Broadcast Electronics FX-50 Instruction Manual Download Page 90

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The drain of Q4 connects to an impedance matching network which operates as: 1) a broad band impedance
step-up transformer to establish an output impedance of 50 Ohms, and 2) a second harmonic notch filter.
Capacitor C36 functions as a dc blocking capacitor.
Power amplifier Q4 operates from an adjustable dc potential of +3 to +24 volts. The adjustable potential is
preset by circuitry on the power supply/control circuit board and is automatically maintained by feedback from
the forward directional coupler. Inductors L7 and L8, and capacitors C28 through C31 provide power supply
isolation.
DIRECTIONAL COUPLER CIRCUITS.  The directional couplers provide two dc signals obtained by rectifying a
sample of the RF output signal. Due to the polarity of the samples, one signal will represent the forward output
signal and the other will represent the reflected.
Forward Directional Coupler. The forward voltage sample is obtained from a microstrip on the circuit board
near the output line. This signal is rectified and filtered by diode D1, capacitors C38 and C39, and resistor R15.
Capacitor C37 establishes the broadband characteristics of the circuit.
Reflected Directional Coupler. The reflected voltage sample is obtained from a microstrip on the circuit board
near the output line. This signal is rectified and filtered by diode D2, capacitors C40 and C41, and resistor R13.
Capacitor C41 establishes the broadband characteristics of the circuit. The directivity of the circuit is adjusted by
null control R12.
INPUT MUTE CIRCUIT.  The input mute circuit consists of transistors Q5 and Q6. During normal operation, +20
volts is routed to input amplifier U2 through Q5. When the exciter is muted, the final +V supply is terminated.
The loss of this potential will bias Q6 OFF and disable Q5 which terminates the +20 volts to U2.

10.5

MAINTENANCE

This section provides maintenance information, electrical adjustment procedures, and troubleshooting
information for the RF amplifier assembly.

10.5.1

ELECTRICAL ADJUSTMENTS.

Although the following controls are not located on the RF amplifier assembly, the controls effect the operation
of the RF amplifier. The adjustment procedure for each control is presented in the power supply/control circuit
board section of this manual.

A. TEMP TRIP (R27)
B. TEMP CAL (R25)
C. FWD CAL (R5)
D. RFL CAL (R9)

REQUIRED EQUIPMENT. The following tools and equipment are required for electrical adjustment procedures.

A. Insulated adjustment tool, shipped with the exciter (P/N 407-0038).
B. Non-inductive, 100 watt, 50 Ohm test load.
C. Adapter, BNC jack to type N plug for test load (P/N 417-3288).
D. Adapter, type N jack-to-jack for test load (P/N 417-3841).
E. Coaxial accessory cable, BNC connectors, shipped with exciter (P/N 947-0017-2).

10.5.2

RFL NULL (R12).

The RFL NULL control on the RF amplifier circuit board adjusts the directivity of the reflected power directional
coupler. Potentiometer R12 is adjusted as follows.