Communication Concepts EB63A Construction Hints - Manual (Page 3)

Install the 0.1 ceramic mono capacitor C4 on top of the PC board as shown in the
component layout.
Install the 470 uF electrolytic capacitor C22 (observe polarity). Solder and clip the
excess leads close to the PC board.
Install the 82 ohm 5 watt (formerly a 33 ohm 5 watt) resistor R4 on top of the PC board.
Mount the resistor approximately 1/8” to ¼” off the PC board for cooling purposes. The
resistor will run hot to the touch.
Mount the relay on the PC board as shown in the component layout.
Mount the diode D2 (MJE243 transistor) on the bottom of the PC board as shown in the
component layout. The diode (transistor) mounts with the metal part of the diode toward
the heatsink. Bend the leads up through the PC board. Solder and trim the leads. A 4-
40 screw can temporarily be placed through the hole in the PC board and the diode to
help position it in the proper position.
This completes the PC board construction except for the RF transistors, C8, C9, R5, and
R6. Clean the PC board if so desired, and inspect closely for solder bridges between
the PC board foil lands. Also look for any cold solder joints.
At this point, an intermediate electrical check can be performed to ensure that the bias
supply is operating correctly. Too large a bias voltage will destroy the RF transistors!!!
The bias supply check is performed before the RF transistors are mounted. Temporarily
solder a 10 ohm resistor to one of the base pads and the ground plane. The resistor will
provide a small load for the bias voltage. Apply 13.65 volts to the amplifier and monitor
the bias voltage across the temporary 10 ohm resistor with a digital voltmeter. If the bias
supply is working properly, the bias voltage should be around 0.66 to 0.74 volts. A
higher bias voltage may cause the RF transistors to run very hot when no signal is
applied. Remove the temporary 10 ohm load resistor. This completes the bias voltage
check.
Install the PC board to the heatsink with 4–40 x 3/8” screws and 0.10” spacers or thin #6
nuts used as spacers (not supplied). An insulating mica washer must be placed
between the diode D2 (MJE243 transistor) and the heatsink to prevent shorting out the
bias supply. Use thermal compound on both sides of the mica washer.
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…available at Radio Shack). Bolt or screw the RF transistors into place using 4-
40 x ¼” screws. Solder the transistors in place.
Install the 10 ohm ½ watt resistors, R5 and R6, on top of the PC board as shown in the
component layout. The resistors should stand approximately ¼” above the PC board.
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