Antenna Preamplifier

Reference:    NAVSHIPS 91875(A), para 2b(3)

Refer to figure 7-14.  The input circuit accomodates a low- or high-impedance position, the full primary of one of the five transformers (T501 through T505) is connected to the input circuit.  A tap and one side of the primary are connected to the input circuit when the link is in the low-impedance position.  Each  of these transformers has a primary winding, a secondary winding, and is coupled to an associated coil L501 through L505.  Transformer primary and secondary windings are selected by S502A and S502B which are controlled by the band selector.  The band selector also controls S501, which selects the appropriate coil and shorts out the unused coils and transformers.

The receiver should be isolated from a second receiver if two are to be operated from a common antenna by the setting of link O501 (see figures 3-8 and 3-10).  This link connects a resistor-capacitor combination R505-C524 in series with the antenna lead to prevent the impedance of the input circuit from becoming too low and effectively shorting the input of the companion receiver.

Two parallel-resonant circuits are incorporated to provide greater selectivity.  The first of these circuits includes the secondary winding of one of five transformers T501 through T505, associated fixed capacitors, section C651E of the tuning capacitor, and the ANT COMP capacitor C656;  the second resonant circuit includes one of five output coils L501 through L505, section C651D of the tuning capacitor, and associated fixed and trimmer capacitors.  The inductance of the first tuned circuit and the inductance and capacitance of the trimmers in the second, are varied by the screwdriver adjustments shown in figures 7-8 and 7-9c.  C651E and C651D are varied by the tuning knob on the front panel.

Signals are amplified in V501 and fed to the r-f stage through terminals J501L and J501K.  The r-f seciton of gain control, R651A, provides a variable cathode bias to control the gain of V501 (and V526, r-f amplifier, as will be described in a later paragraph).  R651A is connected in series with L651 and C653 is connected across this series network.  This filter arrangement reduces  r-f currents in the gain control circuit.  L651 and C653 are located on filter board E653.  An alternate AGC bias voltage is applied to the control grid of V501 through J501D, to adjust the gain when the RECEPTION control is set to receive A3 or F1 signals.  See paragraph 2b(5) for discussion of the AGC system.