AM RADIO KIT MODEL SUPERHET AM-550TK 7 TRANSISTORS Assembly and Instruction Manual ELENCO Copyright © 2010, 1999 by ELENCO® All rights reserved. ® Revised 2010 REV-G 752550T 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 AM radio kit from a distributor, catalog, etc., please contact ELENCO® (address/phone/e-mail 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/ Switch with Nut and Washer Discap Electrolytic Radial Tuning Transistor COILS Color Dot Coil Ferrite Core Plastic Holders Antenna Assembly Coil MISCELLANEOUS Knob (pot) Knob (dial) Screw M2.5 . 3.8mm Nut 2-56 Screw 2-56 x 1/4” Screw M2.
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 BAND 2 2nd Digit Digit 0 1 2 3 4 5 6 7 8 9 Color Black Brown Red Orange Yellow Green Blue Violet Gray White Multiplier Digit 0 1 2 3 4 5 6 7 8 9 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 550T AM Radio is a “superheterodyne” receiver of the standard AM (amplitude modulated) broadcast frequencies. The unique design of the Superhet 550T allows you to place the parts over its 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 • Turn off iron when not in use or reduce temperature setting when using a soldering station. The most important factor in assembling your Elenco® Superhet 550T AM Transistor 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.
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 a NPN transistor. Refer the Figure C (page 8) 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 Theory of Operation - 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. In a Class A amplifier (transistor on over entire cycle) the maximum theoretical efficiency is .
ASSEMBLY INSTRUCTIONS - AUDIO AMPLIFIER We will begin by installing resistor R14. Identify the resistor by its color code and install as shown on page 3. 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. Electrolytics have a polarity marking indicating the (–) lead.
ASSEMBLY INSTRUCTIONS Figure F Your kit may contain a different type of earphone jack. Before installing the jack, determine which one you have. Foil Side J1 - Earphone Jack with Nut (see Figure F) Jack Nut 2 3 Speaker Speaker Pad 4” Wire (see Figures G & H) 1 1 - GND 2 - Tip 3 - N.C. Tip Part # 622130 GND Pad Foil Side Battery Holder 3 Screws 2-56 x 1/4” 3 Nuts 2-56 Solder and cut off excess leads. Jack 2 Nut 1 - GND 2 - Tip 3 - N.C.
You have completed wiring the Audio Amplifier. We shall proceed in testing this circuit. You will need a Volt-OhmMilliammeter, preferably a digital type. STATIC MEASUREMENTS RESISTANCE TEST reverse multimeter leads. If you get a reading lower than 20kΩ, 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 150kΩ, check for open copper or bad solder connections on resistors R13 and R14.
OUTPUT BIAS TEST Adjust your VOM to read 9 volts and connect it to test point 8 (TP8) as shown in Figure 5. Make sure that the battery, or a 9 volt power supply (if available), is properly connected and turn the power ON. The voltage at TP8 should be between 4 to 5 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 all of the transistors are correctly inserted in the correct locations.
DYNAMIC MEASUREMENTS DC GAIN Adjust your VOM to read 9 volts DC. Connect the positive lead of the VOM to TP6 and the negative lead to any ground. Turn the power ON and record the voltage at TP6 here: Once again, parallel resistor R13 with resistor R4 as shown in Figure 6. The voltage at TP8 should also drop to a lower voltage. Record the new reading at TP8 here: V1=________ volts. V4=__________ volts. Place resistor R4 across resistor R13 as shown in Figure 6.
AC GAIN Connect the VOM and generator to TP6 as shown in Figure 7. Turn the power ON. Normally the AC gain is measured at a frequency of 1 kilohertz (kHz). Your VOM, however, may not be able to accurately read AC voltages at this frequency. It is recommended, therefore, that this test be performed at 400Hz. Set the generator at 400Hz and minimum voltage output. Set your VOM to read an AC voltage of 1 volt at the output of your Audio Amplifier.
AC BANDWIDTH Connect the oscilloscope and generator to your circuit as shown in Figure 8. Set the 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 TP8. Move the oscilloscope probe to TP6 and record the input voltage here: waveform on the oscilloscope drops to .7 of its original reading, 1.4 Vpp or 2.8 divisions.
DISTORTION Connect the generator and oscilloscope as shown in Figure 8. Set the generator at a frequency of 1kHz, turn the power ON and adjust the generator output until the peaks of the sinewave at TP8 are clipped as shown in Figure 9A. Clipped Crossover Distortion A B Figure 9 Measure the maximum voltage peak to peak when clipping first occurs and record that value here: The waveform on your oscilloscope should resemble Figure 9B.
SECTION 2 AM DETECTOR AND AGC STAGES THEORY OF OPERATION is to maintain a constant audio level at the detector, regardless of the strength of the incoming signal. Without AGC, the volume control would have to be adjusted for each station and even moderately strong stations would clip in the final IF amplifier causing audio distortion. AGC is accomplished by adjusting the DC bias of the first IF amplifier to lower its gain as the signal strength increases.
ASSEMBLY INSTRUCTIONS - DETECTOR C6 - 100μF Lytic (see Figure B) R8 - 100Ω Resistor (brown-black-brown-gold) R5 - 27kΩ Resistor (red-violet-orange-gold) T3 - IF Coil (black) T1 - IF Coil (yellow) TP3 - Test Point Pin (see Figure A) C4 - 10μF Lytic (see Figure B) TP5 - Test Point Pin (see Figure A) R11 - 3.3kΩ Resistor (orange-orange-red-gold) D1 - 1N4148 Diode (see Figure E) C9 - .02μF or .022μF Discap (marked 203 or 223) C15 - .001μF Discap (marked 102) R12 - 2.
DYNAMIC MEASUREMENTS DETECTOR AND ACG TEST Turn the power OFF and connect the VOM and RF generator as shown in Figure 13. Set the VOM to accurately read 2 volts DC and set the output of the RF generator for 455kHz, no modulaton, and minimum amplitude. Turn the power 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.
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 15.
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 16. 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 17A. 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.
STATIC MEASUREMENTS Q2 BASE BIAS With the power turned OFF, reconnect your VOM to test point 3 (TP3) as shown in Figure 12. 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 frequencies except those near 455kHz. T1 also couples the 455kHz signal to the base of Q2 to be processed by the IF amplifiers. 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.
ASSEMBLY INSTRUCTIONS - ANTENNA, MIXER AND OSCILLATOR R1 - 56kΩ Resistor (green-blue-orange-gold) L1 - Antenna with Holders (see Figures I & J) L2 - Oscillator Coil (red) C2 - .02μF or .022μF Discap (marked 203 or 223) Q1 - 2N3904 Transistor NPN (see Figure C) TP1 - Test Point Pin (see Figure A) C3 - .01μF Capacitor (marked 103) R2 - 12kΩ Resistor (brown-red-orange-gold) C1 - Tuning Gang Capacitor 2 Screws M2.5 x 3.8mm Knob (dial) Screw M2.5 x 8mm Label (dial knob) (see Figure K) R3 - 3.
Your kit may contain a 3 lead or a 4 lead capacitor. Bend the leads as shown. Fasten C1 into place on the top side of the PC board with two M2.5 x 3.8mm screws. Knob Post Fasten the knob to the shaft of the capacitor with one M2.5 x 8mm screw. M2.5 x 8mm Screw C1 Turn the dial fully clockwise. Remove the protective backing from the label and align the 1600 with the arrow on the PC board.
STATIC MEASUREMENTS Q1 BIAS With the power turned OFF, connect the VOM to your circuit as shown in Figure 19. 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 With the power turned OFF, connect the RF generator and the oscilloscope to your circuit as shown in Figure 21. 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. Set the oscilloscope to read 0.1Vpp and turn the power ON. Increase the amplitude of the RF signal until the oscilloscope registers 0.5Vpp.
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).
AM ALIGNMENT WITH NO TEST EQUIPMENT It is best to use an earphone for this alignment procedure. Rotate the tuning knob fully counterclockwise and place the label on the knob with the white arrow pointing at the 540kHz marking. their broadcast frequency is announced. If no stations are present at the low side of the AM band, adjust L2 until a station is heard.
SCHEMATIC DIAGRAM -30-
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