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SECTION IV

MECHANICAL AND ELECTRICAL CHARACTERISTICS

16. CIRCUITS.

Electrically, lhe receiver comprises two stages of tuned radio frequeacy amplification preceding the first detector, a temperature-compensated heterodyne oscillator, three intermediate frequency amplifier stages, a second desector and one stage of audio-frequency amplification vwith a transformer output circuit. A crystal band-pass filter and beat-frequency oscillator are also included. The former is for increasing selectivity and the latter for receiving c-w signals. The schematic and wiring diagrams are shown in Section Vlll.

17. FREQUENCY RANGE AND BANDS

Six bands controlled by a band change switch are covered. The frequency range for each of the six bands is given in the following table:

BandFrequency Range
200500 Kilocycles
1.53.5 Megacycles
3.56.0 Megacycles
6.09.5 Megacycles
9.513.5 Megacycles
13.518.0 Megacycles

18. INPUT COUPLING.

The antenna input circuit is designed to operate prop erly with antennas having capacitics ranging from 50 to 250 mmf. A one rnegohm resistor is connectet across the antenna and ground posts to disharge static charges.

19. RADIO FREQUENCY AMPLIFIER.

The radio frequenq preselector comprises three tuned circuits coupled by two Tubes VT-117. Separate inductors are employed for each frequency band.

20. FIRST DETECTOR

The first detector employs Tube VT 150 which also functions as the hetcrodyne oscillator. The low signal level at the grid of the first detector, together with the r-f preselection, insures a minimum of undesired rcsponse Fixed bias is provided by returning the control grid circuit through a filter resistor to the low potential end of a

 

25 ohm resistor (Refs. 108-l and 108-2 in parallel). The bias consists of the drop icross this resistor which is in the negative plate supply line.

21. HETERODYNE OSCILLATOR.

The heterodyne oscillator employs a tuned grid cir circut. Excitation is secured by means of a cathode winding tightly coupled to the grid winding. The high value grid resistor and the low grid coupling capacity used, together with the inherent stability of Tube VT-150, makes a voltage regulator unnecessary. The effects of wide variations in ambient temperatures under service conditions on the oscillator frequency have been reduced to a minimum by the use of a highly stable tuning capacitor and temperarure-compensation with ceramic fixed capacitors (35-1, 35-2, 42-, 42-2, 45, 48, 49-2), Individual inductors and trimmers are employed for each frequency band. On the four lower frequeng tuning bands, the oscillator frequency is higher than the desired signal by the intermediate frequency. On the two higher frequency bands 5 and 6, the oscillator is on the low frequency side of the desired signaL The latter results in an improvement in the image rejection ratio.

22. INTERMEDIATE FRSQUENCY AMPLIFIER

The intermediate frequency amplifier comprises two low-gain amplifying stages coupled by three highly se lective, double-tuned circuit transformers and one resistance coupled stage. The intermediate frequency em ployed is 915 kc. The i-f transformers are tuned by means of adjustable iron cores and fixed capactors. The lowered tuned circuit impedance, secured by the relatively large fixed tuning capacitors, provides an inherently stable amplifier. Two Tubes VT 117 function as the first and second i-f amplifiers and Tube VT 116 is employed as the third i-f amplifier. A relatively high level signal is supplied to the second detector diodes of Tube VT 233 which also functions as the c w oscillator.

23. C.W. OSCILLATOR

a. The c-w oscillator employs the triode section of Tube VT-233 (second detector) in a tuned grid, plate feedback circuit. The variable iron core in the grid inductance 151 is used for frequeng adjustment, and is so mounted that about one turn of the beat frequency knob

Revise 30 July 1945

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