©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 5/3/04
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pc board in the positions shown, ex-
cept do not install bias resistors R3
and R7 until directed to do so later in
the text. Ground leads are tack sol-
dered to the
ground plane. Use short, direct lead
dressing, and install parts as close as
possible to positions shown so you
have enough room for all of them.
Observe polarity of components,
such as diodes and electrolytic ca-
pacitors. Note that leads may be tack
soldered to the top side of power tran-
sistor leads if necessary.
o. Check to make sure all parts are
installed except bias resistors R3 and
R7. Several 1/4W and 1/2W resistors
will be left for this purpose.
p. Check all connections against
diagrams, check all solder connec-
tions, and check polarity of diodes
and electrolytic capacitors.
CONNECTIONS FOR
CLASS-C OPERATION.
If the unit is to be used exclusively
on fm or cw, you may wish to change
the bias circuit for class-C operation
for slightly higher efficiency and out-
put power and for zero idling current.
To do that, the cold end of R1 and Z3
should be connected to ground in-
stead of the bias source, and resistors
R3 and R7 should not be installed. R5
is not used. If the unit has not been
built up, and if you never intend to
use linear operation, the other bias
circuit components need not even be
installed. If you intend to use ssb or
other modes requiring linear operation
at times, it is best to build the unit up
as a linear amplifier as designed and
not worry about changing back and
forth between linear and class-C.
There really isn't much advantage to
try to switch modes on the PA.
BIAS RESISTOR
CONNECTIONS.
For linear operation, values must
be chosen and installed for R3 and R7
to provide the proper bias for the two
stages. Each resistor actually is a
parallel combination of several resis-
tors to arrive at the correct amount of
bias to just begin to turn on the tran-
sistor (biased slightly beyond class-B).
To install the resistors, proceed as fol-
lows.
a. Temporarily unsolder Z2 from
the B+ bus on the pc board and re-
connect through milliameter (50-100
mA range). Apply 13.6 Vdc and
ground from power supply to points
on right side of pc board indicated in
component location diagram. (As a
check, there should be no current
drawn from the power supply yet.)
b. Tack solder a 330 ohm, 1/2W
resistor in one position indicated for
R3. (Temporarily turn off power sup-
ply each time a part is soldered in.)
Check Q1 collector current on meter.
The desired collector current is 20-40
mA. If you have obtained this level of
current with the first resistor, stop at
this point. If the current is too high, a
higher value resistor should be used
instead. If the current is too low,
which is likely to be the case, add an-
other resistor and check again. First
try a 330 ohm, 1/2W resistor. If still
not enough, add a 1.2K, 1/4W resis-
tor or a 680 ohm, 1/4W resistor or
both until the proper idling current is
obtained. Note that as base voltage in-
creases beyond the voltage required to
turn on the transistor, the collector
current will rise rapidly. At low volt-
ages, though, no collector current will
be drawn.
c. When the proper resistor values
have been determined, dress the leads
neatly, and solder the resistors in
permanently. Double check the idling
current, and then remove the meter
and resolder Z2 to the pc board B+
bus.
d. In similar fashion, lift the lead of
Z5 from the B+ bus and connect mil-
liameter in series. Try various combi-
nations of resistors for R7 as done
before, using one or two 330 ohm,
1/2W resistors first. Apply B+ and
look for 50-100 mA idling current for
the collector of Q2. As done before,
add or subtract resistors to arrive at
the desired idle current. Then, dis-
connect meter and solder everything
together neatly.
e. Connect milliameter in series
with B+ line to pc board bus to check
overall idling current of the two stages
and the bias circuits. This value
should be approximately 200-400 mA.
INPUT/OUTPUT
CONNECTIONS.
The input and output connections
are made with lengths of 50 ohm coax
cable connected to the appropriate in-
put and output pads and ground
(shield) of the pc board as shown on
the component location diagram.
Connect cables by stripping as il-
lustrated and tack-soldering to board.
Keep leads short and neat. Connect
the shield by pretinning all around the
shield and then tack soldering just the
part of the shield which contacts the
board. Avoid melting polyethelene in-
sulation on cable by pretinning board
and cable and then tacking them to-
gether quickly. If desired, light gauge
wire can be wrapped around the
shield before soldering to make a
neater shield termination.
Connectors were deliberately
avoided at the pa, since connectors
must be used at the other ends of the
cable at the Exciter and rear panel
and short connections to the pa board
are desired. RG-174/u cable is con-
venient to use for short connections to
the pa. (See catalog.)
The unit does not have provisions
for t/r switching of the antenna. If one
antenna is to be used with the pa and
a receiver, some form of coax relay
must be provided between the pa and
the antenna.
Note: The input circuit of the LPA
2-40 is such that bias voltage will be
present at the input coax cable. Check
to be sure the output circuit of the
Exciter is dc blocked by a capacitor so
the bias voltage is not shorted to
ground.
POWER CONNECTIONS.
+13.6Vdc should be connected to
the B+ pad at the top of the pc board.
A ground return cable should be con-
nected from the power supply to the
ground plane of the pc board as
shown in the component location dia-
gram. The cable should be #16 or lar-
ger 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 for any ssb equip-
ment, including the PA. Current drain
of the PA at full output is about 6
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 ca-
ble losses so that you have full B+
available at the PA.
If the unit is used in a mobile ap-
plication, or on anything other than a
well regulated and protected power
supply, a hash filter consisting of a
choke and large electrolytic capacitor
should be connected in series with the
B+ line and a rectifier diode, such as a
1N4001, should be connected across
the line with reverse polarity to shunt
any reverse voltage transients which
may occur on the B+ line. This is in
addition to the fuse mentioned above.
The rest of the transmitter may also
be operated on the same filtered line
output if desired.
In addition, both power lines
should be run directly to the battery
or power supply. Do not depend on
other conductors for a good, low-
impedance ground. the battery in a
