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SCOPE MODULE A2
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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Section VIII. POWER SUPPLY STAGE ANALYSIS AND SWITCHING
TM 11-5805-387-34-2
1-61. INTENSITY SQUELCHING
The intensity squelching circuit biases off the display when the signal in the vertical channel falls below a predetermined
level. The emitter output of vertical amplifier A2A1Q2 is sampled across SQUELCH ADJ potentiometer A2A1R25 and
amplified by transistors A2A1Q4 and A2A1Q3.  Thermistor A2A1RT1, in the emitter path of amplifier A2A1Q3,
compensates for variations in emitter current gain with temperature. The amplified signal at the collector of A2A1Q3,
directly proportional to the output level of vertical amplifier A2A1Q2, is applied to high-voltage isolation transformer
A2A1T3. Diodes A2A2CR4, CR5 and capacitor A2A2C9 form a full-wave rectifier which develops a dc level proportional
to the signal coupled through transformer A2A1T3. This dc level is applied to the base of squelch switch A2A2Q1,
causing the transistor to conduct.  Squelch switch A2A2Q1 is connected between the wiper (A2J1-3) of SCOPE
INTENSITY control R3 and its negative most end (A2J1-1). When A2A2Q1 is conducting, the portion of R3 between the
wiper and the negative most end is shorted out, causing a more positive voltage to be applied to control grid, pin 2 of V1.
As R3 Is rotated clockwise, the scope intensity becomes brighter; counterclockwise rotation dims the intensity. When the
vertical input signal level falls below a level sufficient for good display, the amplified signal across transformer A2A2T1
cannot develop a dc level large enough to keep squelch switch A2A2Q1 conducting. Consequently, switch A2A2Q1 cuts
off and the full -900 volt potential is applied to the control grid of the crt, squelching the display. The collector of A2A2Q1
is returned to -750 volts through SCOPE INTENSITY control potentiometer A2R3, which allows adjustment of emitter
current flow and resulting display intensity.
1-62. POWER SUPPLY
High voltage for the crt bias circuit Is provided by the scope power supply, which consists of a dc-to-ac inverter, a
voltage doubler, and a divider network. The dc-to-ac Inverter consists of oscillator A2Q1, Q2, and A2T1, starting resistor
A2A2R13, feedback diode A2A2CR3, rfi filter A2A2C8, L1, slow-down circuit A2A2C6, C7, and overvoltage zener diodes
A2A2VR3 and VR4. Except for the overvoltage zener diodes, the circuit Is Identical with the dc-to-ac inverter of power
supply module A5, and operates as described In paragraph 1-40. Overvoltage zener diodes A2A2VR3 and VR4 limit the
collector voltages of transistors A2Q1 and A2Q2 to +68 volts, preventing the emitter-collector voltage ratings of the
transistors from being exceeded. Transformer A2T1 provides a 12 volt, peak-to-peak heater voltage for the crt and a
1,000 volt output Is divided by deflection supply voltage-divider chain A2A2VR1, VR2 and resistors A2A2R5 through R9.
Potentiometers A2A2R2 and A2A2R4 are connected between the + 100 and -100 volt potential points, to allow centering
of the vertical and horizontal scope traces.
1-53

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