©1995 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 12/14/02
- Page 3 -
eral, the gate-2 voltage 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, assuming there
are no problems in the coax cabling, the
transistor may have been damaged by
transmitter rf or lightning discharge at
the antenna. Such damage often does
not cause a change in the dc characteris-
tics 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 re-
verse B+ voltage, 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 pre-
amps are effective in improving sensitiv-
ity of receiver systems in weak signal
areas. However, it is normally consid-
ered inadvisable to use a preamp, even
with a well designed receiver, in very
strong signal areas, such as the center of
a city or other locations with high pow-
ered transmitters in the area.
Adding gain ahead of a receiver de-
grades the selectivity of a receiver by an
equivalent amount by boosting undesir-
able signals as well as desirable 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 addi-
tional 20 dB.
If you use a preamp with a repeater
receiver, you will need to have addi-
tional rejection in your duplexer to at-
tenuate your transmit signal that much
more to prevent desense.
TRANSISTOR
REPLACEMENT.
Transistor replacement is complicated
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.
/
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 solder-
ing, 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 precautions above. The
transistors are all factory tested and
wrapped in foil to ensure that they arrive
in good condition. There is no warranty
coverage 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 before 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.
PARTS LIST.
Ref # Description
C1
10 pf disc
(LNW-28 & LNW-50)
6 pf disc
(LNW-75 & LNW-100)
C2
50 pf cer. variable
(28 & 50)
20 pf cer. variable
(LNW-75)
10 pf cer. variable
(LNW-100)
C3
.001 µf disc (1nK or 1nM)
C4 not
used
C5
.001µf feedthrough capacitor
C6 not
used
C7
0.15µf mylar capacitor
C8
.001µf feedthrough capacitor
L1-L2 3µH coil [exposed wires]
(LNW-28)
0.33µH coil [orn-orn-sil-red]
(LNW-50)
0.22µH coil [red-red-sil-red]
(LNW-75 & LNW-100)
Q1
N.E.C. 3SK122 mosfet
R1-R2 100K
U1
78L08 Voltage Regulator IC
Z1 Ferrite
Bead
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
