©2005 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 11/11/05
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8Vdc power is applied to the ic at pin
4. The 455 kHz i-f signal is con-
verted to audio by a quadrature de-
tector at pins 6-8. Quadrature coil
L8 sets the center frequency of the
detector, and resistor R4 sets the
modulation acceptance bandwidth.
It is important to note that all the
circuits along the bottom of the ic on
the schematic are referenced to B+
bus and not to ground. C6 is a mas-
ter bypass capacitor which ties this
B+ bus to ground all at one point.
C7 and C8 bypass parts of the inter-
nal circuitry in the detector.
The signal path continues right to
left across the top of U1 on the sche-
matic. Detected audio or white noise
at pin 9 is applied to the VOLUME
control through blocking capacitor
C12 and partial de-emphasis net-
work R5/C11.
Pin 14 is the collector of a squelch
muting transistor switch inside the
ic. This transistor is controlled by
the dc signal at pin 12 and effectively
grounds the audio line to squelch it
when only noise is present in the de-
tected signal. The signal at pin 12 is
derived from two sources, bias from
the SQUELCH control and detected
noise from pin 11 through voltage
doubler rectifier C15, CR1, CR2.
C16 averages signal peaks. Com-
bined with hysteresis action in the ic
at pin 12, this prevents the squelch
from bouncing on and off with fading
signals and makes the squelch ac-
tion positive acting.
Inside the ic, there is an op amp
between pins 10 and 11 which acts
as an active bandpass filter peaked
at about 10 kHz, looking for white
noise above the normal audio fre-
quency range. The active filter is
formed by R6, R7, R8, C13, and C14.
Audio from the VOLUME control
is applied to the speaker and com-
puter’s sound card through power
amplifier U2. C5/R3 is a parasitic
suppression network, which nor-
mally is not used in this application,
but we left space for it just in case.
Channel selection is done by the
scan circuit, consisting of U4 and
U5. Ring counter U5 is clocked at a
4 pps scanning rate by U4B-U4C.
This turns on each channel for a ¼
Sec period, which is long enough for
the receiver squelch circuit to re-
spond if it hears a signal for that sat-
ellite frequency, even if there is some
fading. When pin U5-13 is enabled
(low), U5 continually sequences from
one output to another. The counter
stops when the enable signal at pin
13 is removed.
The selected output from the
counter is a positive current to oper-
ate the appropriate LED and channel
oscillator. The output voltage is lim-
ited by the LED to about 2Vdc. This
voltage acts as bias for the base of
the selected oscillator transistor
shown on sheet 1.
Scanning control is done by OR-
gate U4-D. When the SCAN switch is
in the AUTO position, pin 12 is held
low, and the output of U4-D is con-
trolled by the COS (carrier operated
switch) signal from the squelch cir-
cuit. When the squelch is closed, the
ring counter is enabled and it
changes channels. When the
squelch circuit opens due to a re-
ceived signal, the ring counter stops
on that channel.
With the SCAN switch set to MAN,
a positive voltage from R12 is fed to
U4 pin 12 constantly to lock on the
enable signal for the ring counter.
However, the positive voltage through
R13 disables the clock oscillator; so
the ring counter stays on one chan-
nel. When you want to change
channels, operating the SCAN switch
to the STEP position grounds U4-B
and allows the clock to run. If you
flick the switch momentarily to the
STEP position, the clock only cycles
once. If you hold the switch in the
STEP position, the clock continues to
run until the switch is released.
PARTS LIST.
Ref Desig Description
(marking)
C1-C3
4.7 uF electrolytic
➊
C4
47 µF electrolytic
➊
C5 not
used
C6-C8
➐
0.1
µF
C9
➐
220
pF
C10 68
pF
C11
➐
.0047
µF
C12
0.15 µF mylar
(red)
C13-C14
➐
.001 µF
C15
0.15 µF mylar
(red)
C16
2.2 µF electrolytic
➊
C17
➐
.0047
µF
C18
0.47 µF electrolytic
➊
C19
➐
.0047
µF
C20
➐
27
pF
C21 68
pF
C22-C23
➐
.0047 µF
C24
➐
18
pF
C25
➐
0.5
pF
C26
➐
15
pF
C27
➐
4
pF
C28-C30
➐
.0047 µF
C31-C35
2-30 pF variable
➋
(green)
C36-C40
➐
22 pF
C41-C50
➐
150 pF
C51-C55
➐
.0047 µF
C56 82
pF
C57
➐
1
pF
C58 82
pF
C59
➐
.0047
µF
C60
➐
27
pF
C61
➐
0.5
pF
C62
➐
22
pF
CR1-CR2 1N4148 diode
➊
DS1-DS5 T-1 red L.E.D.
➊
FL1
10.7 MHz cer filter (10.7MA)
FL2
455 kHz ceramic filter (455B)
J1
DB-9 male jack
J2
BNC right-angle jack
L1-L3
2½ turn slug-tuned
(red)
L4-L5
6½ turn slug-tuned
(blue)
L6-L7
2½ turn slug-tuned
(red)
L8
455 kHz i-f xfmr
(T1003 or RLC-352)
Q1 2N3904
Q2-Q3
BF998 mosfet
➊
➎
Q4-Q9 2N5770
➊
R1-R2
100K panel mount pot
R3 not
used
R4
➐
15K
R5
➐
510K
R6 4.7K
➌
R7
➐
680
Ω
R8
➐
510K
R9
➐
150K
R10 150K
R11
➐
6.8K
R12
➐
27K
R13
➐
150K
R14
➐
330K
R15-R16
➐
100K
R17 100K
➌
R18 270
Ω
R19-R23
➐
6.8K
R24-R28
➐
270
Ω
R29 1K
➌
R30
➐
3.9
meg
S1
mom-off-on toggle sw
➍
S2
on-on toggle switch
➍
U1
MC3361BP IF ampl
➊
U2
LM-380 audio out
➊
U3 78L08
voltage regulator
➊
U4 4001B
quad nor gate
➊
➎
U5
4017B ring counter
➊
➎
Y1 137.100
➏
(14.044,444)
Y2 137.400
➏
(14.077,778)
Y3 137.500
➏
(14.088,889)
Y4 137.620
➏
(14.102,222)
Y5 137.9125
➏
(14.134,722)
Y6 10.245
➏
Z1-Z5
Ferrite bead, prestrung
Notes:
➊
Observe polarity.
➋
Install flat end oriented as shown.
➌
Leave 1/16" test point loop at top
and face as shown.
➍
There are two different switches.
➎
Caution: Static sensitive part!
➏
Caution: Fragile part!
➐
Surface mount part under board
