HAMTRONICS LPA 4-10R User manual

©1995 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark.
Revised:
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GENERAL INFORMATION.
The LPA 4-10R is designed to be
installed as an integral part of a
transmitter enclosure in a repeater
installation with a two Watt exciter
module. The transistor is biased for
class-C operation to give 8-10 Watts
output. The LPA 4-10R operates on
+13.6 Vdc at about 2 Amp. It has a
50-ohm input and output impedance
and is designed for continuous duty.
CONSTRUCTION.
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 pos-
sible.
Note: This series of power amplifiers
is designed to be mounted in an rf tight
enclosure with the exciter in such appli-
cations as our REP-200 Repeater or an
RF Tight box. 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 pull-
ing the leads off the ceramic case. The
LPA 4-10R is designed to have the
thickness of a standard #8 SAE flat
washer or two thin #4 flat washers
(about 0.050 inch total) as a spacer be-
tween the pc board and the chassis
which the transistor is mounted on.
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. Place flatwashers as spacers
over the screws, as previously de-
scribed.
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.
g. Cut tabs of variable mica ca-
pacitor C6 and piston variable capaci-
tors C2, C3, C7, and C8 as shown in
the detail on the component location
diagram, and solder them to the board
in the exact positions shown. Doing
so leaves adequate space for coil con-
nections. Mount the capacitors ori-
ented as shown so the rotor screw is
connected to the proper side of the
circuit.
Note: There are two sizes of bus
wire in the kit: #22 is the finest and
#14 is the heaviest.
h. Ferrite choke Z2 is threaded
with 2-1/2 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 as shown,
not in a zig-zag fashion. The choke is
mounted flat against the pc board, and
the leads are tack soldered to the
board.
i. Tack solder R2 across Z2 as
shown, being careful not to short to
turns on the choke.
j. Install chip capacitors as fol-
lows. Use small tweezers to handle
them. Be careful not to drop them; they
are difficult to find. Since they have no
markings, be sure to leave them in the
package until installed so you can tell
the values apart. Note where capaci-
tors are to be positioned. The chip
capacitors must straddle the area be-
tween the pad and the ground plane,
with one electrode soldered to each.
Apply a little solder to the pads ad-
jacent to the transistor leads where
one end of each capacitor will be posi-
tioned. Do not apply solder to the
ground plane yet.
Pick up one capacitor at a time.
Set the capacitor in place. Then,
heat the solder on the pc board pad,
and allow the solder to bond to the
electrode on the capacitor. When the
solder melts, the capacitor will seat
down on the board in the molten sol-
der. It is essential that this process
be done relatively quickly so the sol-
der doesn't oxidize and so there is still
a little flux left where the capacitor
electrode sits.
After one end of each of the ca-
pacitors is soldered and the positions
have been confirmed to be correct,
solder the ground plane end of each
capacitor.
k. Wind the coils exactly as speci-
fied in the component location dia-
gram, and tack solder them to the
board. Note that all pertinent details
of coil winding are given in the dia-
gram. Any rod of the proper diameter
(such as the shank of a drill bit) can
be used as a forming tool for coil wind-
ing.
HAMTRONICS® LPA 4-10R UHF REPEATER POWER AMPLIFIER

©1995 Hamtronics, Inc.; Hil
ton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised:
11
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l. Tack solder electrolytic capaci-
tor C12. Bend the leads at right an-
gles, and observe polarity.
m. Tack solder R1 and L2 as
shown. Ferrite bead Z1 is installed on
the ground lead of choke L2.
n. Check to be sure all parts are
installed according to parts list, and
check all solder connections.
RF INPUT/OUTPUT
CONNECTIONS.
The input and output connections
are made with RG-174/u 50-ohm coax
cable connected to the appropriate in-
put and output pads and ground plane
of the pc board. Lengths shown as-
sume that PA will be installed in REP-
200 Repeater or an A16 RF Tight Box.
Connect cables by stripping as il-
lustrated and tack-soldering to board.
Note that stripped length of coax is
inductive; so keep leads short and
neat.
Connect the shields by pretinning
all around the shield and then tack
soldering just the part of the shield
which contacts the board. Avoid melt-
ing polyethelene insulation on cable
by pretinning board and cable and
then tacking them together quickly.
POWER CONNECTIONS.
+13.6Vdc 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.
A ground return cable should be
connected from the power supply to
the ground plane of the pc board
through the mounting hardware. The
cable should be #18 or larger wire to
minimize voltage drop. A 3 or 4 Amp,
quick acting fuse should be connected
in the positive supply line for protect-
ion.
A well regulated power supply
should be used. Current drain of the
PA at full output is about 2 Amp.
Note that the output capability of
the PA drops rapidly as the voltage is
reduced below 13.6Vdc; therefore, you
should try to use a power source of
sufficient voltage and minimize cable
losses so that you have full B+ avail-
able 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
and they want to be sure you get good
parts with your kit. Therefore, they
do not honor claims that "the transis-
tor must have been bad from the fac-
tory". For your protection, please be
sure to observe the following precau-
tions:
1. Transistors are made to oper-
ate in specific circuits. Do not try to
check with ohmmeter, etc. Some-
times, you can blow a transistor when
you reverse polarity.
2. 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 9 or 10 Volts when you
first apply power until the unit is
tuned. Then, turn up to full 13.6Vdc.
Of course, final tuning should be done
at full 13.6V.
3. Never exceed 13.6Vdc, as even
a small over-voltage causes strain on
transistors.
4. Be sure you have a low imped-
ance connection to the power supply,
i.e., short, heavy cable.
5. 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. Con-
nect the input to an exciter which
has already 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 capacitor should
be screwed down tight and then
backed off about three turns. The pis-
ton trimmer capacitors in the output
circuit should be adjusted so that
5/16 inch of piston screw is exposed
at top. The piston trimmer capacitors
in the input circuit should be ad-
justed so that 1/2 inch of piston
screw is exposed at top.
Apply B+ and moderate rf drive.
First, adjust mica variable capacitor
C6 for maximum output. Then, al-
ternately tune the various mica and
piston trimmer capacitors for maxi-
mum output. Continue repeaking
capacitors until maximum output is
achieved and all interactions between
capacitors are worked out.
Note: If the output is less than 8-10
Watts, check to be sure that the input
tuned circuits are not tuned to a false
peak, which can happen if the piston
capacitors in the input circuit are ad-
justed with only about 1/4 inch of pis-
ton exposed. The true peak (assuming
operation in the 440-470 MHz range)
will occur with about 1/2 inch of piston
exposed.
If you happen to have a spectrum
analyzer (not required), you can fine
tune C8 for lowest harmonic level and
repeak C6 and C7 for maximum; oth-
erwise, just peak all the capacitors for
maximum output.
Note: Do not retune exciter with PA
connected. Once the exciter is tuned
into a 50-ohm load, it should never be
tuned again. Tuning the input of the PA
takes care of matching the PA to the ex-
citer.
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. Power
output may sag about 5% as the tran-
sistor heats up, but no more. If ex-

©1995 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark.
Revised:
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cessive power sag occurs after heat-
ing and retuning the capacitors does
not correct the problem, check to be
sure the transistor is mounted prop-
erly to the heat sink surface with
heat sink compound.
TROUBLESHOOTING.
Since the unit has only one sim-
ple amplifier stage, there isn't much
which can go wrong. The circuitry is
straightforward, with shorted coax ca-
bles or incorrect or shorted pc board
component connections being the
first things to suspect should there be
no output.
Should it be necessary to replace
rf power transistor Q1, 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, re-
move the mounting hardware and
gently push the old transistor out of
the heatsink. Clean all the old solder
off the pc board. Add new heatsink
compound, and install new transistor
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 components removed
for access to the transistor 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 similar inductors can
cause transients up to several hun-
dred volts. This is the most common
problem related to damaged semi-
conductors. 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 os-
cilloscope and watch the B+ line when
you turn the switch on and off.
PARTS LIST.
Ref Desig Description (marking)
C1 15 pf chip capacitor
C2-C3 10 pf piston trimmer cap.
C4-C5 15 pf chip capacitor
C6 mica variable #703
C7-C8 10 pf piston trimmer cap.
C9 15 pf chip capacitor
C10 .001 uf chip capacitor
C11 0.1 uf chip capacitor
C12 47 uf electrolytic cap
L1-L4 form per diagram
L2 0.33 uh rf choke
Q1 Philips BLW-81 or
Mot. MRF-653
R1 270 ohms, 1/4W
R2 10 ohms, 1/4W
Z1, Z3 Ferrite bead (remove lead
before using)
Z2 6-hole ferrite balun core
wound as per text

©1995 Hamtronics, Inc.; Hil
ton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised:
11
/
27
/
01
-
Page
4
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©1995 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark.
Revised:
11
/
27
/
0
1
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Page
5
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CHIP PARTS FOR CONSTRUCTION OF LPA 4-10R POWER AMPLIFIER:
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4 ea 15pf chip caps:
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1 ea .001 uf chip cap:
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1 ea 0.1 uf chip cap:
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