Communication Concepts EB27A User Manual

CONSTRUCTION HINTS FOR THE EB27A

These construction hints for the EB27A are only to supplement the Engineering Bulletin
and are not meant to be used as a construction manual.

HEATSINK PREPARATION

A heatsink of sufficient size to dissipate over 300 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, lay the PC board on the heatsink in the
desired position and mark the four mounting holes and the mounting hole for D1. Drill
and tap the four mounting holes and the hole for D1 for 4-40 screws. Then bolt the bare
PC board flush on the heatsink and place the RF transistors through the PC board so
they rest on the heat sink. Mark the mounting holes for the RF transistors. Remove the
PC board from the heatsink and 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.

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BROADBAND TRANSFORMERS
Just a quick word about the broadband transformers. The low impedance winding of the

input (T1) and output (T3) transformers consist of one winding. Two metal 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

Flow some solder (a very light coat) on the PC board where the input end of T3
will be mounted and install the chip capacitor, C5. See Figure 1.

Figure 1

Strip the leads of the output transformer (T3) to ¼ inch of insulated wire from the
transformer end.

Place the leads of T3 into the proper holes on the output side (the transformer
must be installed with the wide gap on the input end placed down on the PC board).

Place a piece of Mylar, Teflon, or similar piece of insulating material under the
transformer, T3 to prevent it from shorting to the ground plane. Position the transformer,
T3 close to the chip capacitor, C5 on the input end of the PC board. Solder the split­ends (two places) to the PC board with a fillet of solder (make sure the transformer is
held flat against the PC board). See Figure 2.

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Figure 2

Solder the wire leads from the output of T3 to the PC board. Clip the excess
leads close to the PC board.

The input transformer T1 is mounted next. Strip the wire leads back to
approximately 3/16 inch of insulated wire from the transformer end. If swage pins are
used (optional) mount them in the two holes on the PC board and solder on the bottom.
Insert the insulated wires in their holes and position the split-end of the transformer up
against the swage pins and solder. The transformer must be insulated from the PC
board using a piece of Mylar, Teflon, or some similar insulating material. In lieu of
swage pins, short pieces of #20 bus wire or something similar can be inserted into the
two holes on the PC board and soldered. Then bend the ends of the wires over to the
split-ends of the transformer and solder. Clip all Excess leads.

Install the regulator (MC1723). Match the TAB orientation of the regulator with
the notch on the PC board and push the device down to within 3/8 inch of the PC board.
Solder the regulator in place. (Be very careful …!!!, the PC lands in this area are
extremely close and solder bridges are easy to make.) Clip the excess leads.

Install the potentiometer (push it firmly down in place on the circuit board and
solder it in place). Note: The pot must be installed BEFORE
because of an interference fit between the pot (certain manufacturer) and the 1K resistor.
Clip the excess leads.

resistor R7 is installed

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