Elenco Electronics AR-2N6K User guide

FM RECEIVER KIT
MODEL AR-2N6K
Assembly and Instruction Manual
ElencoTM Electronics, Inc.
Copyright © 2003, 1995 ElencoTM Electronics, Inc. Revised 2003 REV-D 753001

PARTS LIST
If you are a student, and any parts are missing or damaged, please see instructor or bookstore. If you purchased this FM
receiver kit from a distributor, catalog, etc., please contact ElencoTM Electronics (address/phone/e-mail is at the back of this
manual) for additional assistance, if needed.
RESISTORS
Qty. Symbol Value Color Code Part #
1R9 0ΩJumper Wire (use a discarded resistor lead)
1 R15 10Ω5% 1/4W brown-black-black-gold 121000
2 R10, R12 1kΩ5% 1/4W brown-black-red-gold 141000
2 R3, R7 3.3kΩ5% 1/4W orange-orange-red-gold 143300
2 R4, R6 4.7kΩ5% 1/4W yellow-violet-red-gold 144700
1 R8 8.2kΩ5% 1/4W gray-red-red-gold 148200
1 R11 10kΩ5% 1/4W brown-black-orange-gold 151000
1 R5 20kΩ5% 1/4W red-black-orange-gold 152000
1 R2 47kΩ5% 1/4W yellow-violet-orange-gold 154700
1 R1 68kΩ5% 1/4W blue-gray-orange-gold 156800
1 VR2 5kΩ20% linear 192450
2 VR1, VR3 100kΩknurl shaft 192614
-1-
Qty. Symbol Value Marking Part #
1 C5 3.9pF Discap 3.9 203921
1 C8 7pF 20% 25V 7 207000
1 C6 33pF 10% 50V Disc 33 213317
1 C6A 39pF 39 213917
1 C17 51pF 10% 51K 215110
1 C12 68pF 10% 68K 216816
1 C1 120pF 121 221280
1 C7 470pF 10% 50V Disc 471 224717
Qty. Symbol Value Marking Part #
1 C10 1000pF 10% 50V 102 231035
1 C2 .005µF 20% 25V 502 or .005 235025
3 C11,C13,C16 .01µF +80, –20% 103 241031
1 C22 .047µF +80, –20% 50V 473 244780
6 C3,C4,C9 .1µF +80, –20% 50V 104 251010
C18,C19,C20
2 C14, C21 4.7µF 50V Radial 4.7µF 50V 264747
1 C15 220µF 16V Radial 220µF 16V 282244
SEMICONDUCTORS
Qty. Symbol Value Marking Part #
2 D1, D2 Diode 1N914 1N914 310914
1 U2 IC LM386N-1 Audio Amp LM386N-1 330386
1 U1 IC MC3362P FM Receiver MC3362P 333362
1 U3 IC 78L05 Regulator +5V 78L05 338L05
Qty. Symbol Value Part #
1 L4 Coil (1.5 turns coated wire) 430170
1 L1 Coil (4.5 turns coated wire) 430180
1 L5 Coil (metal can yellow) 430260
2 L2, L3 Coil (on form with core) 468752
MISCELLANEOUS
Qty. Symbol Description Part #
1 PC Board 517021
2 SW1, SW2 Switch Slide DPDT 541021
1 F2 Filter Ceramic 10.7MHz 560107
1 F1 Filter Ceramic 455kHz 560455
1 Y1 Crystal 10.245MHz 561024
1 Battery Snap 9V 590098
1 Battery Holder 9V 590099
1 SPK Speaker 8Ω590102
3 Bracket L-shaped 613000
1 Front Panel 614107
4 Nylon Clips 621012
1 Phono Plug 621017
2 Knob Push-on 622009
1 Knob Large 622080
1 Phono Jack 622103
1 Case Plastic 623240
Qty. Symbol Description Part #
1 Alignment Tool 629011
4 Screw 2-56 x 1/4” 641237
3 Screw 4-40 x 1/4” 641433
3 Nut 7mm Hex 644101
4 Nut 2-56 Hex 644201
3 Nut 4-40 Hex 644400
3 Flat Washer 645101
3 Lock Washer 5/16” internal tooth 646101
3 Lock Washer #4 internal tooth 646401
1 IC Socket 8-pin 664008
1 IC Socket 24-pin narrow 664025
1 Tape Double-sided Foam 740004
1 Wire #22 Solid Orange 84” 814320
1 Cable Shielded 2-conductor 876090
1 Solder Tube 9ST4

It is the goal of this project to educate the builder in
all of the principles needed to design and build this
kit. The radio is broken down into four blocks. Each
block contains:
1. Explanation of circuit to be assembled
(Theory of Operation).
2. Detailed assembly instructions for each
circuit.
3. Specifications and testing procedure for each
circuit.
4. Troubleshooting guide for each circuit.
5. Quiz on circuit (answers included).
A final quiz is included (with answers) to help
demonstrate the overall knowledge gained by
building this kit. With this in mind, let’s start by
defining exactly what this radio kit is.
THE AR-2N6
-2-
Figure 1
Dual Conversion The original radio frequency in converted first to a 10.7MHz intermediate
frequency (i-f) and amplified (Block 4). The 10.7MHz signal is then converted to
455kHz and amplified (Block 3). Noise is removed and the modulated signal is
recovered from the 455kHz i-f signal (Block 2). The audio signal is amplified to
drive a speaker (Block 1).
The AR-2N6 is a dual conversion narrow band
FM radio receiver designed to detect signals in
the 2 meter and 6 meter bands.
Why Convert Twice? In the attempt to obtain the desired intermediate frequency (i-f) signal by mixing
the local oscillator with the desired radio frequency, an unwanted output may
result due to a transmission spaced one intermediate frequency (i-f) on the
opposite side of the oscillator (image). By using a large i-f frequency, this image
is moved further out of the band of desired frequencies. The second conversion
provides the selectivity to filter out the desired narrow band transmission.
Figure 2
BLOCK 4
CONVERT TO 455
AND AMPLIFY REMOVE NOISE &
DEMODULATE
CONVERT TO 10.7
AND AMPLIFY AUDIO
AMP
BLOCK 3 BLOCK 2 BLOCK 1
Antenna Speaker
Desired Band of Frequencies
Local Oscillator
Image
Frequency
Desired
Frequency
Single Conversion with 455kHz
Intermediate Frequency Double Conversion puts the Image Outside of Desired Band and still allows Narrow Band Output
SECOND CONVERSION
TO 455kHz
Crystal
Oscillator
at 10.245 10.7MHz
10.7MHz
Desired Frequency
Desired Band of Frequencies
Local Oscillator #1
Image
Frequency

-3-
Narrow Band The selectivity of the entire system is limited to only enough frequencies to pass
voice or low frequency data. A normal FM receiver would have a bandwidth large
enough to pass music and high frequency data transmissions.
FM Radio Receiver The letters “FM” stand for Frequency Modulation . The other popular forms of
modulation are
AM (Amplitude Modulation) and PM (Phase Modulation).
Frequency modulation means the data or voice changes the frequency of the
radio wave.
Amplitude ModulationFrequency Modulation
Figure 4
Figure 3
Narrow Band
Wide
Bandwidth
3kHz 200kHz
2 Meter Band If the speed of a wave (meters per second) is divided by the number of waves that
pass a given point (cycles per second), the seconds cancel and you obtain the
wavelength (λ= meters per cycle). The speed of radio waves is approximately
300,000,000 meters per second. If the frequency is 150MHz, the wavelength
becomes 300,000,000/150,000,000 or 2 meters.
6 Meter Band To find the frequency in MHz, divide the speed in millions of meters per second
by the wavelength in meters. A 6 meter wave has a frequency of 300/6 or 50MHz.
Actual bands are: 2 Meter covers 144MHz to 148MHz.
6 Meter covers 50MHz to 54MHz.
All licensees, except Novices, are allowed to use these bands.
50MHz150MHz
Figure 5
2 Meters 6 Meters

-4-
The audio in this radio is amplified by using an
integrated circuit audio power amplifier. The LM-386
specifications are as follows:
• Single supply voltage (4-12V)
• Idle current - 4 milliamps
• Inputs referenced to ground
• Input resistance - 50kΩ
• Self-centering output voltage
• Total harmonic distortion less than 0.2%
• Output power with 9 volt supply voltage
• Voltage gain with 10µF from pin 1 to 8 - 200 or
46dB
• Voltage gain with pins 1 and 8 open - 20 or
26dB
• Bandwidth with pins 1 and 8 open - 300kHz
The output impedance of the amplifier is low
enough to drive an 8Ωspeaker directly. The
coupling capacitor value is picked to pass audio
signals down to 100 cycles by matching the
reactance of the capacitor with the speaker
impedance. In other words, 8Ω= 1/2πfC, where f =
100Hz. By solving for C we get:
C = 1/(2πf)(8Ω)
C = 1/(6.28)(100)(8)
C = 1/5026
C = 0.00019896 or C ≈200µF (220µF used)
Due to the high input resistance of the amplifier
(50kΩ), the audio coupling capacitor C3 can be as
small as 0.1µF. The equivalent resistance at the
junction of R8 and VR3 is approximately 6.6kΩ(the
parallel combination of R8, VR3 and the 50kΩinput
impedance of the LM-386). The capacitor C2 and
this equivalent resistance sets the 3dB corner used
to attenuate any IF voltage at pin 13. A simple RC
filter attenuates at a rate of 6dB per octave (an
octave is the same as doubling the frequency). By
using 6.6kΩas the equivalent resistance and
0.005µF as the capacitance, we get a 3dB corner at
approximately 4.8kHz. To get to 455kHz, you must
double 4.8kHz approximately 6.6 times. This
equates to a reduction of the IF voltage at the R8 -
VR3 junction of 39.6dB (6dB per octave times 6.6
octaves), or 95 times.
BLOCK 1 - THE AUDIO AMPLIFIER THEORY OR OPERATION
Figure 6
R12 = 1kΩ
LM-386
Audio Amplifier
R8 = 8.2kΩ
8Ω
C2 = 0.005µF
Pin 11
Carrier
Detect
Pin 13
Detector
Output
VR3 = 100kΩC3 = 0.1µFC1 = 220µF

Figure 7
ASSEMBLY INSTRUCTIONS FOR BLOCK 1
Solder the parts to the PC board and put a check mark in the box (!) next to each step after it is completed.
The parts should be similar to the sketch in each box, but will differ in size. Be sure to check each solder point
for shorts and cold solder connections. Be careful to prevent static discharge when handling integrated circuits
U2 and U3.
-5-
C3 - .1µF Capacitor
C4 - .1µF Capacitor
104
Insert the IC socket into
the PC board with the
notch in the same
direction as the marking
on the PC board.
Insert the LM-386 IC into
the IC socket with the
notch in the same
direction as the notch on
the socket.
Notch
PC Board
Marking
C15 - 220µF Lytic
Polarity
Mark
(–) (+)
C14 - 4.7µF Lytic
C21 - 4.7µF Lytic
Polarity
Mark
(–) (+)
U3 - 78L05 Voltage
Regulator
Bend the leads as shown
and make sure that the
flat side matches the PC
board marking.
Flat Side
R8 - 8.2kΩResistor
(gray - red - red - gold)
R12 - 1kΩResistor
(brown - black - red - gold)

C2 - .047µF
Capacitor (473)
-6-
C2 - .005µF Capacitor
(502)
CONNECTING PARTS TO THE BACK SIDE OF THE PC BOARD
Twist two leads of a 10Ωresistor and a .047µF capacitor as shown in Figure 8a. Solder the wires close to the
components and clip off any excess wire.
Mount the components shown in Figure 8 to the foil side of the PC board. Place a check mark in the box (!)
after each part is in place.
502
R15 - 10ΩResistor
(brown-black-black-gold)
473
Figure 8
473
(brown-black-black-gold)
Figure 8a
C22 - .047µF Capacitor (473)
R15 - 10ΩResistor
Solder

-7-
CONNECTING THE FRONT PANEL PARTSTO THE PC BOARD
Solder the parts to the PC board and put a check mark in the box (!) next to each step after it is completed.
The parts should be similar to what is shown in Figure 9. To wire the speaker, pot and switch, cut the indicated
length of wire from the roll of 22 gauge solid wire and strip 1/4” of insulation off of each end. Before soldering,
mechanically connect the wire to the parts as shown in the figure. For the jumper wire, cut 2” of 22 gauge solid
wire, strip 1/4” of insulation off of each end and solder to the points indicated.
Speaker Leads - 4” wires
Battery Snap
ON/OFF Switch
Attach the wires to the switch
and points A & B on the PC
board as shown.
VR3 - 100kΩVolume Control
Attach the wires to points C, D,
& E on the PC board. Cut off
tab on the pot.
Jumper Wire (temporary)
Incorrect
(Lead too long) Correct
Figure 9
+
_
Make a good mechanical connection before
soldering.
Tab
3”
4”

-8-
TEST PROCEDURE - BLOCK 1
Procedure 1
If you do not have an audio generator and
multimeter, skip to procedure 2.
Power Test
Make sure that the power switch is in the OFF
position (handle away from the 2 wired terminals).
Connect a fresh 9V alkaline battery to the battery
snap. Set the multimeter to read on the 20V scale
and connect to the circuit as shown in Figure 10.
Connect the positive voltage probe to the positive
lead of capacitor C4. Connect the common probe to
the negative lead from the battery. Turn the power
ON. The multimeter should read between 4.5V and
5.5V. If not, turn off the power and check that U3 is
correctly installed. Also, check for solder shorts and
unsoldered leads.
Audio Gain
Make sure that the power switch is in the OFF
position. Connect the audio generator and
multimeter as shown in Figure 11. Turn the volume
control fully clockwise (maximum gain). Put the
multimeter in the AC 20V position, turn the
generator to minimum at 400Hz sine wave output,
and turn the power ON. Slowly increase the
generator output until the multimeter reads 2Vrms.
Move the multimeter lead to the generator output as
shown by the dashed lines in Figure 11. Adjust the
multimeter for the best reading and record here:
Input Voltage = ___________________
The gain is the output voltage divided by the input
voltage. Since the output voltage was set at 2V, the
gain is equal to 2 divided by the reading recorded
above. Gain should be approximately equal to 17.
If gain test fails, turn off the power and check the
connections on all external parts. Check for shorts
between pins on IC U2 and make sure that all of the
IC pins are properly inserted in the socket. Replace
the battery with a fresh alkaline battery.
Figure 10
V
20A
A
COM
VΩΩ
9V Battery
Temporary Connection

Power Output
If you do not have an audio generator and an
oscilloscope, skip this test. Make sure that the
power is OFF and connect the audio generator and
the oscilloscope as shown in Figure 11 (replace the
multimeter leads with scope leads). Set the
generator to 400Hz minimum output and set the
oscilloscope to read 1V/div. Put the volume control
in the fully clockwise position. Turn the power ON
and set the audio generator output until a waveform
similar to the one shown in Figure 12 is obtained.
The flat area indicates clipping. You may have to
adjust the oscilloscope time base for correct
comparison. Measure the peak to peak voltage at
which the clipping first occurs and record here:
Peak to Peak Reading = __________________
The power output before clipping is equal to the
square of the rms voltage across the speaker
divided by the speaker impedance. The rms voltage
for a sine wave is equal to the peak voltage times
0.707. To get the peak voltage, divide the peak to
peak reading recorded above by 2.
Multiply the peak by 0.707 and square the result.
Divide by 8Ω(speaker impedance) to obtain the
maximum power output before clipping distortion
occurs. Maximum power output should be 1/2W or
greater. If this test fails, replace the battery with a
power supply capable of delivering 1/2A @ 9V or
replace the battery with a fresh alkaline battery.
Example
If the peak to peak voltage - 5.66
Peak Voltage - 5.66 x 0.5 or 2.83
rms Voltage - 2.83 x 0.707 = 2.0
rms Voltage Squared - 2 x 2 = 4
Dividing by speaker impedance of 8Ωgives a power
of 0.5W.
Figure 12 Peak to Peak
Flat Area
Figure 11
V
20A
A
COM
VΩΩ
9V Battery
-9-
455kHz

-10-
Frequency Response
If you do not have an audio generator and an
oscilloscope, skip this test. Make sure that the
power is OFF and connect the audio generator and
the oscilloscope as shown in Figure 11 (replace the
multimeter leads with scope leads). Set the
generator to 400Hz minimum output and set the
oscilloscope to read 1V/div. Put the volume control
in the fully clockwise position. Turn the power ON
and adjust the audio generator output until a
waveform similar to the one shown in Figure 13 is
obtained. You may have to adjust the oscilloscope
time base for correct comparison. Without changing
the generator output level, slowly lower the
frequency of the audio generator until the sine wave
at the speaker falls to 0.707 of the original setting
(3dB point or 1/2 power point). Record that
frequency here:
Speaker Frequency = __________________
Without changing the generator output level, slowly
raise the frequency of the audio generator above
the original 400Hz until the sine wave at the speaker
falls to 0.707 of the original setting and record here:
Speaker Frequency = __________________
During this second measurement, the voltage at the
speaker may go higher than the original value due
to speaker resonance. The frequency at which a
speaker resonates changes with speaker size,
mounting method, and speaker enclosure. The
bandwidth of your audio circuit is equal to the
frequency difference between these 2 half power
points and should be no less than 3,000Hz. If you
fail this test, check that capacitors C2, C3, and C15
are the proper values and are correctly installed.
Example
3dB Point Low Side = 100Hz
3dB Point High Side = 4,000Hz
Bandwidth = 4,000 - 100Hz
Bandwidth = 3,900Hz
Bandwidth = __________________
Figure 13
0.707 of
original peak to
peak voltage Original
Voltage
Procedure 2
If you do not have any test equipment, you can test
the audio circuit as follows:
Make sure that the power switch is turned OFF.
Connect a fresh 9V battery to the battery snap. Put
the volume control in the mid position. While holding
one lead of a 0.1µF capacitor on the R8 resistor
lead, place the other lead on the speaker wire
marked “+” and turn the power ON (see Figure 14).
The audio should make a buzzing sound. The pitch
will change if you vary the volume control setting. If
this test fails, turn the power OFF and check for
solder shorts between the pins on U2 (LM-386
integrated circuit). Check that all of the parts are the
correct value and are installed properly. Check that
regulator U3 is installed properly.

-11-
QUIZ 1
1. The impedance of a 200µF capacitor at 100Hz is:
(a) 50Ω(b) 79.6Ω(c) 2Ω(d) 7.96Ω
2. If the frequency is 150MHz, the wavelength is
approximately . . .
(a) 20 meters (c) 6 meters
(b) 2 meters (d) need more information
3. Frequency modulation means the data or voice
changes the . . .
(a) amplitude (c) phase
(b) frequency (d) power
4. The audio in this radio is amplified using . . .
(a) an IC audio power amplifier
(b) a discrete power amplifier
(c) a preamplifier
(d) increased bandwidth
5. Double conversion is used to . . .
(a) remove undesired image
(b) reduce noise
(c) provide greater gain
(d) increase bandwidth
6. The first IF frequency in the AR-2N6 Radio Kit is:
(a) 455kHz (b) 10.7MHz (c) 150MHz (d) 50MHz
7. In order to get the second IF frequency, a crystal
oscillator must be set at:
(a) 455kHz (c) 10.7MHz
(b) 545kHz (d) 10.245MHz
8. The integrated circuit audio power amplifier used
in the AR-2N6 Kit has an input resistance of . . .
(a) 50kΩ(b) 5kΩ(c) 8Ω(d) 7.96Ω
9. Resistor R8 and capacitor C2 attached to the
audio volume control are used to . . .
(a) peak the audio (c)
flatten audio response
(b) provide bass boost (d) limit the bandwidth
10. When soldering electronic components, always
use . . .
(a) high wattage iron (c) rosin core solder
(b) acid core solder (d) paste flux
Answers: 1. d, 2. b, 3. b, 4. a, 5. a, 6. b, 7. d, 8. a, 9. d, 10. c
Figure 14
9V Battery
Feedback Capacitor
Volume control in
mid-position
104

BLOCK 2 - RECOVERING THE DATA
Theory of Operation
In dealing with FM receivers, there are some terms
that must be defined. First, let’s determine the term
deviation as the frequency swing of the incoming
FM signal. When no modulation is present, the
incoming signal is a fixed frequency
carrier wave
(Fc) . Positive deviation (Fp) is the increase in Fc
due to modulation, and negative deviation (Fn) is
the decrease in Fc due to the modulation. The
detector must be linear over the maximum total
deviation (Fp max. - Fn max.) produced by the
transmitter or distortion will occur.
Before the second i-f signal reaches the detector, it
is applied to a
limiting amplifier inside the
integrated circuit. A limiting amplifier is designed to
remove any amplitude variations in the signal by
amplifying the signal well beyond the limit of the
amplifier. The frequency variations (FM) are not
affected by the limiter. The limiter removes the
affects of fading (driving through a tunnel) and
impulse noise (lightning and ignition). These affects
produce significant unwanted amplitude variations
in the received signal, but minor frequency
variations. Noise immunity is one of the great
advantages of frequency modulation over amplitude
modulation. The remaining signal contains only the
frequency modulation as shown in Figure 15.
Figure 15
Original Transmitted
Signal Received Signal with Noise and Fading Received Signal After
Limiting Amplifier
The AR-2N6 Radio Kit uses a quadrature detector
to demodulate the FM signal. After the noise is
removed by the limiter, the signal is internally
connected to the quadrature detector. A parallel
tuned circuit must be connected from pin 12 to the
power supply. This circuit produces the 90Ophase
needed by the quadrature detector. A resistor
shunting this coil sets the peak separation of the
detector. If the value of the resistor is lowered, it will
increase the linearity, but decrease the amplitude of
the recovered audio as shown in Figure 16.
-12-

Figure 16
Notice how the linearity improves for a given amount
of frequency deviation when the positive and
negative peaks are further apart. Figure 16 also
shows how the amplitude of the output would drop
for the same frequency deviation on a more linear
detector.
The quadrature detector combines two phases of
the i-f signal that are 90Oapart, or in quadrature, to
recover the modulation (see Figure 17a). The
shifted signal is used to gate the non-shifted signal
in a synchronous detector. When the frequency
increases above the carrier frequency (positive
deviation), the phase shift increases causing a
decrease in the width of the gated impulse output
(see Figure 17b). In a similar manner, when
negative deviation occurs, the gated impulse output
will widen (see Figure 17c). The gated output is
then filtered to remove the pulses and recover the
modulating signal.
-13-
Figure 17
(a)
(b)
(c)
Signal from Limiter
No Modulation
90OPhase Shift
Gated Output
Signal from Limiter
Positive Deviation
Greater than 90O
Phase Shift
Gated Output
Signal from Limiter
Negative Deviation
Less than 90O
Phase Shift
Gated Output
Linear Area
Narrow Band
Amplitude
Frequency Frequency
Amplitude
Same Deviation
Lower R

ASSEMBLY INSTRUCTIONS FOR BLOCK 2
Turn the switch OFF and remove the battery. Solder the parts to the PC board and put a check mark in the box
(!) next to each step after it is completed. The parts should be similar to the sketch in each box, but will differ
in size. Be sure to check each solder point for shorts and cold solder connections. Be careful to prevent static
discharge when handling the integrated circuit U1.
-14-
C18 - .1µF Capacitor
C19 - .1µF Capacitor
C20 - .1µF Capacitor
104
R11 - 10kΩResistor
(brown - black - orange - gold)
R1 - 68kΩResistor
(blue - gray - orange - gold)
Remove this jumper wire.
R2 - 47kΩResistor
(yellow - violet - orange - gold)
L5 - 0.64mH Coil (yellow)
Solder all leads
and tabs.
Insert the IC socket into
the PC board with the
notch in the same
direction as the marking
on the PC board.
Insert the MC3362 IC into
the IC socket with the
notch in the same
direction as the notch on
the socket.
Notch
PC Board
Marking
Figure 18
F1 - 455kHz Filter

-15-
CONNECTING THE SQUELCH CONTROLTO THE PC BOARD
Cut two 3” pieces of wire from the roll of 22 guage
soild wire and strip 1/4” of the insulation off of each
end. Using these wires, solder the 100kΩ
potentiometer (VR1) to the PC board and put a
check mark in the box (!) after both wires are in
place. This part should be similar to the sketch
shown in the box, but may differ in size.
Be sure to
mechanically connect each wire as shown in
Figure 19 before soldering.
VR1 - 100kΩPotentiometer
Squelch Control
Attach wires to points R & Q
on the PC board as shown.
Cut tab off
3” 3”
Make a good mechanical connection before
soldering.

V
20A
A
COM
VΩΩ
TEST PROCEDURE - BLOCK 2
Procedure 3
If you do not have a multimeter, skip to procedure 4.
DC Test
Make sure that the power switch is in the OFF
position. Reconnect the 9V battery to the battery
snap. Set the multimeter to read on the 20V scale
and connect to the circuit as shown in Figure 20.
Connect the positive voltage probe to pin 12 of the
integrated circuit U1. Connect the common probe to
the negative lead from the battery. Turn the power
ON. The multimeter should read between 4.5V and
5.5V. If not, turn off the power and check that coil L5
is correctly installed. Also, check for solder shorts,
unsoldered leads, or an open copper run between
the positive side of C14 and coil L5.
-16-
Figure 20

AC Test
If you do not have a signal generator, skip to
procedure 4.
Make sure that the power switch is in the OFF
position. Turn the squelch control fully
counterclockwise . Set the mulitmeter to read
20VDC full scale and connect to the circuit as
shown in Figure 21. Connect the positive voltage
probe to pin 13 of integrated circuit U1. Connect the
common probe to the negative lead from the battery.
AC couple a generator capable of producing a
455kHz signal to pin 5 and attach the generator
ground lead to the ground lead of the volume control
(see Figure 21). Set the generator frequency to
455kHz and the amplitude to between 50 and
100mV peak to peak. Turn the power ON. Turn coil
L5 to its maximum counterclockwise position. The
multimeter should read approximately 2.4V. Turn L5
slowly clockwise. The multimeter voltage should
rise to a maximum of about 3.0V, then fall to a
minimum of about 1.5V and then rise again. Record
the maximum (Vmax) and minimum (Vmin)
voltages. Set L5 midway between Vmax and Vmin.
For example, ifVmax = 2.95V andVmin = 1.45V, set
L5 to (Vmax + Vmin)/2 = (2.95 + 1.45)/2 = (4.40)/2
= 2.2V.
A small increase in generator frequency (positive
deviation) should cause the DC voltage at pin 13 to
drop. Likewise, a small increase in generator
frequency (positive deviation) should cause the DC
voltage to rise. Careful point-by-point plotting (use
increments of about 1kHz) should reproduce a
curve similar to the one shown in Figure 16. If not,
turn the power OFF and check that the 455kHz filter
F1 is installed correctly. Also check for solder
shorts, unsoldered leads, and parts you may have
missed in the block 2 assembly instructions.
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Figure 21
V
A
COM
VΩΩ
104
455kHz
20A

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Squelch Control Check
The function of the squelch control is to eliminate
static when there is no signal present. This control
sets the level of passable signals. The squelch
control works by raising the voltage to the (–) input
to the audio amplifier and thus disabling the
amplifier output.
The squelch control can be checked with the circuit
shown in Figure 21. Turn the squelch control fully
counterclockwise. Set the signal generator to
455kHz and the output amplitude to minimum.
Move the multimeter probe from pin 13 to pin 11 of
U1. The voltage should be near 0V. Turn the
squelch control clockwise until the voltage jumps to
about 2.6V, disabling the audio amplifier. If the
voltage does not jump, use a voltage divider to
reduce the signal generator output to 5mV peak to
peak and try again. If the voltage still does not jump,
check the wiring to the squelch control pot. Also, be
sure that the temporary jumper installed in block 1
was removed.
Procedure 4
If you do not have test equipment, it is possible to
check block 2 with the following procedure:
1. Make sure that the ON/OFF switch is in the
OFF position.
2. Reconnect the battery.
3. Turn the volume control fully clockwise.
4. Turn the squelch control fully counter-
clockwise.
5. Hold a 0.1µF capacitor on pins 5 and 13 as
shown in Figure 22.
6. Turn the power ON.
The audio should produce a hissing sound. Turning
the squelch control clockwise should quiet the
audio. If you do not get a hissing sound, check that
the integrated circuit U1 is properly installed. Check
that all of the leads installed in block 2 have been
soldered and no shorts exist. If the hissing is
present, but the squelch control does not remove it,
check the leads going to the control and compare
them to Figure 20.
Turn the ON/OFF switch to the OFF position and
remove the 9V battery. Before moving on to the next
block, try answering the questions in Quiz 2 to see
if you missed an important fact about the operation
of the second i-f, squelch control, or quadrature
detector.
Figure 22
104

QUIZ 2
1. When no FM modulation is present, the incoming
signal frequency is . . .
(a) DC
(b) fixed
(c) minimum
(d) maximum
2. An increase in the carrier frequency due to
modulation is called . . .
(a) detection
(b) negative deviation
(c) positive deviation
(d) peaking
3. The amplifier known as the “limiter” is used to . . .
(a) remove noise
(b) detect FM signals
(c) increase audio output
(d) improve linearity
4. Lowering the value of the resistor across the
quadrature coil will . . .
(a) reduce bandwidth
(b) remove noise
(c) improve gain
(d) improve linearity
5. Positive deviation will cause the gated output
pulse to . . .
(a) decrease in width
(b) increase in width
(c) decrease in amplitude
(d) increase in amplitude
6. When the quadrature detector is made more
linear by separating the peaks, the output . . .
(a) increases
(b) remains the same
(c) produces more distortion
(d) decreases
7. The function of the squelch control is to remove:
(a) high frequencies
(b) fading
(c) static during transmissions
(d) non-transmission static
8. A decrease in the carrier frequency due to
modulation is called . . .
(a) detection
(b) negative deviation
(c) positive deviation
(d) peaking
9. An FM detector converts frequency changes to:
(a) DC changes
(b) resistance changes
(c) capacitance changes
(d) modulation index
10. The coil in the quadrature detector is used to
produce . . .
(a) DC voltage
(b) phase shift
(c) frequency shift
(d) amplitude peaking
Answers: 1. b, 2. c, 3. a, 4. d, 5. a, 6. d, 7. d, 8. b, 9. a, 10. b
BLOCKS 3 & 4 - CONVERTING TO 455kHz
Theory of Operation
The first local oscillator should be set at 133.3MHz
to 137.3MHz for the 2 meter band and 39.3MHz to
43.3MHz for the 6 meter band. This oscillator is
free-running varactor-tuned. The first mixer
converts the RF input to an i-f frequency of
10.7MHz. This i-f signal is then filtered through a
ceramic filter and fed into the second mixer. If the
oscillator of the second mixer is precisely set at
10.245MHz, it will produce an output containing the
sum (20.945MHz) and the difference (0.455MHz or
455kHz) frequencies. This 455kHz signal is then
sent to the limiter, detector, and audio circuits.
The mixers are doubly balanced to reduce spurious
(unwanted) responses. The first and second mixers
have conversion gains of 18 to 22dB respectively.
Conversion gain is the increase in a signal after the
signal has been converted to a new frequency and
amplified. For both converters, the mixers are
designed to allow the use of pretuned ceramic
filters.
After the first mixer, a 10.7MHz ceramic bypass filter
is used. This eliminates the need for special test
equipment for aligning i-f circuits. The ceramic filter
also has a better aging and temperature
characteristic than conventional LC tuned circuits.
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