AM/FM RADIO KIT MODEL AM/FM-108K INTEGRAL CIRCUIT, 9 TRANSISTORS, 4 DIODES Assembly and Instruction Manual Elenco Electronics, Inc. TM TM Copyright © 2003, 1989 by Elenco Electronics, Inc. All rights reserved. Revised 2003 REV-X No part of this book shall be reproduced by any means; electronic, photocopying, or otherwise without written permission from the publisher.
The AM/FM Radio project is divided into two parts, the AM Radio Section and the FM Radio Section. At this time, only identify the parts that you will need for the AM radio as listed below. DO NOT OPEN the bags listed for the FM radio. A separate parts list will be shown for the FM radio after you have completed the AM radio. PARTS LIST FOR THE AM RADIO SECTION If you are a student, and any parts are missing or damaged, please see instructor or bookstore.
IDENTIFYING RESISTOR VALUES Use the following information as a guide in properly identifying the value of resistors. BAND 1 1st Digit Color Black Brown Red Orange Yellow Green Blue Violet Gray White Digit 0 1 2 3 4 5 6 7 8 9 Multiplier BAND 2 2nd Digit Color Black Brown Red Orange Yellow Green Blue Violet Gray White Resistance Tolerance Color Multiplier Black 1 Brown 10 Red 100 Orange 1,000 Yellow 10,000 Green 100,000 Blue 1,000,000 Silver 0.01 Gold 0.
INTRODUCTION TM The Elenco Superhet 108 AM/FM Radio Kit is a “superheterodyne” receiver of the standard AM (amplitude modulation) and FM (frequency modulation) broadcast frequencies. The unique design of the Superhet 108 allows you to place the parts over their corresponding symbol in the schematic drawing on the surface of the printed circuit board during assembly. This technique maximizes the learning process while keeping the chances of an assembly error at a minimum.
CONSTRUCTION Introduction The most important factor in assembling your Superhet 108 AM/FM Radio Kit is good soldering techniques. Using the proper soldering iron is of prime importance. A small pencil type soldering iron of 25 - 40 watts is recommended. The tip of the iron must be kept clean at all times and well tinned. Safety Procedures • Wear eye protection when soldering. • Locate soldering iron in an area where you do not have to go around it or reach over it. • Do not hold solder in your mouth.
SEMICONDUCTOR PARTS FAMILIARIZATION This section will familiarize you with the proper method used to test the transistors and the diode. TRANSISTOR TEST Refer to the parts list and find transistors. These are NPN transistors. Refer to Test A for locating the Emitter, Base and Collector. Using an Ohmmeter, connect the transistor as shown in Test A. Your meter should be reading a low resistance. Switch the lead from the Emitter to the Collector. Your meter should again be reading a low resistance.
SECTION 1 AUDIO AMPLIFIER The purpose of the Audio Amplifier is to increase the audio power to a level sufficient to drive an 8 ohm speaker. To do this, DC (direct current) from the battery is converted by the amplifier to an AC (alternating current) in the speaker. The ratio of the power delivered to the speaker and the power taken from the battery is the efficiency of the amplifier. For the Audio Amplifier, we use the integrated circuit (IC) LM-386.
ASSEMBLY INSTRUCTIONS We will begin by installing resistor R43. Identify the resistor by its color and install as shown on page 4. Be careful to properly mount and solder all components. Diodes, transistors and electrolytic capacitors are polarized, be sure to follow the instructions carefully so that they are not mounted backwards. Check the box when you have completed each installation.
ASSEMBLY INSTRUCTIONS Note: Mount the Pot/SW, earphone jack, and speaker to the foil side of the PC board. Battery Holder 3 Screws 2-56 x 1/4” 3 Nuts 2-56 Solder and cut off excess leads.
STATIC MEASUREMENTS POWER TEST For all measurements, connect your equipment GND to circuit GND TP15. Set your VOM (Volt-OhmMillimeter) to read 2 amps DC. Connect the meter to the circuit as shown in Figure 6. Make sure that the volume control is in the OFF position (turned fully counter-clockwise). While watching your VOM, turn the volume to the ON position (rotate clockwise until a “click” is heard). The VOM should indicate a very low current. Adjust your meter for a more accurate reading if necessary.
If you do not have an audio generator, skip the following test and go directly to Section 2. DYNAMIC MEASUREMENTS GAIN Connect the VOM and audio generator to the circuit as shown in Figure 8. lead of your VOM to the Jumper J3. Record the AC input voltage to the amplifier here: Normally the AC gain is measured at a frequency of 1kHz. Your VOM however, may not be able to accurately read AC voltages at this frequency. Therefore, it is recommended that this test be performed at 400Hz.
If an oscilloscope is not available, skip the following test and go directly to Section 2. AC BANDWIDTH or 2.8 divisions. This frequency is called the low frequency 3dB corner. Record your answer. Connect the oscilloscope and audio generator to your circuit as shown in Figure 9. Set the audio generator for a frequency of 1kHz and minimum voltage output. Set the oscilloscope to read .5 volts per division. Turn on the power and slowly increase the volume control to a comfortable level.
MAXIMUM POWER OUTPUT The maximum power output before distortion due to “clipping” can be calculated using the voltage Vclp Clipped obtained in the Distortion Step as follows: Crossover Distortion Vpeak (Vp) = Vclp/2 Vroot mean squared (Vrms) = Vp x .7 Max power out = (Vrms)2/8 ohms = (Vclp x .35)2/8 Maximum power output should be greater than 200 milliwatts.
Generator If you do not have a power supply, use a 9 volt battery instead. Power Supply Hz Amps Amps COM TP15 TP15 Figure 11 Vp = Vclp/2 Vp = ______ Vrms = Vp x .
SECTION 2 AM DETECTOR AND AGC STAGE The purpose of the detector is to change the amplitude modulated IF signal back to an audio signal. This is accomplished by a process called detection or demodulation. First, the amplitude modulated IF signal is applied to a diode in such a way as to leave only the negative portion of that signal (see Figure 12). The diode acts like an electronic check valve that only lets current pass in the same direction as the arrow (in the diode symbol) points.
ASSEMBLY INSTRUCTIONS C34 - 100mF Lytic (see Figure B) R38 - 100W Resistor (brown-black-brown-gold) T6 - AM IF Coil (Yellow Dot) TP3 - Test Point Pin (see Figure A) R35 - 27kW Resistor (red-violet-orange-gold) T8 - AM IF Coil (Black Dot) TP5 - Test Point Pin (see Figure A) D4 - 1N4148 Diode (see Figure D) C32 - 10mF Lytic (see Figure B) C38 - .01mF Discap (103) R42 - 2.2kW Resistor (red-red-red-gold) R36 - 3.3kW Resistor (orange-orange-red-gold) C33 - .02mF Discap (203) or .
DYNAMIC MEASUREMENTS AM DETECTOR AND AGC TEST Connect your VOM and RF generator as shown in Figure 14. .001mF V GENERATOR COM V TP15 Hz Figure 14 Set the VOM to accurately read 2 volts DC and the output of the RF generator for 455kHz, modulation, and minimum voltage output. Turn power ON and slowly increase the amplitude of set no the the TP15 generator until the voltage at TP5 just starts to drop. This point is called the AGC threshold with no IF gain.
AM DETECTOR BANDWIDTH TEST Connect your test equipment as shown in Figure 15. Set the generator at 455kHz with 80% modulation at a modulation frequency of 1kHz. Set the oscilloscope to read .1 volts per division. Turn the power ON and set the volume at the minimum. Increase the amplitude of the generator until the signal on the oscilloscope is 4 divisions peak to peak. Check the signal to make sure that it is free of distortion.
ASSEMBLY INSTRUCTIONS R39 - 39kW Resistor (orange-white-orange-gold) T7 - AM IF Coil (White Dot) C36 - .02mF Discap (203) or .022mF Discap (223) TP4 - Test Point Pin (see Figure A) Q9 - 2N3904 transistor (see Figure C) R40 - 10kW Resistor (brown-black-orange-gold) C37 - .02mF Discap (203) or .022mF Discap (223) R41 - 470W Resistor (yellow-violet-brown-gold) STATIC MEASUREMENTS Q9 BIAS Connect your VOM as shown in Figure 17. Set the VOM to read 9 volts DC and turn the power ON.
DYNAMIC MEASUREMENTS AC GAIN Connect your test equipment as shown in Figure 18. .001mF GENERATOR TP15 Hz TP15 Figure 18 BANDWIDTH Set the generator at 455kHz, no modulation and minimum voltage output. Set the oscilloscope at 1 volt per division. The scope probe must have an input capacitance of 12pF or less or it will detune T8. Turn the power ON and slowly increase the amplitude of the generator until 4 volts peak to peak are seen on the scope.
SECTION 4 FIRST AM IF AMPLIFIER The operation of the first IF amplifier is the same as the second IF amplifier with one important difference. The gain of the first IF amplifier decreases after the AGC threshold is passed to keep the audio output constant at the detector and prevent overload of the second IF amplifier. This is accomplished by making the voltage on the base of transistor Q8 lower as the signal strength increases.
DYNAMIC MEASUREMENTS Short TP3 to R38 as shown below. .001mF Generator Hz TP15 TP15 Figure 19 AC GAIN probe to the base of Q8 and record the peak to peak level of the 455kHz signal here: Connect your test equipment as shown in Figure 19. Vb=___________Vpp. The scope probe must have an input capacitance of 12pF or less, otherwise it will detune transformer T7. Using a clip lead, short TP3 to R38 as shown. This short prevents the AGC from lowering the gain of the first IF amplifier.
The “difference frequency” is used as the intermediate frequency in AM radios. The collector of Q7 also contains an IF transformer (T6) tuned only to the difference frequency. This transformer rejects all frequencies except those near 455kHz. T6 also couples the 455kHz signal to the base of Q8 to be processed by the IF amplifiers. The antenna and the oscillator coils are the only two resonant circuits that change when the radio is tuned for different stations.
IMPORTANT: Before installing the antenna coil, determine if you have a 3 wire coil or a 4 wire coil. Assemble it to the PC board as shown below. Mount the antenna assembly to the PC board. Put the tab of the first holder into the right hole and twist the tab 90O. Put the tab of the second holder into the left hole and twist the tab 90O. Punch out one antenna shim from the front flap of the box. Insert the cardboard antenna shim between the ferrite core and the Slide the ferrite core through the holders.
ASSEMBLY INSTRUCTIONS Figure M Screw Knob Fasten the knob to the shaft of the capacitor with a screw. Rotate the knob fully clockwise. Peel off the protective backing on the label. Line up the long white lines on the label with the arrows on the PC board. PC Board Stand Insert the PC board into the stand as shown.
STATIC MEASUREMENTS Q7 BIAS Connect your VOM to the circuit as shown in Figure 20. Short TP6 to the collector of Q7 as shown. V COM V TP15 Figure 20 Connect a clip lead from TP6 to the collector of Q7. This short prevents Q7 from oscillating. Set the VOM to read 2 volts DC and turn the power ON. The DC voltage at TP7 should be about 1.6 volts. If the voltage in your circuit differs by more than .5 volts, leave the power ON and check the battery voltage. If the battery voltage is greater than 8.
AM FINAL ALIGNMENTS AM ALIGNMENT EQUIPMENT There are two different AM alignment procedures. The first alignment procedure is for those who do not have test equipment and the second is for those who do have test equipment. WITHOUT TEST It is best to use an earphone for this procedure. Make sure that the switch is in the AM position. With an alignment tool or screwdriver, turn coils L5, T6, T7 and T8 fully counter clockwise until they stop. DO NOT FORCE THE COILS ANY FURTHER.
ANTENNA ALIGNMENT Tune the radio for a station around 600kHz. With the “magic wand” place the brass end near the antenna coil as shown in Figure 22. If the signal heard at the output increases, it means that the antenna coil needs less inductance. To remove inductance, carefully slide the antenna coil along it’s ferrite core in the direction shown in Figure 22. Place the iron end of the “magic wand” near the antenna coil.
collector of Q7 to TP6. Set the RF generator at 455kHz, modulation of 400Hz 80% and minimum voltage out. Set the oscilloscope to read .1 volts per division and turn the power ON. Increase the amplitude of the generator until the oscilloscope shows a 400Hz sinewave 5 divisions or .5 volts pp. With an alignment tool or screwdriver adjust T6 for a peak. Reduce the generator amplitude so that 5 divisions are maintained. Adjust T7 for a peak and reduce that amplitude again if necessary.
brass end of the “magic wand” near the antenna coil. If the signal increases, it means that the antenna coil needs less capacitance. Adjust the antenna trimmer for a peak. Refer to Figure L for the location of the AM antenna trimmer. Since the adjustment of both the antenna alignment is optimized. This process sets the AM tracking of the Superhet 108. Once the antenna is properly aligned, carefully apply candle wax or glue the antenna coil to the ferrite rod to prevent it from moving as shown in Figure 23.
QUIZ - AM SECTION INSTRUCTIONS: Complete the following examination, check your answers carefully. 1. The number of cycles produced per second by a source of sound is called the ... A) amplitude. B) vibration. C) sound wave. D) frequency. 6. When the two metal plates on a variable capacitor are unmeshed the ... A) capacitance is minimum. B) capacitance is maximum. C) capacitance is not affected. D) inductance is increased. 2. The radio frequencies used by AM broadcast stations are between ...
PARTS LIST FOR FM SECTION RESISTORS Qty. 2 1 1 5 1 2 2 7 2 2 3 2 Symbol Value Color Code Part # R9, 23 R25 R3 R18, 22, 24, 26, 27 R11 R15, 6 R2, 7 R10,12,14,16,19,20,28 R1, 8 R4, 5 R13, 17, 21 R29, 30 100W 5% 1/4W 220W 5% 1/4W 470W 5% 1/4W 1kW 5% 1/4W 1.8kW 5% 1/4W 2.2kW 5% 1/4W 6.
SECTION 6 THE FM RADIO Section 6 begins the construction of the FM radio. The stages that we will build are shown in the block diagram below. We will begin with the FM Ratio Detector and work back to the FM Antenna. Each stage will be tested before proceeding to the next stage.
ASSEMBLY INSTRUCTIONS R24 - 1kW Resistor (brown-black-red-gold) R23 - 100W Resistor (brown-black-brown-gold) C21 - .01mF Discap (103) C23 - .02mF Discap (203) or .022mF Discap (223) R21 - 47kW Resistor (yellow-violet-orange-gold) R25 - 220W Resistor (red-red-brown-gold) C19 - .
STATIC MEASUREMENTS V Battery COM V TP15 Figure 28 FM VOLTAGE Connect your VOM as shown in Figure 28. Switch the AM/FM switch to the FM position. Set your VOM to read 9 volts DC. Turn the power ON. The voltage at this point should be between 7 and 9 volts. Turn the power OFF. If you do not get this reading, check R25, C24 and the battery voltage. TRANSISTOR CURRENT TEST V COM V TP15 Figure 29 Connect your VOM to the circuit as shown in Figure 29. Turn the power ON.
If you don’t have an RF generator and oscilloscope, skip to Section 7. DYNAMIC MEASUREMENTS AC GAIN The AC gain of the ratio detector is set by the AC impedance of the primary side of T4 and the current through Q6. The current is set by R20, R21 and R22. Capacitors C22 and C19 bypass the AC signal to ground.Connect your RF generator and oscilloscope to the circuit as shown in Figure 30.
METHOD #2 ALIGNMENT OF RATIO DETECTOR USING A RF GENERATOR AND OSCILLOSCOPE Turn the power ON and set the volume control to a minimum. Increase the amplitude of the sweep generator until an “S” curve is seen (refer to Figure 32). Using an alignment tool or screwdriver, adjust the blue coil T5 until the “S” curve is centered, until each half of the “S” is equal. Repeat these steps until the alignment is optimized. Turn the power OFF.
ASSEMBLY INSTRUCTIONS R17 - 47kW Resistor (yellow-violet-orange-gold) R19 - 10kW Resistor (brown-black-orange-gold) Q5 - 2N3904 Transistor (see Figure C) TP10 - Test Point Pin (see Figure A) C20 - .01mF Discap (103) T2 - FM IF Coil (Green Dot) C18 - .01mF Discap (103) C17 - .01mF Discap (103) R16 - 10kW Resistor (brown-black-orange-gold) R18 - 1kW Resistor (brown-black-red-gold) STATIC TESTS Q5 BIAS OFF. If you do not get this reading, check R17, R16, R18, Q5 and T2.
.001mF GENERATOR Hz TP15 TP15 Figure 35 AC GAIN BANDWIDTH Connect the RF generator and oscilloscope to the circuit as shown in Figure 35. The scope probe must have an input capacitance of 12pF or less otherwise the probe will detune T3 resulting in a false reading of the AC gain. Set the generator at 10.7MHz no modulation and minimum voltage output. Set the scope to read 50mV per division and turn the power ON. Slowly increase the generator until 150mVpp or 3 divisions are seen on the scope.
SECTION 8 FIRST FM IF AMPLIFIER The operation of the first IF amplifier is the same as the second IF amplifier except that the gain is different. The gain is set by the AC impedance of the primary side of T2 and the current in Q4. The current in Q4 is set by the resistors R12, R13 and R15. Capacitors C14 and C15 bypass the AC signal to ground. C13 and C16 are bypass capacitors from V+ to ground to prevent feedback on the V+ line. R19 is used to widen the bandwidth of the transformer T2.
If you don’t have an RF generator and oscilloscope, skip to Section 9. .001mF GENERATOR TP15 Hz TP15 Figure 37 BANDWIDTH Connect the RF generator and oscilloscope and oscilloscope to the circuit as shown in Figure 37. The scope probe must have an input capacitance of 12pF or less otherwise the probe will detune T2 causing an incorrect measurement of AC gain. Set the generator at 10.7MHz no modulation and minimum voltage output. Set the scope to read 20mV per division and turn the power ON.
SECTION 9 FM RF AMPLIFIER, MIXER, OSCILLATOR, AND AFC STAGE In a superheterodyne receiver, the radio waves are emitted and then mixed with the local oscillator to produce the intermediate frequency (IF). The first stage is the RF amplifier which selects a radio station and amplifies it. The second stage is the local oscillator which oscillates at a frequency 10.7MHz above the desired radio station frequency.
STATIC MEASUREMENTS Q3 BIAS V COM V TP15 Figure 38 With the power turned OFF, connect your VOM to the circuit as shown in Figure 38. Set your VOM to read 9 volts DC and turn the power ON. The DC voltage at the base of Q3 should be approximately 1.8 volts. If your answer varies by more than 2 volts, turn the power OFF and check components R7, R8, R11 and Q3. generator until 4 divisions or 40mVpp are seen on the scope. With an alignment tool or a screwdriver, adjust T1 for peak.
.001mF GENERATOR TP15 Hz TP15 Figure 39 BANDWIDTH TEST Turn the power OFF. calculated as follows: Connect your test equipment to the circuit as shown in Figure 39. Set your generator at 10.7MHz no modulation and minimum voltage output. Set the scope for 10mV per division. Turn the power ON and slowly increase the amplitude of the generator until 40mVpp are seen on the scope. Increase the frequency until the voltage drops .707 of its original value, 2.8 divisions or 28mVpp.
STATIC MEASUREMENTS Q2 BIAS volts. Turn the power OFF. If you do not get this measurement, check R4, R5 and Q2. Connect your VOM to the circuit as shown in Figure 40. Set your VOM to read 9 volts and turn the power ON. The voltage at the base of Q2 should be about 4 V COM V TP15 Figure 40 AFC diode called a varactor. A varactor will change its internal capacitance when a voltage is applied. The ratio detector diodes are positioned in such a way that when the 10.
If you don’t have an RF generator, skip to the RF Amplifier Assembly Procedure. Generator .001mF V Hz COM V TP15 TP15 Figure 41 Connect the RF generator and VOM to the circuit as shown in Figure 41. Set your VOM to read 9 volts DC. Set your generator at 10.7MHz no modulation and moderate signal strength output. Turn the power ON. Record the voltage of D1 here: Increase the frequency of the generator until the voltage is equal to V(D1).
STATIC MEASUREMENTS Q1 BIAS Connect your VOM to the circuit as shown in Figure 42. Set your VOM to read 9 volts and turn the power ON. The voltage at the base of Q1 should be about 1.6 volts. If you do not get this reading, check R1, R2, R3 and Q1. Turn the power OFF. V COM V TP15 Figure 42 ANTENNA FM ASSEMBLY Antenna FM 2 Antenna Screws M2 3.5” Wire #22 Insulated (extra wire in AM Section) (see Figure O) Threaded Holes Mount the antenna to the PC board with two screws as shown.
ALIGNMENT WITH NO TEST EQUIPMENT With an alignment tool or screwdriver turn coils T1, T2 and T3 fully counter-clockwise. DO NOT FORCE THE COILS ANY FURTHER. Turn each coil in about 1 1/4 to 1 1/2 turns. IF ALIGNMENT V COM V TP15 Figure 43 With an alignment tool or screwdriver turn coils T1, T2 and T3 fully counter-clockwise. DO NOT FORCE THE COILS ANY FURTHER. Turn each coil in about 1 1/4 to 1 1/2 turns. Use the earphone provided for best results. Switch to the FM position.
is heard. Place the iron end of the “magic wand” near L3. If the station is heard, it means that L3 needs less capacitance. Carefully adjust the FM oscillator trimmer located on the back of the gang until the station is heard. Repeat this step until the pointer is aligned to the station’s frequency. Adjusting both the oscillator coil L3 and the oscillator trimmer capacitor will effect the oscillator’s frequency, so it is advisable to repeat this procedure until the FM oscillator alignment is optimized.
OSCILLATOR ALIGNMENT RF ALIGNMENT Remove the clip lead and set your generator at 88MHz modulator at 1kHz, 22.5kHz deviation and minimum voltage output. Tune the radio until a 1kHz signal is seen on the scope. It may be necessary to increase the amplitude of the generator. Rotate the dial until the white pointer is aligned to 88MHz. Using the “magic wand” place the brass end near L3 as shown in Figure 44. If the signal seen on the scope increases, this means L3 needs less inductance.
DC VOLTAGES The voltage readings below should be used in troubleshooting the FM section. (Switch at FM position.) Q1 B E C 1.6 .9 7.0 Q4 B E C 1.3 .7 7.5 Q2 B E C 3.3 3.0 7.1 Q5 B E C 1.3 .6 7.5 Q3 B E C 1.6 1.3 7.0 Q6 B E C 1.2 .6 6.6 Test Conditions 1. Volume set to minimum. 2. Connect TP14 to TP15 with a jumper wire. 3. Battery voltage = 9V 4. All voltages are referenced to circuit common. 5.
QUIZ - FM SECTION INSTRUCTIONS: Complete the following examination, check your answers carefully. 1. The FM broadcast band is . . . A) 550 - 1,600kHz. B) 10.7MHz. C) 88 - 108MHz. D) 98.7 - 118.7MHz. 6. The ratio detector is used because ... A) it is sensitive to noise. B) it is insensitive to noise. C) it provides amplification. D) it doesn’t need a filter. 2. The maximum audio frequency used for FM is ... A) 7.5kHz. B) 15kHz. C) 20kHz. D) 10.7MHz. 7.
AM/FM-108 Radio Baffle NOTICE: Keep the box the kit came in. After you have completed the radio and it operates satisfactorily, you may want to install a baffle to improve the sound. it creates positive pressure on the air in front of it and negative pressure on the rear. At low frequencies, out of phase front and rear waves mix causing partial or total cancellation of the sound wave. The end result is a speaker less efficient and distorted.
3. Bend the top side upward as shown. 4. Bend the two sides upward. Attach the three sides using scotch tape or glue (Elmer’s, Duco Cement, or other). 5. Bend the bottom side upward and attach it to the other sides using scotch tape or glue. Bend the two mounting flaps down. Side View 6. Place the baffle over the speaker. Align the mounting holes and attach it using the two screws and two nuts supplied. The screws should go through the X mark on the baffle flap.
SCHEMATIC DIAGRAM - AM/FM RADIO -54-
TM Elenco Electronics, Inc. 150 W. Carpenter Avenue Wheeling, IL 60090 (847) 541-3800 http://www.elenco.com e-mail: elenco@elenco.
