©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 5/3/04
- Page 3 -
car acts as a huge filter capacitor for
the electrical system, and it is the only
place good, clean power can be ob-
tained in a vehicle. If ignition switch
operation is desired, use a relay to
switch the power to the radio.
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 factory
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 transistor
must have been bad from the factory".
For your protection, please be sure to
observe the following precautions:
1. Transistors are made to operate
in specific circuits. Do not try to check
with ohmmeter, etc. Sometimes, you
can blow a transistor when you re-
verse polarity.
2. Observe power and duty cycle
ratings in the specifications published
in our catalog. some units are not de-
signed for continuous operation. Keep
heatsink fins in free air, not closed in,
and not upside down on solid surface
blocking air circulation. When tuning
on bench, allow for cooling periods to
avoid overheating.
3. 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, bias adjustments and final tun-
ing should be done at full 13.6V.
4. Never exceed 13.6Vdc, as even a
small over-voltage causes strain on
transistors.
5. Be sure you have a low imped-
ance connection to the power supply,
i.e., short, heavy cable.
ALIGNMENT.
Alignment is very simple. Connect
the input to a 220 MHz transmitting
converter or exciter which has already
been tuned into a 50-ohm dummy
load. Connect the output to a 50-ohm
load of sufficient power rating. Apply
moderate drive and B+. Alternately
tune the various pairs of mica trim-
mer capacitors for maximum output.
Continue increasing drive slightly and
repeaking capacitors until maximum
output is achieved.
At this point, the current drain
should not exceed about 6 Amp, and
the exciter or transmitting converter
should be within proper current limit
(i.e., no more than 450 mA). Of
course, during normal operation, you
would not drive the PA to its limit
such as this unless you were running
fm or cw; you would stay in the linear
region. However, for alignment, you
want to tune for absolute maximum
output to establish the proper load for
the pa transistors for best linearity on
ssb.
Note: Do not retune exciter or
transmitting converter with PA con-
nected. Once it 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 exciter or
transmitting converter.
OPERATION.
Operation is quite simple. B+ can
be applied either just during transmit
or all the time if desired. Merely apply
a signal to the PA when you want to
transmit.
It is necessary to avoid overdriving
the PA. Moderate overdrive will not
damage the unit, but it will cause ex-
cessive intermodulation distortion of
ssb signals. About 1 to 1-1/2 Watts of
drive should be sufficient to obtain
30W p.e.p. output. Do not drive the
PA to the saturation point on ssb. For
cw or fm operation, up to 2 Watts of
drive can be used for 40W output. Of
course, avoid drive levels in excess of
2 Watts entirely, as PA transistor
damage may occur on severe overdrive
(over 2-1/2 to 3 Watts).
TROUBLESHOOTING.
Since the unit has only two simple
amplifier stages, there isn't much
which can go wrong. It is helpful to
know that the Q1 and Q2 base volt-
ages should be about +0.5 to +0.7
Vdc. The rest of 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 or Q2, be sure to
use an exact replacement. There are
other transistors rated at similar out-
put levels, but they may have lower
gain or different impedance character-
istics.
To replace a transistor, carefully
peel each lead away from the pc board
while melting the solder. Then, remove
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 com-
pound, and install new transistor with
collector lead in correct location. Care-
fully tighten nut on Q1 transistor
without over-torquing or tighten the
screws on Q2. Then, flatten leads
against the board, and sweat solder
them to the board. Remember to re-
solder 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.
MOUNTING.
If desired, the PA can be mounted
to a panel with screws in the left and
right hand edges of the heatsink as
indicated in the component location
diagram. It can be mounted with
standoffs to clear the components, or
a cutout can be made in the rear
panel to clear the pc board and the
heatsink can then be mounted flush
to the panel. Standard 6-32 or #6 self-
threading screws can be threaded into
the aluminum heatsink if 1/8 inch pi-
lot holes are drilled, or screws can be
used with nuts if you are careful to
clear the fins.However the unit is
mounted, the fins should be in free air
to allow for good convection cooling
and keep the heat away from the os-
cillator on the Exciter or Transmitting
Converter board. Do not mount the PA
with the fins inside a cabinet.
