©1992 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Manual revised: 05/03/04 2:51:00 PM- Page 1 -
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. It is a new
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
transmitting 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 50
Ω
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) for more
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 for TV
transmissions.
CIRCUIT DESCRIPTION.
Refer to the schematic diagram.
Amplifier transistors Q2 and Q3 are
the new generation, high gain, 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 efficiency printed
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 now.
Therefore, only two stages are used,
providing better stability and linearity
as well as reduced complexity and
cost.
The transistors are normally
biased slightly above class-B for
linearity. However, when the unit is
to be used only for fm or cw, greater
efficiency and power output is
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 regulate the base bias
according to the transistor
temperature for a stable idle current
over a wide temperature range to
prevent thermal runaway. As the
transistors warm up, they tend to
conduct more; but the diodes reduce
the bias accordingly to counteract the
drift in idle current.
CONSTRUCTION.
Most of the pertinent construction
details are given on the component lo-
cation and schematic diagrams and in
the parts list. Following are details of
special procedures and techniques
and a general construction sequence
to be used. Note that 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.
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
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
eyelets connect the pc board front and
rear foils together at critical locations.
Be sure to solder all around the heads
of the eyelets to provide low
impedance.
b. Turn the pc board over, and
solder the other side of the eyelets to
the ground plane. Be sure all are
soldered, and check to make sure
none have slipped back through the
other side. If so, heat eyelet flange on
top side of board until solder on both
sides melts, and push eyelet gently
back in place. Note that it is
especially important that the 8 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 in-
stalled 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 low inductance
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.
Place five #6 x
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 cutting screws,
making sure that the washers stay in
place and that transistor holes are
aligned over heatsink holes.
Note: If you are building the pa for
class-C operation (fm or cw) instead of
linear mode, refer to separate
paragraph on differences later in this
text before beginning construction.
e. Next, mount the transistors as
follows. Make sure you know which
lead is the collector. Carefully open
the package of heatsink compound
with scissors. Apply it
sparingly
with
a small piece of wire or a toothpick,
etc. It is necessary only to use
enough compound to make a smooth
thermal surface between the heatsink
and transistor. Using too much only
makes a mess. Be sure not to over
tighten the nut on the transistor stud;
the gold-plated brass stud could
break.
f. Unpack the stud-mount
transistor used for Q2. Use scissors
to trim each lead to about 1/4 inch.
Be careful to note which lead is the
collector; it has a notch at the end.
Cut a similar notch in the corner of
the lead after trimming. Apply a light
HAMTRONICS
LPA 4-35 UHF LINEAR POWER AMPLIFIER
CONSTRUCTION, ALIGNMENT, & OPERATION INSTRUCTIONS
