crystals control the oper-

ating frequencies of the reference and interpolation oscillators. The switch rotor turns only in one

direction. After 360 degrees rotation, the rotor is again at the original position.

OSCILLATOR BUFFER AND RF AMPLIFIER MECHANICAL TUNER

The oscillator buffer and rf amplifier tuning shafts are rotated by the output of the gearbox differential

the same way as described for the vhf tuner. The rf amplifier tuning shaft is mechanically coupled to

the oscillator buffer tuning shaft. Tuning of the vhf tuner, oscillator buffer, and rf amplifier is synchro-

nized since the oscillator buffer tuning shaft is coupled off the vhf tuner tuning shaft through a 1 to 1

gear ratio in the gearbox.

CHANNEL TONE AND CRS DELAY CONTROL CIRCUITS

See FO-19 while reading the text in this section.

During tuning operations, an 800 Hz tone is supplied to the receiver-transmitter audio output circuits

to indicate to the radio set operator that the tuning system is selecting a new channel. While either

drive motor B9703 or B9702 is running, a ground is supplied by transistors Q9703 and Q9704 or Q9701

and Q9702 through diode CR9711 or CR9723.

This ground causes relay K9605 in the receiver-transmitter to energize. Contacts 5 and 2 of K9605

connect + 16 vdc through pin 19 of connector P5501 on the audio-frequency amplifier. Application of

+ 16 vdc activates the 800 Hz oscillator in the receiver-audio amplifier. The 800 Hz tone is connected

through pin 2 of P5501 to the audio line where it is applied to the aircraft interphone set.

The ground used to energize relay K9605 is also sensed by diode CR9605. This ground activates time-

delay circuits in the voltage regulator, which in turn energizes relay K3001 in the crs.

When relay K3001 energizes, the crs control voltage normally fed to the receiver local oscillator is mo-

mentarily grounded. This prevents the receiver local oscillator from locking on the incorrect frequen-

cy. The energizing of relay K9605 also prevents the receiver-transmitter from being keyed during the

tuning cycle.

In normal transmit operation, K9605 is deenergized and supplies a ground to the receiver-transmitter

transmit-receive relays through contacts 6 and 8.

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