Audio Unit Assembly.

TM 11-5895-1141-34

(3) Connector J5. This is a 5 pin connector for use with a type NT-409985A or equivalent M-109 headset having

a transistor preamplifier and requiring a voltage source for the preamp. The voltage source (+12 vdc) is derived from

zener diode CR1 and current limiting resistor R8 connected to the +24 v supply. Decoupling is provided by R7 and C5.

Resistor R6 provides the required 30 ohm microphone load while capacitor C4 blocks dc from passing through R6.

Connector J5 may also be used with an M-109 microphone equipped with a matching 5 pin connector. Isolation between

the high level microphone and the headset microphone as well as attenuation of these microphone signals to the correct

level for input to the microphone amplifier circuit is provided by resistor R2 and R5. Capacitors C2 and C3 are dc blocking

capacitors. When the keying switch of the transistorized high level microphone is depressed, +12 v is applied to the base

of Q1 via R9, saturating Q1 and effectively grounding terminal P1-C.

b. Audio Unit Assembly. Refer to figure FO-6. Connectors J101 through J105, located on the rear panel provide for

connection to the headset/microphone jack panel assembly, the selector unit assembly, external telephone circuits (if

used), a tape recorder (if used), and the 24 vdc power. Receptacles J106, J107, and J108 are located within the

enclosure and mate with the audio unit modules as follows:

(1) Connector J106, located near the loudspeaker, connects to the microphone amplifier/lamp brightener

module assembly.

(2) J107 is for a recorder-monitor module (if used).

(3) J108 connects the volume control module assembly.

(4) The speaker amplifier board assembly, not accessible from the front panel as are the other modules, uses

connector J109.

DC power is delivered to the audio unit assembly via decoupling resistor R101. Capacitor C103, insures the loudspeaker

blocks the dc present at the driver amplifier output. Filter choke L101 and capacitor C101 make a low pass filter which

removes any residual noise or hum from the dc supply voltage before it is delivered to the circuits in the various modules.

Switch S101 located on the front panel, allows the operator to switch the 24 v power at his position. Indicator lamp DS101

goes on when power is present.

c. Microphone Amplifier/Lamp Briqhtener Module Assembly. See figure FO-8. This module is located in the audio

unit enclosure. For the sake of brevity, the basic component part numbers will be used in place of the actual part

numbers. On this module, the basic number is added to 1200 to form the actual number. For instance, the module

connector

2-14