©1999 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 12/11/02
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much as possible, turn power supply
voltage down to about 10 Volts when
you first apply power until the unit is
tuned. Then, turn up to full 13.6Vdc.
Of course, final tuning should be
done at full 13.6V.
2. Never exceed 13.6Vdc, as even
a small over-voltage causes strain on
transistors because of additional
heat.
3. Be sure you have a low imped-
ance connection to the power supply,
i.e., short, heavy cable.
4. Do not attempt to operate PA
until exciter has been properly
aligned by itself, operating into a
50-ohm load.
ALIGNMENT.
Alignment is very simple. Connect
the input to an exciter which has al-
ready been tuned into a 50-ohm
dummy load. Connect the output to
a 50-ohm load of sufficient power rat-
ing. Use an in-line power meter, or
monitor output with a dc voltmeter
connected to rf detector test point pad
on pc board.
Preset variable capacitors as fol-
lows if this is the first time tuning
from a kit; otherwise, they should be
left where previously tuned. The
large mica variable capacitors should
be screwed down tight and then
backed off about three turns. The
small ceramic variable capacitors
should be rotated 90° from where
they are set from the factory.
Apply B+ and moderate rf drive.
Alternately tune the four variable ca-
pacitors for maximum output. Con-
tinue increasing drive slightly and
repeaking capacitors until maximum
output is achieved and all interac-
tions between capacitors are worked
out.
Note: Do not retune exciter with PA
connected. Once the exciter is tuned
into a 50
Ω
load, it should never be
tuned again. Tuning the input of the
PA takes care of matching the PA to
the exciter.
With 13.6Vdc power applied and
2W drive, the 144 MHz unit should
put out about 17-20W or 25-30W,
depending on model, the 220 MHz
unit should put out about 17-23W,
and the 50 MHz unit should put out
about 20-25W. Current drain should
be about 4-5 Amp.
To minimize stress on the transis-
tor, avoid running the pa over these
maximum levels. A good way to re-
duce the output power and the cur-
rent drain is to tighten loading
capacitor C9 slightly and repeaking
tuning capacitor C8. It is usually
possible to reduce both the power
level and the current drain that way.
Watch both meters while tuning to be
sure that is what is happening.
You can tell if the transistor is
overheating by watching the output
power and current drain as the unit
heats up. Neither should change
much. If the output power sags by
more than a few watts as the transis-
tor heats up, there is insufficient
heatsinking. Either the heatsink is
too small or the thermal interface be-
tween the transistor and heatsink is
deficient. There should be heatsink
compound between the two surfaces
and the nut on the transistor stud
should be tight (but not strained to
the breaking point).
OPERATION.
Operation is quite simple. B+ can
be applied all the time if desired.
Merely apply an rf signal to the PA
when you want to transmit.
TROUBLESHOOTING.
Since the unit has only one simple
amplifier stage, there isn't much
which can go wrong. The circuitry is
straightforward. The first things to
suspect should there be no output
are shorted coax cables or incorrect
or shorted pc board component con-
nections.
Should it be necessary to replace
rf power transistor Q1, be sure to use
an exact replacement. There are
other transistors rated at similar out-
put level, but they may have lower
gain or different impedance charac-
teristics.
To replace the 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 and remove the old
heatsink compound. Add new heat-
sink compound, and install new tran-
sistor with collector lead in correct
location. Carefully tighten nut on
transistor without over-torquing.
Then, flatten leads against the board,
and sweat solder them to the board.
Remember to resolder any compo-
nents removed for access to the tran-
sistor 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 simi-
lar inductors can cause transients up
to several hundred volts. This is the
most common problem related to
damaged semiconductors. 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 oscilloscope and
watch the B+ line when you turn the
switch on and off.
PARTS LIST.
(See separate sheet for 6M version.)
Ref Desig Description
C1
39 pf disc capacitor
[22 pf for 220 MHz band]
C2
20 pf (pink) ceramic var.
C3
20 pf (pink) ceramic var.
[.001 disc cap for 220 MHz
band, marked 102 or 1n]
C4-C5
For LPA 2-15R:
47pf for C4, none for C5
[For 220 MHz: 110 pf disc
marked "111"]
C6-C7 not
used
C8-C9
mica variable marked
"703"
C10
.001 uf chip capacitor
C11
.047 uf chip capacitor
C12
47 uf electrolytic cap
C13-C14 56 pf disc capacitor
[33 pf for 220 MHz band]
C15
5pf disc capacitor
[not used on 220 MHz]
L1, L3-L5 wind per text
L2
0.22 uh rf choke marked
red-red-silver-red
Q1
2N6081 (Motorola) or
BLY-88C (Philips)
[BLW-40 for 220 band]
R1 3.3
Ω
, ¼W resistor
R2 10
Ω
, ¼W resistor
R3, R4
27
Ω
, ¼W resistor
Z1
Ferrite bead over B+ lead
Z2
2½ turn ferrite choke
Chip Capacitors taped here for kit:
1 ea .001 uF ------
1 ea .047 uF ------
