Recorder-Monitor Module

TM 11-5895-1141-34

supply voltage and minimizes the effect on other circuits by supply current fluctuations in the output stage. A description of

both input and output stages follows.

(1) Input Stage. Transistor Q1 is a class A amplifier whose base biasing network is completed to ground at contact

D by resistor R1147 of the volume control module assembly when the latter is installed. Without the latter in place,

transistor Q1 is cut off and no voltage appears at its collector to provide drive for the output stage. With resistor R1147 in

the circuit, current flowing through resistor R1 through resistor R5 and R6 produce approximately 12 volts at the collector

of transistor Q1.

(2) Output Stage. Output transistors Q3 and Q4 are arranged in a quasi-complementary configuration. When the

collector of transistor Q1 is quiescent, the Darlington pair transistors Q2 and Q3 hold the output (contacts A and B) at

about +11 volts. A capacitor in the audio unit enclosure is in series with the loudspeaker so no dc flows through the

loudspeaker. Under quiescent conditions only a minimal current flows through resistor R5 and R6. Any tendency of this

current to increase tends to cut off transistor Q5 and is then counteracted by the reduced base current supplied to

transistor Q4. Diode CR1 compensates for the base-emitter drop of transistor Q5. When signal appears, positive swings

at the collector of transistor Q1 cause transistor Q3 to conduct and "pull up" the output, driving current through the speaker

via capacitor C103 (fig. FO-6). Since capacitor C103 is large, it does not charge up appreciably, even at the lowest voice

frequencies, before the transistor Q1 collector swings below its quiescent point. This tends to cut off transistor Q2 and

transistor Q3 but at the same time transistor Q5 is enabled to conduct. This, in turn, causes strong conduction of

transistor Q4 whose collector "pulls down" the output, reversing the current in the speaker. The loudspeaker thus

reproduces the audio signal arriving at the input.

h. Recorder-Monitor Module. Refer to figure FO-13. This module when present is contained in the audio unit

enclosure. For this module, the actual component designation numbers are the basic numbers shown on the diagram

added to 1300. Thus the module plug, shown as P1, is actually P1301. For brevity of discussion the basic numbers will

be used. Filtered +24 vdc power is delivered from the microphone amplifier/lamp brightener module to P1-3 via L101 and

C101 in the audio unit enclosure. The filtered 24 v return is brought to P1-20. Variable lamp voltage from the microphone

amplifier/lamp brightener module is brought to P1-4.

Monitor Lamp Driver. Signals from the external recorder are delivered to the monitor input terminals P1-1 and

19 and are subsequently fed to class A amplifier stage Q3 via transformer T4. R9 maintains the overall input impedance

to this circuit at approximately 600 ohms. The output from Q3 is fed to the base of Q1 which, in the absence of signals

from T4, is continuously biased to saturation by current supplied from

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