©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 5/3/04
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GENERAL DESCRIPTION.
The LPA 2-40 is designed to am-
plify the 2W pep output of the XV2
Transmitting Converter or the TA51
Exciter or any other 2W 220 MHz rf
source to 30W p.e.p. on ssb (linear
mode) or 40W on fm or cw (class-C
mode). It is a linear amplifier; so it
may be used on any mode of opera-
tion, including ssb, am, cw, and fm. It
has a 50 ohm input and output im-
pedance. The LPA 2-40 operates on
+13.6Vdc at 8-10 Amp. It may be
tuned to any frequency in the 220-230
MHz range, and has a passband of 5
MHz.
CIRCUIT DESCRIPTION.
Refer to the schematic diagram.
Amplifier transistors Q1 and Q2 are of
the new high gain, emitter ballasted
type. They are both normally operated
well below their full output capability
to remain in the linear range. Imped-
ance matching is done with high Q,
discrete coil-capacitor tuned circuits
to aid signal purity. A low-pass filter is
used in the output circuit for har-
monic suppression.
The transistors are biased slightly
above class B for linearity. However,
when the unit is to be used only for
fm or cw, slightly greater efficiency
and power output is obtained by dc
grounding the transistor bases for
class-C operation. Both bias circuits
(Q1 & Q2) use silicon diodes which
are thermally coupled to the heatsink
to regulate bias according to tempera-
ture for a stable idle current over a
wide range of operating temperatures.
As the transistors warm up, they tend
to conduct more, but CR1 and CR2
reduce the bias, as warm up occurs,
to counteract the drift in idle current.
Feedback networks C4/R4 and
C18/R8 are employed to spoil the ex-
cessive low frequency gain of the tran-
sistors to prevent oscillations at low
frequencies.
CONSTRUCTION.
Most of the pertinent construction
details are given on the component lo-
cation and schematic diagrams. Fol-
lowing are details of coil winding and
special parts mounting procedures.
Note that all parts are tack soldered to
the pc board; so it is necessary to cut
and form leads so that they seat prop-
erly on the board. Use the shortest
possible leads on all bypass capaci-
tors. Be sure to observe polarity on
diodes and electrolytic capacitors.
The markings on disc capacitors
may be confusing; so some explana-
tion may be helpful. Some larger val-
ues may be marked in pf with two
significant figures and a multiplier
much as resistors are marked. For ex-
ample, 101 = 100pf and 103 = .01uf
(10,000 pf).
a. Refer to component location dia-
gram. Set heatsink flat on bench, and
set 1/16 inch thick washers over the
six holes for screws used to mount
board.
b. Carefully set the pc board over
the washers, positioning as shown.
Secure board with six 6-32 thread
cutting screws, being careful not to
move the washers below the board.
Align pc board over transistor and di-
ode holes before tightening screws.
c. 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 stud mount tran-
sistor where it contacts the heatsink.
d. Set the stud mount transistor in
location Q1, and orient the notched
collector lead to the right as shown.
Secure transistor with #8 lockwasher,
solder lug, and 8-32 nut. Orient sol-
der lug next to diode hole. Do not
overtighten nut; tighten only to the
point of being snug. Hold transistor
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. Excessive torque can break
stud or leads.
e. Apply heatsink compound to
flange type transistor and set in posi-
tion Q2. Note that the collector lead is
narrower than the base lead. The col-
lector lead should be facing the right
hand side. Secure transistor with two
4-40 screws. On the lower screw, in-
stall #4 lockwasher and 4-40 nut. On
the upper screw, install solder lug and
4-40 nut. Orient solder lug next to di-
ode hole.
f. Form the transistor leads down
against the board. Then, tack solder
them to the foil, using sufficient solder
so that a bond is formed under the
full length of the leads.
g. Install the 40pf and 250pf metal
clad mica capacitors in the exact posi-
tions shown. The flat side with the
part number should face up. The tab
contact on each capacitor should be
bent down to contact the pc board foil
and the tab should be soldered while
maintaining correct positioning. With
the capacitor firmly held in place by
the tab, the case can then be soldered
along each side to the ground plane
foil with generous amounts of solder.
(Exception: C21 tab should be bent
upward to form a solder terminal for
the junction of Z5 and Z6.) Note that
some pc board pads are not used.
h. Solder the tabs of the variable
mica capacitors to the board in the ex-
act positions shown. Doing so leaves
adequate space for coil connections.
i. Form coils L1, L2, L3, and L4
from #14 (heavy) bus wire exactly as
specified in the component location
diagram, and tack solder them to the
board. Note that all pertinent details
of coil winding are given in the details
above the schematic diagram.
j. Install the two silicon diodes as
shown in the diagram, observing po-
larity. In each case, position the body
of the diode directly resting on the
case of the transistor. Place a small
amount of heatsink compound be-
tween the transistor case and the di-
ode body. Form the cathode lead down
to the ground plane foil next to the
transistor, and tack solder in place.
Reseat the diode in its proper position,
and tack solder the anode lead to the
insulated bus pad on the pc board.
k. Ferrite chokes Z2, Z4, Z5, and
Z6 are wound (threaded) with 2-1/2
turns of #22 bus wire (fine) as shown
in the detail diagram, by feeding the
wire through adjacent holes and pull-
ing tight. One hole will not be used.
Ferrite chokes are mounted flat
against the pc board, and their leads
are tack soldered to the board.
l. The network formed by Z3/R5 is
made by threading 2 turns of #22
(fine) wire through a ferrite choke (4
holes used). Then, one lead of the re-
sistor is passed through a remaining
hole and tack soldered to the bus wire
to form a junction as shown in the de-
tail. (3.3 ohm resistor is coded
org/org/gold.) Be careful that no part
of the winding of the choke touches
other metal parts.
m. Feedback networks C4/R4 and
C18/R8 are tack soldered between the
base and collector leads of the transis-
tors, using short leads. The free leads
are brought together over the transis-
tor and diode to form a junction and
tack soldered together in mid air.
n. Tack solder all other parts to the
HAMTRONICS® LPA 2-40 220 MHZ LINEAR POWER AMPLIFIER:
CONSTRUCTION, OPERATION, AND MAINTENANCE
