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Section VIII. POWER SUPPLY STAGE ANALYSIS AND SWITCHING
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TM-11-5805-387-34-2 Modem Radion Teletypewriter MD-522A/RC (NSN
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OPERATIONAL CONTROL CIRCUITS
TM 11-5805-387-34-2
1-65. +22-VOLT PREREGULATOR
(fig. FO-15)
The +22-volt preregulator consists of reference zeners A9VR3, VR5, VR6; comparator A9Q5; amplifier Q3; series
regulators Q1, 02; transient protectors CR1, A9VR4; and associated resistors and capacitors. Zener A9VR3 provides an
18 volt ( 5 percent) reference voltage to voltage divider A9RT1, R11, R12. The +22V ADJ control A9R11 adjusts the
voltage of the preregulator output from 20 to 24 volts. The preregulator output is sampled through series diodes A9VR5,
VR6, which maintain a constant voltage drop of 8.4 volts from the +22 volt output. As loading or other factors cause the
preregulator output voltage to drop, the voltage drop across emitter resistor A9R9 is lowered, forwarded biasing
comparator A9Q5. This action causes A9Q5 to conduct more, lowering the base voltage of amplifier Q3 with respect to
its emitter. Amplifier Q3 conducts more, raising the base voltage of series regulator Q1, Q2. Series regulator Q1, Q2
conducts more, raising the output voltage of the preregulator and compensating for the original drop in output voltage.
The unit operates the opposite way to counter a rise in output voltage. Transistors Q1 and Q2 are paralleled to provide
required current capacity. Diode CR1 prevents collector-to-emitter breakdown of Q1, Q2 due to starting transients.
Zener diode A9VR4 grounds out transients exceeding 27 volts. The capacitance of A9C2 multiplied by the betas of
A9Q5 and Q3, filters noise. Capacitor A9C3 filters out low-voltage transients and noise. Starting resistor R8 provides a
voltage source to A9VR3 to start the preregulator.
A short across the output of the preregulator will not trip circuit breaker CB1. The preregulator provides its own
short-circuit protection. A short across the preregulator output drops the voltage across reference diode A9VR3 to zero;
therefore, comparator A9Q5, amplifier Q3, and series regulator Q1, Q2 are sequentially cut off. When the short is
removed, A9R8 applies the starting voltage to reference zener A9VR3.  Thus, a short circuit at the output of the
preregulator cannot trip circuit breaker CB1. However, shorts in circuitry beyond the preregulator output can trip the
circuit breaker if they draw a current that exceeds 2.6 amperes.  The preregulator +22 volts Is applied to MODE
SELECTOR switch S4 (para 1-67) and the 20 volt regulator (para 1-66).
1-66. TWENTY-VOLT REGULATOR
(fig. FO-15)
The 20 volt regulator In receive audio module A1 provides a +20 volt dc regulated output to all circuits for
appropriate operating settings (850 HZ, 85 HZ, VOICE, 85 HZ DIV, and 85 HZ+ VOICE) of MODE SELECTOR switch
S4. Twenty-two volts dc is applied to series regulator A1Q1. The effective collector-to-emitter resistance of A1Q1, in
series with the 22 volt dc line, drops the voltage to +20 volts dc for any given current required by the applicable circuits.
Comparator A1A2Q2 compares the output level with a level established by reference zener A1A2VR1. The circuit is
adjusted by +20V REG ADJ A1A2R4 so that the output is maintained at +20 volts dc 0.2. The difference between the
voltage standard and 20 volt dc output is raised in level by comparator A1A2Q2 and applied to amplifier A1A2Q1. Dc
amplifier A1A2Q1 is direct-coupled to series regulator A1Q1; therefore, the conduction of A1Q1 is controlled by the
conduction of A1A2Q1. Thus, the output of dc amplifier A1A2Q1 will alter the amount of conduction (effective collector-
to-emitter resistance) of series regulator A1Q1 causing the output voltage to be maintained at +20 volts dc. The +20
volts output is applied to all modules (A1, A2, A3, A4, A5), board A7, and MODE SELECTOR switch S4F.
1-55

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