AM RADIO KIT MODEL AM-550K INTEGRAL CIRCUIT, 3 TRANSISTORS, DIODE Assembly and Instruction Manual Elenco Electronics, Inc. ® Copyright © 2007, 1999 Elenco® Electronics, Inc. Revised 2007 REV-S No part of this book shall be reproduced by any means; electronic, photocopying, or otherwise without written permission from the publisher.
PARTS LIST If you are a student, and any parts are missing or damaged, please see instructor or bookstore. If you purchased this kit from a distributor, catalog, etc., please contact Elenco® Electronics (address/phone/email is at the back of this manual) for additional assistance, if needed. DO NOT contact your place of purchase as they will not be able to help you. RESISTORS Qty.
PARTS IDENTIFICATION RESISTORS CAPACITORS SEMICONDUCTORS Diode Resistor 50kΩ Potentiometer with Switch Discap Electrolytic Radial Tuning LM-386 IC Transistor COILS Color Dot Plastic Holders Coil Ferrite Core Antenna Assembly Coil MISCELLANEOUS Screw M2.5 x 3.
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 BAND 2 2nd Digit Color Black Brown Red Orange Yellow Green Blue Violet Gray White Digit 0 1 2 3 4 5 6 7 8 9 Multiplier Color Black Brown Red Orange Yellow Green Blue Silver Gold Resistance Tolerance Multiplier 1 10 100 1,000 10,000 100,000 1,000,000 0.01 0.
INTRODUCTION The Elenco® Superhet 550 AM Radio Kit is a “superheterodyne” receiver of the standard AM (amplitude modulation) broadcast frequencies. The unique design of the Superhet 550 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 Assembly of your AM-550 AM Radio Kit will prove to be an exciting project and give you much satisfaction and personal achievement. If you have experience in soldering and wiring techniques, then you should have no problem with the assembly of this kit. Care must be given to identifying the proper components and in good soldering habits. Above all, take your time and follow these easy step-by-step instructions. Remember, “An ounce of prevention is worth a pound of cure”.
A poorly soldered joint can greatly affect small current flow in circuits and can cause equipment failure. You can damage a PC board or a component with too much heat or cause a cold solder joint with insufficient heat. Sloppy soldering can cause bridges between two adjacent foils preventing the circuit from functioning. What Good Soldering Looks Like Types of Poor Soldering Connections A good solder connection should be bright, shiny, smooth, and uniformly flowed over all surfaces. 1.
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 the speaker. Be careful to properly mount and solder all components. Diodes 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. J1 - Earphone Jack with Nut (see Figure C) SP1 - 8Ω Speaker Speaker Pad Wire 4” (see Figures B & D) Battery Holder 1 Screw 2-56 x 5/16” 2 Screw 2-56 x 1/4” 3 Nuts 2-56 Solder and cut off excess leads.
ASSEMBLY INSTRUCTIONS TP6 - Test Point Pin (see Figure E) C12 - 10μF Lytic Capacitor (see Figure F) C14 - 470μF Lytic Capacitor (see Figure Fa C11 - 10μF Lytic Capacitor (see Figure F) C13 - 470μF Lytic Capacitor (see Figure F) R13 - 47Ω 5% 1/4W Resistor (yellow-violet-black-gold) TP7 - Test Point Pin (see Figure E) U1 - IC Socket 8-Pin U1 - Integrated Circuit LM-386 (see Figure I) C16 - .
You have completed wiring the Audio Amplifier. We shall proceed in testing this circuit. You will need for static measurements, a Volt-Ohm-Milliammeter, preferably a digital type. STATIC MEASUREMENTS RESISTANCE TEST multimeter leads. If you get a reading lower than 100kΩ, check the circuit for shorts or parts inserted incorrectly. Check C14 to see if it’s leaky or inserted backwards. If you get a reading higher than 10MΩ, check for open copper or bad solder connections on all components.
OUTPUT BIAS TEST INPUT BIAS Adjust your VOM to read 9 volts and connect it to test point 7 (TP7) as shown in Figure 8. Make sure that the battery, or a 9 volt power supply (if available), is properly connected and turn the power ON. The voltage at TP7 should be between 4 to 6 volts. If you get this reading, go on to the next test. If your circuit fails this test, turn the power OFF and check that the integrated circuit is correctly inserted in the correct locations.
DYNAMIC MEASUREMENTS AC BANDWIDTH Connect the oscilloscope and audio generator to your circuit as shown in Figure 10. Set the audio generator for a frequency of 1kHz and minimum voltage output. Set the oscilloscope to read .5 volts per division. Turn the power ON and slowly increase the generator output until the oscilloscope displays 2 volts peak to peak (Vpp) at TP7.
DISTORTION Connect the generator and oscilloscope as shown in Figure 10. Set the generator at a frequency of 1kHz, turn the power ON and adjust the generator output until the peaks of the sinewave at TP7 are clipped as shown in Figure 11. Clipped Crossover Distortion Figure 11 Figure 12 Measure the maximum voltage peak to peak when clipping first occurs and record that value here: Vclp = _______ Vpp.
SECTION 2 AM DETECTOR AND AGC STAGES THEORY OF OPERATION 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 13). 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 - AM DETECTOR AND AGC STAGES C6 - 100μF Lytic Capacitor (see Figure F) R8 - 100Ω 5% 1/4W Resistor (brown-black-brown-gold) R5 - 27kΩ 5% 1/4W Resistor (red-violet-orange-gold) T1 - IF Coil (Yellow Dot) T3 - Detector Coil (Black Dot) TP3 - Test Point Pin (see Figure E) TP5 - Test Point Pin (see Figure E) R11 - 3.3kΩ Resistor (orange-orange-red-gold) D1 - 1N4148 Diode (see Figure H) C4 - 10μF Lytic Capacitor (see Figure F) C15 - .001μF Discap (102) C10 - .
DYNAMIC MEASUREMENTS DETECTOR AND ACG TEST Turn the power OFF and connect the VOM and RF generator as shown in Figure 15. ON and slowly increase the amplitude of the 455kHz signal from the RF generator until the voltage at TP3 just starts to drop. This point is called the AGC threshold with no IF gain. Make a note of the amplitude setting on the RF generator here: ____________. Turn the power OFF.
SECTION 3 SECOND IF AMPLIFIER THEORY OF OPERATION The purpose of the SECOND IF AMPLIFIER is to increase the amplitude of the intermediate frequency (IF) and at the same time provide SELECTIVITY. Selectivity is the ability to “pick out” one radio station while rejecting all others. The second IF transformer (T3) acts as a bandpass filter with a 3dB bandwidth of approximately 6kHz. The amplitude versus frequency response of the second IF amplifier is shown in Figure 17.
STATIC MEASUREMENTS Q3 BIAS With the power OFF, connect the negative lead of your VOM to any ground and the positive lead to the emitter of Q3 as shown in Figure 18. Set the VOM to read 9 volts DC and turn ON the power. The voltage at the emitter of Q3 should be approximately 1 volt. If your reading is different by more than 0.5 volts, turn off the power and check your battery of power supply voltage. Also check components R7, R9, R10 and Q3.
BANDWIDTH TEST With the power OFF, connect your equipment as shown in Figure 20. Turn the power ON and adjust the RF generator for .4Vpp at the cathode of D1. If necessary, realign transformer T3 for maximum output while adjusting the output of the RF generator to maintain .4Vpp. Slowly decrease the frequency of the RF generator until the signal drops to .707 of its peaked value or .28Vpp. Record the frequency of the RF generator here: FL=___________kHz.
STATIC MEASUREMENTS Q2 BASE BIAS With the power turned OFF, reconnect your VOM to test point 3 (TP3) as shown in Figure 14. Set the VOM to read 2 volts DC accurately and turn the power ON. The voltage should be approximately 1.5 volts. If your circuit fails this test, turn the power OFF and check Q2 and R6. Q2 CURRENT With the power turned OFF, connect the positive lead of the VOM to the emitter of Q2. Connect the negative lead of the VOM to any DC ground and turn the power ON.
SECTION 5 MIXER AND OSCILLATOR THEORY OF OPERATION In a superheterodyne type receiver the radio wave at the antenna is amplified and then mixed with the local oscillator to produce the intermediate frequency (IF). Transistor Q1 not only amplifies the RF signal but also simultaneously oscillates at a frequency 455kHz above the desired radio station frequency. Positive feedback from the collector to the emitter of Q1 is provided by coil L2 and capacitor C3.
ASSEMBLY INSTRUCTIONS - ANTENNA, MIXER AND OSCILLATOR R1 - 56kΩ Resistor (green-blue-orange-gold) L1 - Antenna (see Figure K) L2 - Oscillator Coil (red dot) C2 - .02μF Discap (203) or .022μF Discap (223) Q1 - 2N3904 Transistor NPN (see Figure G) TP1 - Test Point Pin (see Figure E) C3 - .01μF Discap (103) R2 - 12kΩ Resistor (brown-red-orange-gold) C1 - Tuning Capacitor 3 Screws M2.5 x 3.8mm (see Figure J1) Knob (dial) Label, Dial Knob (see Figure J2) R3 - 3.
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. Slide the ferrite core through the holders. Slide the antenna coil through the ferrite core.
STATIC MEASUREMENTS Q1 BIAS With the power turned OFF, connect the VOM to your circuit as shown in Figure 22. Connect a clip lead from test point two (TP2) to the collector of Q1. This short prevents Q1 from oscillating. Set the VOM to read 2 volts DC accurately and turn the power ON. The DC voltage at TP1 should be 1.6 volts. If the voltage in your circuit differs by more than 0.5 volts, leave the power ON and check the battery voltage. If the battery voltage is greater than 8.
FINAL ALIGNMENTS IF BANDWIDTH After IF alignment, lower the frequency from the RF generator until the reading on the VOM drops to 0.707 of its peaked value. Record the frequency of this lower 3dB corner here: With the power turned OFF, connect the RF generator and the oscilloscope to your circuit as shown in Figure 24. Short TP2 to the collector of Q1 with a clip lead to “kill” the local oscillator. Set the RF generator at a frequency of 455kHz, modulation of 400Hz 80%, minimum amplitude output.
After peaking the oscillator trimmer capacitor, return the RF generator to 540kHz, and capacitor C1 to the fully counter-clockwise position and readjust L2. Repeat the last few steps until both settings of the oscillator are correct. This process sets the oscillator range at 995kHz to 2055kHz. If a frequency counter is available, you may verify this alignment by measuring the frequency at the emitter of Q1 for both ends of the tuning capacitor (C1).
FINAL ALIGNMENT WITH NO TEST EQUIPMENT It is best to use an earphone for this alignment procedure. adjust L2 until a station is heard. Once a station is found and its broadcast frequency is known, rotate the dial until the white pointer is aligned with that station’s frequency marking on the dial. Adjust L2 until the station is heard. Tune the radio until a station around 1400kHz is heard. It may be necessary to listen to the station until their broadcast frequency is announced.
AM-550 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. The final step in the radio kit will be to assemble and attach a baffle to the speaker. You will need to remove the baffle located in the bottom of the box. If it does not want to come out easily, use a knife to cut the holding tabs. When a speaker is not enclosed, sound waves can travel in all directions.
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