HAMTRONICS LPA 2-25R Installation instructions

©2000 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised
10/2/07
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Page
1
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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 40- 75
MHz and put out 25-30W.
The Power Amplifier operates on
+ 3.6 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 harmonic
output to very low levels.
CONSTRUCTION.
General.
Most of the pertinent construction
details are given in the component lo-
cation 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 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 pro-
vided 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 thick-
ness of a standard 4-40 nut (about
0. 00 inch) as a spacer between 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 transistor
with #8 lockwasher and 8-32 nut. Do
not overtighten nut; tighten only to
the point of being snug. Hold transis-
tor leads with fingers to prevent rota-
tion. 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. Solder variable mica capacitors
C8 and to the board in the exact posi-
tions shown in figure 2. Mount the
capacitors 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 C close to
the body, as shown in figure , 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 . 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 col-
lector terminals. In fact, they should
be soldered on top of those leads.
e.
e.e.
e. Tack solder 10 ohm resi
Tack solder 10 ohm resiTack solder 10 ohm resi
Tack solder 10 ohm resis
ss
stor
tor tor
tor
R directly across C5 with shor
R directly across C5 with shorR directly across C5 with shor
R directly across C5 with short
t t
t
leads. (It is not shown in the dra
leads. (It is not shown in the draleads. (It is not shown in the dra
leads. (It is not shown in the draw-
w-w-
w-
ings.)
ings.)ings.)
ings.)
e. Tack solder C 3 and C 4 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 C 0 and C
close to the body, as shown in figure
, 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 C 2. 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.
Do not thread the wire through in a
zig-zag fashion. The choke is
mounted flat against the pc board,
and the leads are tack soldered to the
board.
b. Install resistor R2 across Z2 as
shown.
c. Cut the bus wire off the ferrite
bead close to one end of the bead and
discard the leads.
d. The following procedure in-
stalls a power lead of the proper
length for our repeater. If you have a
different installation, you can do
something similar. Cut a 3 inch
length of red hookup wire. Strip one
end ¼ inch and the other end 5/8
inch. Tack solder the ¼inch end to
the pc board power trace as shown.
Better yet, wrap it around the lead of
C 2 and solder so that it is mechani-
cally secure. Slide the ferrite bead
over the long end, and bend the wire
at a slight angle to hold the bead in
place until you solder the lead to the
feedthrough capacitor in the repeater.
e. Twist together and tack solder
one lead each of rf choke L2 and re-
sistor R . Trim the other lead of each
part to about ¼ inch and form down
to reach the board. Then, tack solder
these leads to the board as shown,
with the lead of L2 to the pad area for
HAMTRONICS
®
LPA 2-25R REPEATER POWER AMPLIFIER:
ASSEMBLY, INSTALLATION, & MAINTENANCE

©2000 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised
10/2/07
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Page
2
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the base of the transistor and the
lead of R to the ground plane.
Coil Forming and Placement.
Figures 2 & 3 and 5 & 6 show ex-
actly how coils are formed.
You need to form the coils exactly
as specified, using # 8 bus wire sup-
plied. Tack solder them to the board
in the positions shown.
Any rod of the proper diameter
(such as the shank of a drill bit) can
be used as a forming tool for coil
winding. It is important that they not
only be wound the proper inside di-
ameter but that the leads be the
proper length. Any extra lead length
adds to the inductance and will affect
performance. You don't need to be
super precise, but do form them to
resemble the detailed drawings as
closely as you can.
Remember that the finished coils
should fit on the pc traces as shown;
so that will help you check that you
formed the coils properly.
The coils are all either 9/64 or
/4 inch inside diameter. Spacing
between turns of the coils should be
minimal, with turns separated just
enough to prevent shorting together.
L is /8 inch i.d. and ¼ turns.
The feet are formed just so the bottom
wire of the coil doesn't short to the
ground plane. They should be only
about / 6 inch high. L4 is like L
except 2¼ turns. The other two coils
are ¼ inch i.d. L3 is ¼ turns. L5 is
¾ turns.
Inspection.
This completes assembly. Check
to be sure all parts are installed ac-
cording to parts list. Look for any
short circuits or bad solder joints. RF
power transistors are expensive to re-
place; so now is the time to find prob-
lems, before power is applied.
RF INPUT/OUTPUT
CONNECTIONS.
The input and output connections
are made with RG- 74/u 50-ohm
coax cable connected to the appro-
priate input and output pads and
ground plane of the pc board.
The following lengths assume that
PA will be installed in REP-200 Re-
peater. The input cable should be
5½ inches long and the output cable
should be 3 inches long, measured
before stripping the ends.
Connect cables by stripping and
tack-soldering to board as illustrated.
Note that stripped length of coax is
inductive; so keep pigtail leads as
short as possible.
Connect the shields by pretinning
the shield and then tack soldering
the end which contacts the board.
Avoid melting polyethylene insulation
on cable by pretinning board and ca-
ble and then tacking them together
quickly.
POWER CONNECTIONS.
+ 3.6Vdc should be connected to
the B+ pad at the top of the pc board.
When installed in an REP-200 Re-
peater, a hookup wire should be at-
tached to the B+ pad as shown, using
a ferrite bead on the far end, which
attaches to the feedthrough capacitor
in the PA compartment. The ground
return normally is connected to the
pc board through the mounting
hardware.
The cable should be # 8 or larger
wire to minimize voltage drop. A 6
Amp, quick acting fuse should be
connected in the positive supply line
for protection.
A well regulated power supply
should be used. Current drain of the
PA at full output is about 5 Amp,
sometimes slightly higher (but no
more than 6 Amp), depending on
power level.
Note that the output capability of
the PA drops rapidly as the voltage is
reduced below 3.6Vdc; therefore,
you should try to use a power source
of sufficient voltage and minimize ca-
ble losses so that you have full B+
available at the PA.
CAUTIONS TO PROTECT
TRANSISTORS.
Because it is so easy to damage rf
power transistors in the field due to
accidents and abuse, transistor
manufacturers do not provide any
warranty to cover replacements once
a transistor is installed in the unit.
They test them thoroughly at the fac-
tory because they are expensive
parts. Therefore, they do not honor
claims that "the transistor must have
been bad from the factory".  For
your protection, please be sure to ob-
serve the following precautions:
. Sometimes, transistors may be
destroyed by parasitic oscillations oc-
curring during tuning because of the
extremes of capacitor settings, or due
to accidental shorting of components.
To protect against such damage as
much as possible, turn power supply
voltage down to about 0 Volts when
you first apply power until the unit is
tuned. Then, turn up to full 3.6Vdc.
Of course, final tuning should be
done at full 3.6V.
2. Never exceed 3.6Vdc, as even
a small over-voltage causes strain on
transistors because of additional
heat.
3. Be sure you have a low imped-
ance connection to the power supply,
i.e., short, heavy cable.
4. Do not attempt to operate PA
until exciter has been properly
aligned by itself, operating into a
50-ohm load.
ALIGNMENT.
Alignment is very simple. Connect
the input to an exciter which has al-
ready been tuned into a 50-ohm
dummy load. Connect the output to
a 50-ohm load of sufficient power rat-
ing. Use an in-line power meter, or
monitor output with a dc voltmeter
connected to rf detector test point pad
on pc board.
Preset variable capacitors as fol-
lows if this is the first time tuning
from a kit; otherwise, they should be
left where previously tuned. The
large mica variable capacitors should
be screwed down tight and then
backed off about three turns. The
small ceramic variable capacitors
should be rotated 90° from where
they are set from the factory.
Apply B+ and moderate rf drive.
Alternately tune the four variable ca-
pacitors for maximum output. Con-
tinue increasing drive slightly and
repeaking capacitors until maximum
output is achieved and all interac-
tions between capacitors are worked
out.
Note: Do not retune exciter with PA
connected. Once the exciter is tuned
into a 50 Ω load, it should never be
tuned again. Tuning the input of the
PA takes care of matching the PA to
the exciter.
With 3.6Vdc power applied and
2W drive, the pa should put out
about 25-30W. Current drain
should be about 5 Amp.
To minimize stress on the transis-
tor, avoid running the pa over these
maximum levels. A good way to re-
duce the output power and the cur-
rent drain is to tighten loading
capacitor C9 slightly and repeaking
tuning capacitor C8. It is usually
possible to reduce both the power
level and the current drain that way.
Watch both meters while tuning to be
sure that is what is happening.
You can tell if the transistor is
overheating by watching the output
power and current drain as the unit
heats up. Neither should change
much. If the output power sags by

©2000 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised
10/2/07
-
Page
3
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more than a few watts as the transis-
tor heats up, there is insufficient
heatsinking. Either the heatsink is
too small or the thermal interface be-
tween the transistor and heatsink is
deficient. There should be heatsink
compound between the two surfaces
and the nut on the transistor stud
should be tight (but not strained to
the breaking point).
OPERATION.
Operation is quite simple. B+ can
be applied all the time if desired.
Merely apply an rf signal to the PA
when you want to transmit.
TROUBLESHOOTING.
Since the unit has only one simple
amplifier stage, there isn't much
which can go wrong. The circuitry is
straightforward. The first things to
suspect should there be no output
are shorted coax cables or incorrect
or shorted pc board component con-
nections.
Should it be necessary to replace
rf power transistor Q , be sure to use
an exact replacement. There are
other transistors rated at similar out-
put level, but they may have lower
gain or different impedance charac-
teristics.
To replace the transistor, carefully
peel each lead away from the pc
board while melting the solder. Then,
remove the mounting hardware and
gently push the old transistor out of
the heatsink. Clean all the old solder
off the pc board and remove the old
heatsink compound. Add new heat-
sink compound, and install new tran-
sistor with collector lead in correct
location. Carefully tighten nut on
transistor without over-torquing.
Then, flatten leads against the board,
and sweat solder them to the board.
Remember to resolder any compo-
nents removed for access to the tran-
sistor leads.
 A word about relay coils. Any
relay coil connected to the same B+
line as solid state equipment should
have a reverse diode connected
across it to absorb the inductive
kickback which occurs when the coil
is de-energized. Relay coils and simi-
lar inductors can cause transients up
to several hundred volts. This is the
most common problem related to
damaged semiconductors. You
should also be sure that your power
supply does not have an inductive
surge when you turn it on or off. If in
doubt, borrow an oscilloscope and
watch the B+ line when you turn the
switch on and off.
PARTS LIST, LPA2-25R.
Ref Desig Description (marking)
C1 39 pf disc capacitor
C2 20 pf ceramic var. (pink)
C3 20 pf ceramic var. (pink)
C4-C5 82 pf disc capacitor
C6-C7 not used
C8-C9 mica variable (703)
C10 220 pf disc cap (221)
C11 0.1µf monolithic cap (104)
C12 47µf electrolytic cap
C13-C14 56 pf disc capacitor
C15 not used
L1, L3-L5 wind per text
L2 0.22 uh rf choke marked
red-red-silver-red
Q1 MRF-1946A
R1 3.3Ω, ¼W resistor
R2 10Ω, ¼W resistor
R3 10Ω, ¼W resistor
Z1 Ferrite bead over B+ lead
Z2 2½ turn ferrite choke
Figure 1. Le d Trimming nd Forming Det ils
VERY SHORT LEADS
ON DISC CAPACITORS
Figure 3. Coil Winding Det ils
1/4 IN. I.D.
9/64 IN. I.D.
5/8 IN. END-END 9/64
IN. I.D.
1/4 IN. I.D.
L1 L5
L3
L4 RIGHT SIDE
VIEW
FRONT VIEWS
L4
7/8 IN. END-
END
1-3/8 IN. END-
END

©1999 Hamtronics, Inc.; Hilton NY; USA. All rights
reserved. Hamtronics is a registered trademark. Revised
10/2/07
-
Page
4
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Figure 4. Schem tic Di gr m
_
+
Z1
C12 C11
Z2
C3 C1 C2
L1
L2
C4
C5
C6
C7
L4
C10
C8
C9
C13 C14
Q1
L3 L5
RF OUTPUT
RF INPUT
C15
R1
R2
+
_
+13.6VDC IN
RF INPUT
RF OUTPUT
C3
C1 C2 C8
C9
C13 C14
C10
C11
C12
L1
L2
L4
L3
L5
Q1
3" RED WIRE
STRIP WIRE 5/8" &
INSTALL BEAD Z1
C4
C5
Figure 2. LPA 2-25R, Component Loc tion Di gr m
C15
R1
Z2
R2
FERRITE CHOKE Z2, SEE TEXT
R3