Communication Concepts EB63A Construction Hints - 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 4-
40 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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