Mixer

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

Refer to figure 7-14.  Input to the mixer, V551, is applied to the primary of T551, T552, T553, T554, or T555 by a portion of S551B, the setting of which is controlled by the band selector.  Capacitors C557, C560, C563, and C570 are shunted across the primaries of T551, T552, T553, and T555 respectively, to load the primary circuit.  This arrangement provides a constant input impedance throughout each band.  No such capacitor is included in the primary of T554, as the stray capacitance in that circuit is sufficient to perform this function.  The mixer section of the ganged tuning capacitor, C651B, is shunted across the secondary of the selected transformer to adjust the transformer tuning to the desired frequency.  Fixed capacitors and a trimmer, together with the mixer tuning capacitor, resonate with the secondary coils to provide continuous tuning over the selected band.  Coils and trimmers in the mixer circuits are adjustable by means of a screwdriver at points shown in figures 7-8 and 7-9c.

R-f signals are applied to the first grid (pin 1) of V551 from one side of the selected tuned circuit by the setting of S551A.  Switches S551A and B short the secondary of unused transformers to prevent absorption of signals on the band in use.

Except for different frequencies, the mixer circuit (V551) of this receiver is similar to that for the AN/SRR-12 receiver, as described in paragraph 2d(2) of this section.  Because of the high frequency involved, the oscillator voltage is applied to grid 3 (pin 4) of V551 by way of link O603, accessible directly beneath the cover of the oscillator and mixer boxes.

Output from the mixer pentode, V551, is applied to the first i-f assembly through J551L and J901B.  This output is at a frequency of 1600 kilocycles on all five bands. The pentode also feeds the mixer cathode follower, V552, which supplies a panoramic adapter on all bands. The PAN receptacle J1806 is at the back of the receiver.

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