Communication Concepts EB27A Construction Hints - Manual (Page 6)

Cut the extra short piece of wire (L5) to 1 ¾ inch and scrape the insulation 3/8
inch back from each end. Install this wire through T2 and into the remaining holes in the
PC board (pass the wire through T2 only once!). Solder and clip the excess leads close
to the PC board.
Install the 47 ohm 5 watt resistor. Solder and clip the excess leads close to the
PC board.
Install the 470uF (470uF or 500uF) 6.3 volt capacitor (observe the polarity).
Solder and clip the excess leads close to the PC board.
Install the 10uF 100 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 board
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 47 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 28 volts to the amplifier and
monitor the bias voltage across the resistor with a digital voltmeter. The voltage should
vary approximately 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 to the minimum bias voltage (around 0.5
volts). When the bias is set later, the typical operating bias voltage will be around 0.68
volts. Remove the 15 to 47 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). Fasten the transistors to the heatsink with 4-40 screws. Solder the
transistors in place.
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