GEARBOX ASSEMBLY A9700.

TM 11-5820-670-30

1-20. GEARBOX ASSEMBLY A9700.

Refer to FO-3 in the back of this manual while reading the text in this paragraph.

The electrical portion of gearbox assembly A9700 contains a standard Iedex stepping solenoid having

20 positions of 18 degrees rotation per step, a 24-position switch driven by a dc motor, and a dc motor

to control band switching.

The ledex drive is used to select the last two digits of the frequency selected and positions S2001 in rf

oscillator assembly A2000 which contains reference oscillator Q2001 and interpolation oscillator

Q2002. The 24-position switch (S9705) is driven by dc motor B9703 and provides an accurate

mechanical output to the vhf tuner, oscillator buffer, and rf amplifier assemblies.

The gearbox assembly automatically tunes the radio set to the frequency selected at the control unit.

The tuning control signals are applied to 0.05 MHz select lines in various combinations according to

the frequency selected. Assuming the operating frequency chosen is 30.00 MHz, ground signals are

applied to control lines A and E of both 0.05 MHz select and MHz select circuits.

The ground signals are applied to one side of both code comparators and are connected to contacts 3

and 7 of Iedex switch S9703 and contacts 5 and 6 of switch S9705.

The network consisting of diodes CR9702 through CR9706 and transistors Q9701 and Q9702 detects

mispositioning of switch S9703 for any frequency selected at the control unit. Mispositioning is defined

as that position of the switch rotor which causes either a short or an open condition to exist at both ends

of a particular control line. In normal operation, when a ground signal is applied to one end of the

control unit, the other end should be open at the switch rotor in the gearbox assembly. Those control

lines not grounded at the control unit should be shorted at the switch rotor in the gearbox.

The network of diodes CR9714 through CR9718 with transistors Q9703 and Q9704 perform the same

function for switch S9705. For discussion purposes, only the circuit of switch S9705 will be considered

since the operation of switch S9703 and associated diode transistor network are identical. After tuning

to the desired operating frequency, in this case 30.00 MHz, switches S9703 and S9705 are positioned

as shown in FO-3. This action causes the base and emitter of transistor Q9703 to be connected

together through the rotor of S9705, causing Q9703 to be turned off.

When the radio set is tuning, the ground signal applied to emitters of Q9701 through Q9704 is sensed

by diodes CR9711 and CR9723 and is applied through pin 14 of connector J9701 to the control coil of

relay K9605 (not shown) in the receiver-transmitter. Relay K9605 energizes and applies + 16 vdc to

the 800 Hz relaxation oscillator in receiver-audio amplifier assembly A5400. The 800 Hz tone is

applied to the audio output line and operator's headset during the tuning process.

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