Broadcast Electronics FM-5T, User Manual

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4-12
4-65. Component stress at power-on is eliminated by a step/start circuit which limits plate sup
ply inrush current. The step/start circuit is interlocked through contacts of the filament
circuit breaker and the filament/driver contactor to assure that the filament circuit is en-
ergized before a high-voltage-on sequence can be initiated. The controller will energize
the step contactor via K6. After 100 milliseconds, the controller will energize the start con
tactor via K5. The step contactor will deenergize after it has been energized for 160 milli
seconds. In this manner, the current limiting resistors will only be subject to heating dur
ing a 100 millisecond interval between the step/start relay closures. The limiting resistors
are disconnected from the lines after 160 milliseconds, improving reliability.
4-66.
PA SCREEN GRID POWER SUPPLY.
The screen power supply is a full-wave
bridge-rectified supply with a single L-section filter. The primary of the screen transform
er is connected through CB3 to the step/start contactors. The output of the screen supply
is adjustable with a variable autotransformer connected in the primary of the screen trans
former. A dc motor connected to the variable autotransformer allows both manual and au
tomatic control of the screen voltage. The choke in this supply is connected in the negative
leg of the supply to eliminate the dc potential between the choke and ground. A bleeder
resistor connected across this supply improves regulation and enhances safety by discharg
ing C6.
4-67.
PA CONTROL GRID POWER SUPPLY.
The control grid bias supply is a full-wave bridge-
rectified supply with a single L-section filter. The primary of the supply is connected to
the primary ac input through CB5 and filament/driver contactor K3. A bleeder resistor
connected across this supply improves regulation and enhances safety by discharging C9.
4-68. Hum Null Supply. The ground path for the grid bias supply is routed through the hum-
null circuit which introduces a small 120 Hz pulsating dc component into the control grid
bias supply to cancel hum in the PA tube from the screen supply. The amount of voltage
added to the negative side of the control grid bias supply is adjusted by R10. This voltage
is out-of-phase with the 120 Hz ripple component of the screen supply.
4-69.
PA FILAMENT SUPPLY.
The PA filament supply is a low-voltage high current ac supply.
Overload protection for the circuit is provided by circuit breaker CB5. Filament voltage
regulator VR200 provides a stable ac input voltage environment. Variable transformer T5
provides accurate filament voltage adjustment. A FILAMENT TIME meter indicates
hours of filament circuit operation.
4-70.
RF CIRCUITRY.
4-71.
FM EXCITER.
The modulated FM signal for RF circuit operation is generated by the
FM-250C FM exciter (see Figure 4-4). Approximately 140 watts of drive is required to
operate the PA circuitry. Refer to publication 597-1004 for a complete description of the
FM exciter.
4-72.
POWER AMPLIFIER.
The PA stage contains a single 4CX3500A tetrode operated class C in
a folded half-wave cavity to output 5 kW of RF power with approximately 150 Watts of RF
drive. The following text describes the operation of components and circuits within the PA
stage.
4-73. PA Input Circuit. The grid impedance-matching circuit used in the FM-5T/FM-5TS trans
mitter consists of a combination of series inductor and shunt capacitor circuit board ele
ments, implemented with a printed circuit board. The inductors and capacitors are etched
into the copper-clad laminate. Multiple LC sections match the 50 Ohm source impedance
to the 300 to 400 Ohm input impedance of the grid-driven RF power amplifier.