©1995 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 12/14/02
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FUNCTIONAL DESCRIPTION.
The LNW Preamp was designed as a
miniature low noise receiver preamp.
Being only 5/8" x 1-5/8", the LNW con-
veniently mounts inside many radios.
Models are offered for various vhf
& uhf bands. This manual covers mod-
els shown in table 1.
The LNW uses a dual-gate mosfet for
low noise figure and good stability under
a wide range of load conditions. Typical
noise figure is 1.2 dB, and output 1 dB
compression point typically is +5dBm.
Table 1. Preamps Covered By This Manual
Model
Tuning Range Gain 3dB BW
LNW-28 25-35
MHz 28dB
±2
MHz
LNW-50 35-55
MHz 24dB
±3
MHz
LNW-75 55-90
MHz 22dB
±3
MHz
LNW-100 90-120
MHz 22dB ±5
MHz
ASSEMBLY.
The following instructions will help
you build your preamp with a minimum
of effort. It is a miniature unit, though,
so precise construction is necessary to
get all the parts in the proper positions.
The steps which follow are meant to
serve as a general guide as to the con-
struction sequence and critical opera-
tions. It is assumed that you are an ex-
perienced vhf/uhf kit builder and do not
need basic kit building instructions. If
this is not the case, consider having
someone more experienced help you.
During construction, refer to the dia-
grams and parts list for details.
Note that not all the holes in the
board are used for this model. Other
frequency ranges use slightly different
circuitry. The illustrations clearly show
the locations of the parts to be installed
and the empty holes.
a. The side of the pc board which has
mostly ground plane is the top of the
board. The bottom of the board has sev-
eral conductor patterns for circuit con-
nections.
b. Install feedthrough capacitors C5
and C8 as shown in figures 1 & 3. In-
sert them from the BOTTOM of the
board with the tapered part toward the
board. Hold the soldering iron at the
base of the capacitor until the solder on
the capacitor melts. Once the solder
melts at one point, it is easy to get all the
solder to be molten at one time by run-
ning the iron around the edge of the ca-
pacitor. Then, the ft capacitor will settle
onto the board. If necessary to seat it,
you can press
lightly
on the metal base of
the capacitor with the soldering iron
once the solder is melted; but do not
press on the ceramic center of the
capacitor.
Note that there may be a third 1/8
inch hole on the board, which is not used
in this model. The source of the fet is
connected directly to ground; so the hole
next to the source is not used.
An extra
feedthrough capacitor is supplied in case
you damage one.
CAUTION: The small geometry and
high impedances make FET's heat and
static sensitive; so be careful. It is good
to wear a grounded wrist strap, or at
least discharge your hand to a grounded
metal object just before picking up the
transistor, and the use of a grounded sol-
dering iron is mandatory. A heat sink is
not necessary while soldering, but be
careful not to apply any more heat than
necessary. You should not be overly
anxious about blowing out the fet if you
observe the precautions above. The
transistors are all factory tested and
wrapped in anti-static bags to ensure that
they arrive in good condition. There is
no warranty coverage for damage which
occurs in construction or handling; but
replacement transistors are moderately
priced.
c. Figure 2 shows the placement of
Q1 on the board. The drain is the long
lead. Figure 1 shows how to form the
leads.
Pick up the transistor and orient it as
shown in figure 2
with lettering up
.
Make sure you know which lead is
which. Gently bend the source, gate 1,
and drain leads down at a 90° angle as
shown in figure 1. Then, bend the bot-
tom of the source lead out away from the
transistor to form an "L" shaped foot.
When formed properly, the transistor
will sit flush with the board as shown.
(The gate 2 lead is not bent.)
Insert the drain and gate 1 leads
through the holes in the board. The foot
formed in the source lead should rest on
the top of the board as shown in figures
1 and 2. Tack solder the foot of the
source lead to the board. Check that the
transistor is down against the board as
shown in figure 1. Check again that the
transistor is oriented properly with the
lettering up and the long lead in the hole
for the drain. Then, solder the gate 1
and drain leads to their pads under the
board. Trim off end of drain lead below
board.
Note: The feedthrough capacitors
have a hollow center which is metalized
through from end to end. Therefore, in
order to make a connection to the feed-
thru capacitor, simply insert a short
length of the lead into the hollow center
of the feedthru and solder to the metali-
zation. It is not necessary for the lead to
go all the way to the other end because
there is metalization to conduct from one
end to the other.
Also note that the feedthru capacitor
by the transistor is not used as a feedthru
cap in the normal sense; it is merely used
as a very good vhf/uhf bypass capacitor
and tie point.
d. Install vertical resistors R1 and
R2 in the holes shown in figure 2. Sol-
der their bottom leads. Then, refer to
figure 1 and bend the top leads over, in-
serting them into the center of ft capaci-
tor C5. Trim these leads off at the
bottom. Bend the fet gate 2 lead over to
touch the top of the ft capacitor, and then
solder all three leads to the metalization
in the center of the capacitor. It is not
necessary to completely fill the capacitor
with solder as long as the three leads are
all soldered at the top.
e. Solder metal shield to ground
plane on top of pc board in position
shown in figure 2. Keep the shield as
close as possible to the transistor to leave
room for output coil L2. First, tack sol-
der the shield to the board at each end.
HAMTRONICS® LNW-28, LNW-50, LNW-75, LNW-100 RECEIVER PREAMP:
ASSEMBLY, INSTALLATION, & MAINTENANCE
GATE 1
SOURCE
GATE 2
Figure 1. Transistor Lead Formation
LETTERING
MUST BE UP
GATE 2
SOURCE
GATE 1
DRAIN
IS LONG
LEAD
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