VHF TUNER MECHANICAL TUNING

TM 11-5820-670-30

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

The geneva assembly also drives open-seeking switch S9705 through a 6 to 1 speed-reducing gear

train. Each time the slotted wheel on the geneva assembly is engaged and rotated 90 degrees, switch

S9705 is rotated through 15 degrees which corresponds to a 1 MHz change in frequency.

Operation of the differential is based on the principle that both 1 and 0.05 MHz tuning inputs produce

an output, but neither input affects the other. As the receiver-transmitter is tuned in 1 MHz incre-

ments, rotational motion of the geneva assembly is transmitted to the differential. The differential

drives the vhf tuner tuning shafts, oscillator buffer, and rf amplifier. The vhf tuner tuning shaft and

oscillator buffer tuning shaft are synchronized through a gear train which provides a 1 to 1 coupling

ratio.

The output shafts to the vhf tuner and oscillator buffer rotate 66 degrees for each 1 MHz increment of

frequency. When the receiver-transmitter is tuned across its frequency range, the vhf tuner and

oscillator buffer tuning shafts rotate approximately 1,512 degrees.

0.05 MHz TUNING

Motor B9702 (part of Iedex assembly) drives 0.05 MHz Iedex switch S9703 and positions crystal selec-

tor switch S2001 in the rf oscillator. The Iedex assembly, driven by motor B9702, is coupled directly to

the 0.05 MHz tuning cam which rotates 18 degrees for each 0.05 MHz increment of frequency. The

0.05 MHz tuning mechanism turns in one direction through 360 degrees in 20 steps. When the

receiver-transmitter is tuned from XX.00 to XX.95 MHz, the 0.05 MHz cam rotates through 342

degrees in 19 steps. The 20th step completes 360 degrees of rotation, which returns the cam to its

original position.

As the cam rotates, the cam follower transmits angular motion of the cam by means of a selector gear

coupled to a spur gear through a 5 to 1 step-up ratio, to the differential. The spur gear drives the

differential through a 1 to 1 ratio. As the 0.05 MHz cam rotates through 18 degrees for each 0.05 MHz

increment, the differential rotates the vhf tuner, oscillator buffer, and rf amplifier tuning shafts

through approximately 3 degrees. Switch S9703 is coupled directly to the shaft of motor B9702. Crystal

selector switch S2001 is coupled to the shaft of the 0.05 MHz cam.

VHF TUNER MECHANICAL TUNING

The output of the gearbox differential rotates the tuning shaft of the vhf tuner spiral inductor to the posi-

tion corresponding to the frequency selected at the control unit. For each 1 MHz change in frequency,

the tuning shaft turns through 66 degrees. Each complete revolution (360 degrees) of the tuning shaft

results in a frequency change of 5.5 MHz.

As the receiver-transmitter is tuned through its complete frequency range, the vhf tuning shaft com-

pletes approximately 4.2 revolutions of 1,512 degrees. Frequency changes of 0.05 MHz are also

transmitted to the vhf tuner tuning shaft through the differential. For each 0.05 MHz increment of fre-

quency, the tuning shaft rotates through 3 degrees.

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