Continental Electronics 816R - Filament Voltage Regulator

Q3, and Q4, also energized by the 28 V de,
provide the de tum-on signal to the gate cards.
Resistors R24 and R36, and capacitor Cll modify
this signal to soft-start the high voltage power
amplifier plate power supply. Zener regulator VR4
provides -10 V de to MANUAL power adjust
resistor A2A9R1. Transistors Q4 and Q5 amplify
the control signal and apply the signal to the gating
cards. Capacitor C5 and resistor A6R36 phase
compensate the power control servo loop.
Filament Voltage Regulator, A2A4.
When the Filament Regulator Card is in the
automatic mode, the filament voltage regulator
detects and compensates for sustained fluctuations
in the input ac voltage. The fluctuations are
detected by a true rms detection circuit which in
conjunction with associated circuitry, including
motor control circuits, adjusts the setting of
variable transformer A8A2T1. The output voltage
of the variable transformer is then applied to the
primary of power amplifier filament transformer
A12T1. The variable transformer voltage is also
applied to the primary of filament control
transformer A2T4.
Voltage for the power supply circuits on the
filament regulator board is obtained from the
transmitter 28 VDC supply. This 28 volts source is
supplied to voltage dropping resistor R33.
Capacitor C26 provides filtering. A three-terminal
15 V de regulator U9 supplies voltage to the 15 V
de circuits with additional regulation provided by
capacitors C27 and C28. Voltage dropping resistor
R34 feeds three-terminal 5 V de regulator U10
while capacitors C29 and C30 provide additional
voltage regulation. LED DS5 indicates voltage
present on the 5 V de line which implies that the 15
V de circuits are powered also. Negative supply
voltage is provided via diodes CR1 and CR2 via
resistor R3 and capacitor C24 to card edge
connection 38 for distribution to other circuit cards
in the transmitter.
A sample of the voltage feeding the power
amplifier tube filament transformer is applied via
transformer A2T4. This ac signal is applied to RF
filtering components inductor LI and capacitor
C14. Inductor LI has a 4.7 ^ih parallel resonance
which falls in the FM broadcast band providing a
high impedance path for frequency modulated RF
signals. Capacitor C14 is a 100 pf capacitor whose
series resonance falls in the FM broadcast band
providing a low impedance shunt path for
frequency modulated RF signals. These filtering
components are used in several locations in the
filament regulator card, and provide the same
filtering functions as described here.
The filament voltage sample signal is then applied
to the RMS-to-dc converter circuit via voltage
divider resistors R1 and R2, and through capacitor
C15. This RMS-to-dc converter circuit is based
around U7, a true rms-to-dc convener integrated
circuit. The convener directly computes the true
RMS of any complex input waveform containing ac
components. It has crest factor compensation which
allows very accurate measurements up to 300 kHz.
The crest factor of a waveform is the ratio of the
peak signal swing to the RMS value. Components
C17, R17, R18, C18, R19, and C20 provide time
constant and filtering functions for the convener.
Test point 3 (TP3) provides easy access to the de
voltage representation of the filament RMS voltage.
During normal operation of the filament voltage
regulator, resistor R2 is adjusted so the output of
the RMS-to-dc converter circuit is 5.00 V de when
the filament voltage has been preset to the nominal
value by the operator. The output voltage is then
fed to window comparator composed of U8 and
related devices. The voltage references for the
window comparator are provided by U 11, a very
high precision 10-Volt regulator, and voltage
divider components R20, R21, R22, and R23. In
normal operation, resistor R20 is adjusted to
provide 5.00 V de at TP2. The corrected reference
voltages are then applied to their respective
comparators. Pin 5 of U8 has 5.05 V de applied,
and pin 10 of U8 has 4.95 Volts applied. These
voltages will be correct if resistor R20 has been
properly adjusted for 5.00 V de on TP2. The 50
mV voltage drops are provided by voltage divider
resistors R21 and R22. This 100 mV total window
provides a total + 1 % window for the voltage
comparator, and hence for the voltage regulator
circuitry.
If the voltage from the RMS-to-dc convener circuit
is within the 4.95 to 5.05 V de window, the outputs
of the comparators will both be high and the output
of AND gate U4C will go high illuminating green
LED DS4 indicating proper filament voltage is
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