Communication Concepts EB63A User Manual

CONSTRUCTION HINTS FOR THE EB63A

These construction hints for the EB63A are only to supplement the Engineering Bulletin
and are not meant to be used as a construction manual. Due to parts becoming
obsolete or hard to find, some changes are recommended for the EB63. The 1N4997
bias diode is no longer available and a recommended substitute is the MJE243 transistor
used as a diode. The relay has become very hard to find and if found is very expensive.
An Omron replacement relay is recommended and the board has been modified to
accommodate this relay. Also the original COR circuit did not perform very well as
designed by Motorola. An improved COR circuit has also been implemented on the
board.

HEATSINK PREPARATION

A heatsink of sufficient size to dissipate over 140 watts of power should be used. The
heatsink should be marked, drilled, and tapped before construction begins on the PC
board. Using the bare PC board as a pattern or template, lay the PC board on the
heatsink in the desire position and mark the four mounting holes. Do not mark or drill a
hole for D2. Typically, placing a metal bolt through the mounting hole of the diode D2
will short out the bias supply to the heatsink. Drill and tap the four mounting holes for 4­40 screws. After the four mounting holes are drilled and tapped, mount the bare PC
board flush on the heatsink using 4 – 40 x ¼” or 3/8 “ screws. Place the RF transistors
through the PC board so they rest flat on the heat sink. Mark the mounting holes for the
RF transistors. Remove the RF transistors and PC board from the heatsink. Drill and
tap the mounting holes for the RF transistors (4 – 40 screws). The heatsink can also be
fitted with connectors, cover, etc. at this time if so desired.

BROADBAND TRANSFORMERS

Just a quick word about the broadband transformers. The low impedance winding of the
input (T1) and output (T2) transformers consists of a one-turn winding. Two tubes form
the low impedance winding. The tubes are electrically shorted on one end by a piece of
copper-clad laminate (PC board) with holes for the tubes. The tube ends are soldered to
the copper foil. A similar piece of laminate is soldered to the opposite ends of the tubes
and the copper foil is divided into two sections, thus isolating the ends where the primary
connections are made. The high impedance winding is formed by threading Teflon
insulated wire through the tubes for the required number of turns. The transformers are
loaded with ferrite material to provide sufficient low frequency response.

PC BOARD ASSEMBLY

Install the ¼ watt metal film or carbon resistors R2, R3, and R7 in their proper locations
(refer to the placement layout in the construction) and solder in place. Clip the leads
close to the PC board. R1 is not used.

Install the 1N4148 diodes D3 and D4 as shown. Observe the polarity..!! The banded
end is the cathode.

Install transistors Q1 and Q2 as shown in the component layout. The flat sides of the
transistors face the outside of the PC board.

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Install the 1N4001 diode D1 as shown. Observe the polarity..!! The banded end is the
cathode.

Position the transformer T2 on the PC board as shown in the Engineering Bulletin. The
transformer must be installed with the wide gap on the input end placed down on the PC
board. The split-end of the transformer goes toward the center of the PC board. Solder
the split-ends (two places) and the opposite solid end to the PC board with a fillet of
solder (make sure the transformer is held flat against the PC board). Trim the Teflon
insulated secondary windings of T2 for a proper length for mounting into their respective
mounting holes on the PC board. Then trim about ¼” of the insulation off the wire leads.
Install the leads into their proper mounting holes and solder to the PC board.

The input transformer T1 is mounted next. Trim the wire leads to the proper length to
mount on the PC board. Strip ¼” of the insulation from the wire leads. Mount the
transformer to the PC board with the split ends toward the RF transistors. Solder the
transformer to the PC board similar to T2. Solder the wire leads to the PC board.

Install RFC1 as shown on the component layout. RFC1 is composed of ten beads on a
piece of buss wire bent into a “U” shape. The buss wire is soldered through its
respective mounting holes near the edge of the PC board.

Install the 1 uF tantalum capacitor C6 on top of the PC board as shown in the
component layout.

Install the 10uF electrolytic capacitor C5 on top of the PC board as shown in the
component layout.

Install the .001uF disc capacitor C7 on top of the PC board as shown in the component
layout.

Install the 470pF disc capacitor C11 in its proper location as shown in the component
layout. Clip off the excess leads.

Install the 22uF electrolytic capacitor C12 in its proper location as shown in the
component layout. Clip off the excess leads. This capacitor is used to increase the
relay hold time when SSB mode is used. If a longer delay is desired, increase the size
of C12 to 100 uF for approximately 1-½ seconds of delay. An external switch (not
supplied) can be used to activate or ground on side of the capacitor to increase the
delay time.

Install the 18pF mica capacitor C2 and the 24pF mica capacitor C10 on top of the PC
board as shown in the component layout.

Install the 33pF mica capacitor C1 on top of the PC board as shown in the component
layout.

Install the 910pF mica capacitor C20 on top of the transformer T2 and the 1100 mica
capacitor C21 on top of the transformer T1. The leads of the capacitors are soldered to
the split ends of the transformers and the capacitors lay over the top of the transformers.

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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 4­40 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 4­40 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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