AUDIO AMPLIFIER MODULE A5400 (TRANSMIT MODE).

TM 11-5820-670-30

1-45. AUDIO AMPLIFIER MODULE A5400 (TRANSMIT MODE).

Refer to FO-4 while reading the text in this paragraph.

Amplified audio signals from squelch preamplifier A4300 are applied to amplifier Q5401 (1) in the

audio amplifier for amplification.

In transmit mode, a transmit ground signal is applied to X2 of relay K5501, causing B2 and B1, and A2

and A1, to make contact.

When B1 and B2 make contact, the amplified output of Q5401 is applied to low-pass audio filter FL5501

(2) only. The filter passes frequencies below 3 kHz and applies the filtered audio through potentio-

meter R5405 (3) to amplifier Q5403 (4).

The sidetone gate signal from the sidetone gate detector is applied through A1 and A2 of relay K5501 to

X2 of relay K5502. Relay K5502 becomes energized causing A1 and A2 to make contact. +16 vdc is

then applied to the amplifiers to unsquelch the receiver and allow the audio signals to be amplified.

Once amplified, the audio is coupled through transformer (7) and interphone amp and heard in the

headset.

1-46. SQUELCH AMPLIFIER MODULE A5200 (TRANSMIT MODE).

Refer to FO-4 while reading the text in this paragraph.

The squelch amplifier's function in the transmit mode is to generate a 150 Hz tone signal that can be

transmitted to an unsquelch receiver.

The 150 Hz tone signal is generated using four transistors in the squelch amplifier and a phase shift

network in the audio amplifier. The tone oscillator consists of Q5201 (11), Q5202 (16), Q5203 (12),

Q5204 (13), and a phase shift network (23). All of these components are connected in a feedback

loop that oscillates at 150 Hz.

When the radio set is in transmit mode, B2 and B1 of relay K5501 make contact, applying the output of

phase shift network (23) from Q5204 (13), to Q5201 (11): The 150 Hz tone is applied from B1 of relay

K5501 to audio mixer A8600, where it is combined with the audio signals to modulate the 11.5 MHz

modulator frequency.

1-47. AUTOMATIC GAIN CONTROL (AGC).

The agc amplifiers in module A6500 insure that a constant level of rf output power is being transmitted.

Agc action will keep the level of audio being heard in a distant receiver constant, even though varying

levels of voice signals from the transmitter operator are entering the microphone.

1-45