HAMTRONICS LPA 4-30 Construction, Alignment, & Operation Instructions

HAMTRONICS® LPA 4-35 UHF LINEAR POWER AMPLIFIER
CONSTRUCTION, ALIGNMENT, & OPERATION INSTRUCTIONS
GENERAL INFORMATION.
The LPA 4-35 is designed to be
used either as a linear amplifier with
a transmitting converter or as a class
C amplifier with an fm exciter in the
420-450 MHz band.
version of the popular LPA 4-30
Amplifier.
As a linear amplifier, it will take a
1-1/2 Watt p.e.p. output from a trans-
mitting converter, such as the XV4,
and provide 30-35W output. In class-
C operation, it will amplify the 2W
output from an exciter, such as the
TA451, to provide 35-40W output. It is
designed to match a 50Ω input and
output impedance, and it is rated for
continuous duty.
In linear mode, the LPA 4-35 may
be used for any mode of operation,
including ssb, am, cw, atv, and fm.
For exclusive use on modes such as
fm and cw, where linearity is not
required, the bias circuits can be
modified for class-C (actually between
class-B and class-C)
efficiency and it can be driven to
slightly higher output.
The unit operates on 13.6 Vdc at
8-10 Amp peak. It was designed for
the 420-450 MHz ham band, but it
may also be tuned to other nearby
frequencies with reduced efficiency.
It has a 10 MHz bandwidth without
retuning; so it is ideal
transmissions.
CIRCUIT DESCRIPTION.
Refer to the schematic diagram.
Amplifier transistors Q2 and Q3 are
the new generation,
emitter ballasted rf power transistors.
They are operated well below their full
capability to remain in the linear
range. Impedance matching is done
with high effi ciency
transmission lines (strip lines).
Note that what used to be a pre-
driver stage on the pc board is no
longer needed with the premium high
gain transistors available
©1992 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark.
Therefore, only two stages are used,
providing better stability and linearity
as well as re duced complexity and
cost.
The
biased
It is a new
linearity. However, when the unit is
to be used only for fm or cw, greater
efficiency
obtained by running the driver stage
just on the edge of conduction in
class-B and the output stage in class-
C (no bias).
Silicon diodes CR2 and CR3 are
thermally coupled to the transistor
cases to
according
temperature for a stable idle current
over a wide temperature range to
prevent thermal runaway.
transistors warm up, they tend to
conduct more; but the diodes reduce
the bias accordingly to counteract the
drift in idle current.
CONSTRUCTION.
for more Most of the pertinent construction
details are given on the component
location and schematic diagrams and
in the parts list. Following are details
of special procedures and techniques
and a general construction sequence
to be used.
tack soldered to the pc board; so it is
necessary to cut and form leads so
for TV that they seat properly on the board.
a. Lay the pc board on a box or
hold in a vise horizontally with the
etched side of the board up. Using a
sharp pick or any other convenient
tool, pick up eyelets and place in the
high gain,
small holes. The heads of the eyelets
must rest flat against the foil on the
etched side of the board. then, using
a small amount of solder, solder the
heads of the eyelets to the foil. The
printed
eyelets connect the pc board front and
rear foils
locations. Be sure to solder all around
the heads of the eyelets to provide low
impedance.
now.
b. Turn the pc board over, and sol-
transistors are normally
slightly above class-B
and power output
regulate the base bias
to the transistor
As the
Note that all parts are
together at critical
der the other side of the eyelets to the
ground plane.
soldered, and check to make sure
none have slipped back through the
other side. If so, heat eyelet flange on
for top side of board until solder on both
sides melts, and push eyelet gently
back in place.
is especially important
eyelets under the emitter leads of Q3
(see diagram) are properly seated and
that no solder blobs protrude above
eyelet heads. This is to prevent
stress on the transistor leads when it
is installed later.
c. Locate the thin brass strip
stock. Use scissors to cut two pieces
each 5/8 inch long. Form them with
pliers to fit pc board as shown in the
detail above the component location
diagram. The purpose of these straps
is to provide a
connection between the emitter leads
of Q2 and the bottom side ground
plane foil. The strap should be bent
so that it fits tightly, with about 1/4
inch on the top and bottom of the
board. Refer to component location
diagram to determine positions of the
emitter leads of Q2; these are the
leads under the shield shown on the
diagram. Tack solder the straps in
place on the bottom of the board only.
The straps will not be soldered to the
top of the board until after Q2 is
installed.
d. Position the heat sink on the
bench, oriented so holes align with
the pc board as shown in the diagram.
Note that the heatsink is used for
several products; so there will be extra
holes on the pc board which are not
used in this model.
1/16 inch thick flat washers over the
mounting holes. Carefully set the pc
board over the washers, positioning
as shown in the diagram. Secure the
board to the heatsink with five 6-32 x
3/8 inch thread
making sure that the washers stay in
place and that transistor holes are
aligned over heatsink holes.
Manual revised: 11/27/01 5:43 PM- Page
Be sure all are
Note that it is
that the 8
low inductance
Place five #6 x
cutting screws,