©2000 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 11/27/01
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GENERAL INFORMATION.
The Power Amplifier is a class C
device designed to be installed as an
integral part of a transmitter enclo-
sure in a repeater installation with a
2 Watt exciter module. It is designed
to operate over a range of 140-175
MHz and put out 25-30W.
The Power Amplifier operates on
+13.6 Vdc at about 5 Amp. It has a
50-ohm input and output impedance
and is designed for continuous duty.
A low-pass filter reduces har-
monic output to very low levels.
CONSTRUCTION.
General.
Most of the pertinent construction
details are given in the component
location and schematic diagrams and
parts list.
All parts are tack soldered to the
pc board; so it is necessary to cut and
form leads so that they seat properly
on the board and be sure to keep
leads as short and direct as possible.
This is especially true of the disc ca-
pacitors. Figure 1 shows how to trim
and form the leads of capacitors.
PCB, Transistor, and Heatsinking.
This series of power amplifiers is
designed to be mounted in an rf tight
enclosure with the exciter in such
applications as our REP-200 Re-
peater. The unit is supplied less
heatsink and mounting hardware,
since the enclosure acts as a
heatsink and the hardware normally
is provided with the repeater kit.
If you have purchased the PA for
some other use or mounting method,
it is important to assemble and use
the unit as we do in the repeater to
avoid damage to the transistor by
pulling the leads off the ceramic
case. The pa is designed to have the
thickness of a standard 4-40 nut
(about 0.100 inch) as a spacer be-
tween the pc board and the chassis
on which the transistor is mounted.
a. If you are supplying your own
enclosure, mark and drill four clear-
ance holes for mounting the board
with 4-40 screws and one 8-32 clear-
ance hole to mount the transistor in
the center of the cutout in the board.
The latter hole must be close to the
diameter of the xstr stud to provide
maximum surface for the shoulder of
the transistor to contact the chassis
for heatsinking; so do not make this
hole oversize.
b. Install four 4-40 x 3/8 inch
screws from the bottom of the enclo-
sure and secure with 4-40 nuts.
c. Set pc board over screws, and
align so cutout for transistor is cen-
tered over hole in enclosure. Secure
the board with 4-40 nuts and lock-
washers.
d. Carefully open the package of
heatsink compound with scissors.
Use a toothpick or small piece of wire
to apply a small amount of compound
to the shoulder of the transistor
where it contacts the heatsink.
e. Set the transistor in place, and
orient the notched collector lead to
the right as shown. Secure transis-
tor with #8 lockwasher and 8-32 nut.
Do not overtighten nut; tighten only
to the point of being snug. Hold tran-
sistor leads with fingers to prevent
rotation. If leads still rotate, you are
probably applying too much torque.
Note: Since heatsink compound is
used, it is unnecessary to use a lot of
torque, which could break the stud.
f. Form the transistor leads down
against the board. Then, tack solder
them to the foil, using sufficient sol-
der so that a bond is formed under
the full length of the leads. Note that
other parts will be soldered on top of
the base and collector leads; so it
helps to thoroughly flood those leads
with solder.
Installing Capacitors.
a. Cut tabs (if any present) on
variable mica capacitors C8 and C9
as shown in figure 1 unless already
cut off at factory. (Some capacitors
come already trimmed.) Solder them
to the board in the exact positions
shown in figure 2. Mount the capaci-
tors oriented as shown so the rotor
screw is connected to the proper side
of the circuit (as shown in fig. 2).
b. Bend the leads of C2 and C3
gently at a 90° angle, and solder
them as shown. Make sure the
round end of C2 goes to ground.
c. Form the leads of C1 close to
the body, as shown in figure 1, and
tack solder on the board.
d. Tack solder C4-C5, bending the
leads at right angles, and keeping
them as short as possible, as shown
in figure 1. It is important to angle
the capacitors as shown in figure 2
and position them as close as possi-
ble to the body of the transistor. The
idea is to connect them electrically
as close as possible to the emitter
and base terminals or the emitter
and collector terminals. In fact, they
should be soldered on top of those
leads.
e. Tack solder 27 ohm resis-
tors R3 and R4 directly across C4
and C5. (They are not shown in the
drawings.)
e. Tack solder C13 and C14 as
shown, bending the leads at right an-
gles, and keeping them as short as
possible. Position the capacitors as
shown in figure 2.
f. Form the leads of C10 and C11
close to the body, as shown in figure
1, and tack solder on the board with
short leads. Be careful not to stress
the leads of the capacitors too much
to avoid breakage.
g. Tack solder electrolytic capaci-
tor C12. Bend the leads at right an-
gles, and observe polarity.
RF Choke and Ferrite Beads.
a. Ferrite choke Z2 is threaded
with 2½ turns of #22 bus wire, as
shown in the detail, by feeding the
wire through opposite holes and pull-
ing tight. One hole will not be used.
Be sure to wind the wire around and
around through the holes as shown.
HAMTRONICS® LPA 2-25R REPEATER POWER AMPLIFIER:
ASSEMBLY, INSTALLATION, & MAINTENANCE
