Communication Concepts AN762 Construction Hints - Manual (Page 6)

Install the 100uF 25 volt capacitor (observe the polarity). Solder and clip the
excess leads close to the PC board.
Turn the PC board over and install the chip caps in the locations shown in the
Application Note. Install one capacitor at a time (open the envelopes carefully! – some
of the capacitors are very small). CAUTION: When soldering chip capacitors, flow a
small portion of solder on the PC board circuit and hold the capacitor in place. Put some
solder on the iron and tack one end in place. Do not let the iron touch the capacitor end,
but rather let a fillet build up between the board, the iron, and chip cap. The chip
capacitor ends will not take excessive heat.
This completes the PC board construction except for the RF transistors. Clean
the PC board if so desired, and inspect closely for solder bridges between the PC boar
foil lands.
At this point, an intermediate electrical check can be performed to ensure that the
bias supply is functioning properly. Too large a bias voltage will destroy the RF
transistors..!! The bias supply check is performed before the RF transistors are mounted
on the PC board. Temporarily solder a 15 to 100 ohm 1//4 watt resistor between one of
the RF transistor base pads and the ground plane on the PC board. The resistor will
provide a small load for the bias voltage regulator. Apply 12 volts to the amplifier and
monitor the bias voltage across the resistor with a digital voltmeter. The voltage should
vary from about 0.5 to 1.0 volts when the variable pot is varied through it range. If the
bias supply is working properly, set the pot for a bias voltage of 0.5 volts. The typical
operating bias voltage is around 0.68 volts. Remove the 15 to 100 ohm ¼ watt load
resistor. This completes the bias voltage regulator check.
Install the PC board on the heat sink with screws and 1/8 inch spacers or #6 nuts
used as spacers (not supplied). Apply thermal compound to the bottom of transistor D1.
The outmost 1/16” to 1/8” end of each RF transistor lead can be bent up at a 90
degree angle (this procedure is optional but will aid in removal of the RF transistor
should it ever become necessary).
Install the RF transistors with the clipped collector lead toward the output
transformer. CAUTION: Be sure to use thermal compound under the transistors (not
supplied). Solder the transistors in place.
This completes the assembly of the amplifier PC board. The unit may now be
tested.
1. Hook up a power supply of suitable current output to the amplifier. A regulated
power supply capable of delivering 13.6 volts DC at 25 amps is recommended for
this amplifier operation.
2. Connect the RF output of the amplifier through a power meter to a suitable 50
ohm dummy load of a rating capable of handling the power output (use a quality
50 ohm coax cable).
3. The potentiometer should be turned fully clockwise when the bias voltage was
set to 0.5 volts at the end of the bias voltage regulator test.
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