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MARK HOLD SWITCHING, RECEIVER MODULE A4
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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INTENSITY SQUELCHING
TM 11-5805-387-34-2
1-60. SCOPE MODULE A2
(fig. FO-14)
The scope module displays the phase relations between the mark and space signals in tty channel 1 or 2. The
scope module consists of amplifying and channel switching, intensity squelch and power supply circuits. The amplifying
and channel switching are discussed below.
General. Amplification of horizontal and vertical input signals is provided by amplifiers A2A1Q1 and A2A1Q2,
respectively. Voltages from mark and space tones in the frequency discriminators of receiver module A4 are applied to
the scope amplifiers.
Channel Switching . Voltages from the fsk/nsk discriminator are applied to channel 1 input; those from the nsk
only discriminator are applied to channel 2 input. The voltages are coupled through contacts of relay A2A1K1, which
selects the channel to be displayed. Relay A2A1K1 contacts normally connect the channel 2 inputs to the amplifiers.
The relay is controlled by channel switch A2A1Q5. When channel 2 is squelched during 850-Hz mode of operation, or
because a stronger signal exists on channel 1 during 85-Hz diversity mode of operation, +20 volts dc is supplied through
pin 6 of connector A2J1 to zener diode A2A1VR1. Zener diode A2A1VR1 conducts and biases on channel switch
A2A1Q5. Collector current of A2A1Q5 energizes relay A2A1K1, connecting the channel 1 input to the horizontal and
vertical amplifier paths.
Horizontal Amplification. Horizontal input signals are applied from contact A2 of relay A2A1K1 and coupling
capacitor A2A1C14 to an rc network composed of resistors A2A1R22, R23 and capacitors A2A1C3, C4. This network is
one-half of a circuit, which is tuned to 2,805 Hz; this is the center frequency of the frequency discriminators in receiver
module A4. The other half of the tuned circuit is a coil connected to the primary of the frequency discriminator being
used. In the 850-Hz mode of operation, ground is applied to resistor A3A1R24 to decrease the output amplitude of the
network which compensates for greater voltage amplitudes at the fsk discriminator. HORIZONTAL GAIN potentiometer
A2A1R8 feeds a portion of the voltage across the tuned circuit to horizontal amplifier A2A1Q1. The output of A2A1Q1 is
coupled through transformer A2A1T1 and coupling capacitor A2A2C1 to one horizontal deflection plate of crt A2V1.
Capacitor A2A1C17 allows adjustment of the phase of the horizontal voltage to correct phase differences between the
horizontal and vertical signals caused by cable capacitance.
Vertical Amplification. Input signals to the vertical channel are applied from contact B2 of relay A2A1K1 to
VERTICAL GAIN potentiometer A2A1R13. A portion of the voltage across the potentiometer is fed to vertical amplifier
A2A1Q2 which develops two separate outputs. The emitter signal, developed across emitter resistor A2A1R2, is used by
the intensity squelching circuit (para 1-61). The collector signal is coupled through transformer A2A1T2 and coupling
capacitor A2A2C2 to the vertical deflection plate of the CRT Capacitor A2A1C16 allows the phase of the signal to be
adjusted to correct phase differences between the vertical and horizontal signals caused by cable capacitance.
1-52

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