©1995 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 10/23/02
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though protection is provided in the
LNW in the form of a voltage regula-
tor ic, avoid such conditions as a
matter of principle. Care should be
taken especially to install a re-
verse diode across any inductive de-
vices, such as relays, on the same B+
line to absorb transients.
ALIGNMENT.
Factory assembled preamps are
tuned at the center of the band. The
bandwidth is wide enough so that re-
tuning normally is not necessary be-
cause of frequency. However, you
may want to retune the input cir-
cuitry to optimize noise figure with
the unit connected to your antenna.
Simply retune the input variable ca-
pacitor for best reception of weak
signals. No test equipment is neces-
sary. If you happen to have access to
a signal generator and sinadder, they
may be used; otherwise, just do it by
ear.
TROUBLESHOOTING.
Since the unit is fairly simple,
troubleshooting usually is limited to
checking the dc voltages on the tran-
sistor. These will vary somewhat;
however, in general, the gate-2 volt-
age should be about 4 Vdc, and the
drain should be at about 8 Vdc. The
source and gate 1 should be at
ground potential.
If the dc voltages are OK but the
unit is no longer amplifying, assum-
ing there are no problems in the coax
cabling, the transistor may have been
damaged by transmitter rf or light-
ning discharge at the antenna. Such
damage often does not cause a
change in the dc characteristics of
the transistor.
If the drain voltage is much lower
than 8 Vdc, first disconnect ferrite
bead Z1 to see if the transistor is
shorting the output of the voltage
regulator. In the case of a severe
voltage transient or reverse B+ volt-
age, it is possible to also damage the
voltage regulator.
If the unit is amplifying OK but
you are experiencing intermod, you
may be overloading your receiver by
adding gain ahead of the rf stage.
Low noise preamps are effective in
improving sensitivity of receiver sys-
tems in weak signal areas. However,
it is normally considered inadvisable
to use a preamp, even with a well de-
signed receiver, in very strong signal
areas, such as the center of a city or
other locations with high powered
transmitters in the area.
Adding gain ahead of a receiver
degrades the selectivity of a receiver
by an equivalent amount by boosting
undesirable signals as well as desir-
able ones. In severe cases, strong
signals which do not cause intermod
by themselves will create intermod in
the rf stage or mixer of your receiver
after being amplified an additional 20
dB.
If you use a preamp with a re-
peater receiver, you will need to have
additional rejection in your duplexer
to attenuate your transmit signal
that much more to pr event desense.
TRANSISTOR
REPLACEMENT.
Transistor replacement is compli-
cated a little by the fact that the pc
board has plated-through holes. You
must remove all the solder from the
drain and gate-1 leads before trying
to pull the transistor off the board.
This can be done with solder-wick or
a vacuum desoldering tool, as long as
you remove all the solder within the
holes. Then, melt the solder on the
top of ft cap C5, and lift the gate-2
lead of the transistor.
L
CAUTION: The small geometry
and high impedances make fet's heat
and static sensitive; so be careful. It is
good to discharge your hand to a
grounded metal object just before picking
up the transistor, and the use of a
grounded soldering iron is mandatory.
A heat sink is not necessary while sol-
dering, but be careful not to apply any
more heat than necessary.
You should
not be overly
anxious about
blowing out the
transistor if you
observe the pr e-
cautions above.
The transistors
are all factory
tested and
wrapped in foil to
ensure that they
arrive in good
condition. There
is no warranty cove rage for damage
which occurs in handling.
Refer to ASSEMBLY section of
manual for details of how to install a
transistor.
REMOVING OTHER PARTS.
Because the pc board uses plated
through holes, all of the solder
within the holes must be removed be-
fore a lead can be removed from the
board. This can be done with solder-
wick or a vacuum desoldering tool, as
long as you remove all the solder
within the holes.
Z1
L1
Figure 4. LNW Preamp, Schematic Diagram
8-VOLT REG.
U1
E5
B+
(10-15V)
C7
R1
R2
E3
E4
INPUT
RF
C5
C1
C2
Q1
L2
C8
C3
E2
E1
OUTPUT
RF