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INTRODUCTION

MIXER

LOCAL

OSCILLATOR

FIRST

IF AMPLIFIER

SECOND

IF AMPLIFIER

DETECTOR

TRANSISTOR

AUDIO

AMPLIFIER

AGC

Figure 1

Speaker

Antenna

Section 1

Section 2

Section 3

Section 4

Section 5

The Elenco

®

Dual Audio Superhet 550C AM Radio is

a  “superheterodyne”  receiver  of  the  standard  AM

(amplitude  modulated)  broadcast  frequencies. The

unique design of the Superhet 550C 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. It is very important,

however, that good soldering practices are used to

prevent bad connections. The Soldering Guide should

be reviewed before any assembly is attempted.

The  actual  assembly  is  broken  down  into  five

sections. The theory of operation for each section, or

stage, should be read before the assembly is started.

This will provide the student with an understanding of

what that stage has been designed to accomplish,

and how it actually works. After each assembly, you

will  be  instructed  to  make  certain  tests  and

measurements  to  prove  that  each  section  is

functioning  properly.  If  a  test  fails  to  produce  the

proper results, a troubleshooting guide is provided to

help  you  correct  the  problem.  If  test  equipment  is

available, further measurements and calculations are

demonstrated to allow each student to verify that each

stage meets the engineering specifications. After all

of  the  stages  have  been  built  and  tested,  a  final

alignment  procedure  is  provided  to  peak  the

performance of the receiver and maximize the Dual

Audio Superhet 550C’s reception capabilities.

The  Dual  Audio  Superhet  550C  can  best  be

understood by analysis of the block diagram shown

in Figure 1.

The purpose of section 1, the Audio Amplifier Stage,

is to increase the power of the audio signal received

from the detector to a power level capable of driving

the  speaker.  The  audio  amplifier  is  switchable

between transistor or IC function.

Section 2 includes the detector circuit and the AGC

(automatic gain control) circuit. The detector converts

the amplitude modulated IF (intermediate frequency)

signal to a low level audio signal. The AGC stage

feeds back a DC voltage to the first IF amplifier in

order to maintain a near constant level of audio at the

detector. Section 3 is the second IF amplifier. The

second IF amplifier is tuned to 455kHz (kilohertz) and

has a fixed gain at this frequency of 100. The 3dB

bandwidth  of  this  stage  should  be  approximately

6kHz. Section 4 is the first IF amplifier which has a

variable  gain  that  depends  on  the  AGC  voltage

received from the AGC stage. The first IF amplifier is

also tuned to 455kHz and has a 3dB bandwidth of

approximately 6kHz. Section 5 includes the mixer,

oscillator and antenna stages. When the radio wave

passes  through  the  antenna,  it  induces  a  small

voltage  across  the  antenna  coil.  This  voltage  is

coupled  to  the  mixer,  or  converter,  stage  to  be

changed to a frequency of 455kHz. This change is

accomplished  by  mixing  (heterodyning)  the  radio

frequency signal with the oscillator signal. Each of

these blocks will be explained in detail in the Theory

of Operation given before the assembly instructions

for that stage.

GENERAL DISCUSSION

IC AUDIO

AMPLIFIER

AM-550CK_REV-A_010416.qxp  1/19/16  9:03 AM  Page 5

Содержание AM-550CK

Страница 1: ...S 2 DIODES Assembly and Instruction Manual Copyright 2016 2011 by ELENCO Electronics Inc All rights reserved REV A Revised 2016 753008 No part of this book shall be reproduced by any means electronic...

Страница 2: ...3 Identifying Capacitor Values Qty Symbol Value Description Part r 1 C1 Variable Tuning 211677 r 1 C15 0 001mF Discap 102 231036 r 2 C3 C10 0 01mF Discap 103 241031 r 5 C2 C5 C7 C8 C9 0 02mF or 0 022m...

Страница 3: ...bly Coil Color dot Earphone Jack with Nut Knob dial Radio Stand COILS MISCELLANEOUS Label Screw M1 8 x 7mm Screw M2 5 x 4mm Nut M1 8 Speaker Pad 50kW Potentiometer Switch with Nut and Washer Knob pot...

Страница 4: ...egative lead is indicated on the packaging by a stripe with minus signs and possibly arrowheads Also the negative lead of a radial electrolytic is shorter than the positive one Polarity marking BAND 1...

Страница 5: ...alignment procedure is provided to peak the performance of the receiver and maximize the Dual Audio Superhet 550C s reception capabilities The Dual Audio Superhet 550C can best be understood by analy...

Страница 6: ...industry but it is now being replaced by lead free solder for health reasons This kit contains lead free solder which contains 99 3 tin 0 7 copper and has a rosin flux core Lead free solder is differe...

Страница 7: ...Your meter should again be reading a high resistance Typical results read approximately 1MW to infinity Refer to parts list and find a PNP transistor refer to Figure B page 8 for locating the Emitter...

Страница 8: ...istors are working as Class A amplifiers for very small levels of power to the speaker but they slide toward Class B operation at larger power levels Transistor Q4 is a Class A amplifier that drives t...

Страница 9: ...gray red brown gold D2 1N4148 Diode see Figure E R16 1kW Resistor brown black red gold TP10 Test Point Pin see Figure F R15 100W Resistor brown black brown gold Electrolytics have a polarity marking...

Страница 10: ...solder side of the PC board Step 1 Step 3 Step 2 PC Board solder side J1 Earphone Jack with Nut see Figure G Speaker Speaker Pad 4 Wire see Figures H I Battery Holder 3 Screws M1 8 x 7mm 3 Nuts M1 8...

Страница 11: ...r shorts or parts inserted incorrectly Check C14 to see if it s leaky or inserted backwards If you get a reading higher than 150kW check for open copper or bad solder connections on resistors R13 and...

Страница 12: ...The voltage should be between 0 5 and 0 8V higher than the voltage at TP8 All silicon transistors biased for conduction will have approximately 0 7V from the base to the emitter If your circuit fails...

Страница 13: ...voltage at TP8 here V3 __________ volts 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 her...

Страница 14: ...d minimum voltage output Set your VOM to read an AC voltage of 1 volt at the output of your Audio Amplifier Slowly increase the output of the generator until the VOM reads 1 volt AC Leave the audio at...

Страница 15: ...ncy from the generator until the waveform on the oscilloscope drops to 0 7 of its original reading 1 4 Vpp or 2 8 divisions Use the oscilloscope probe to check TP6 to make sure the input voltage did n...

Страница 16: ...imum power output should be greater than 200 milliwatts MAXIMUM POWER OUTPUT Connect the generator and oscilloscope as shown in Figure 8 Set the generator at a frequency of 1kHz turn the power ON and...

Страница 17: ...Figure 12b if a capacitor is placed between pins 1 and 8 The gain can be set to any value from 20 to 200 if a resistor is placed in series with the capacitor The amplifier in our kit with a gain of 5...

Страница 18: ...tch Notch marking STATIC MEASUREMENTS IC AUDIO AMPLIFIER SW2 on the down IC position RESISTANCE TEST You have completed wiring the IC Audio Amplifier We shall proceed in testing this circuit You will...

Страница 19: ...gure 16 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...

Страница 20: ...TP10 10mF Generator GND TP10 Connect the oscilloscope and audio generator to your circuit as shown in Figure 18 Set the audio generator for a frequency of 1kHz and minimum voltage output Set the osci...

Страница 21: ...nnect the generator and oscilloscope as shown in Figure 18 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...

Страница 22: ...o at the volume control as shown in Figure 19 The purpose of the automatic gain control AGC circuit is to maintain a constant audio level at the detector regardless of the strength of the incoming sig...

Страница 23: ...the VOM is adjusted to read 9 volts DC and turn the power ON The voltmeter should read approximately 1 5 volts DC If your reading varies more than 0 5 volts from this value turn the power OFF and che...

Страница 24: ...clockwise position Slowly adjust the amplitude of the RF generator output until you hear the 1kHz on the speaker If this test fails turn the power OFF and check C11 R12 volume control D1 and TP3 SYSTE...

Страница 25: ...in Figure 23 Both IF amplifiers are tuned to a frequency of 455kHz and only need to be aligned once when the radio is assembled These amplifiers provide the majority of the gain and selectivity needed...

Страница 26: ...ulation and minimum amplitude output Set the oscilloscope vertical sensitivity at 1 volt division The scope probe must have an input capacitance of less than 50pF or it will detune transformer T3 Turn...

Страница 27: ...23 BANDWIDTH TEST FIRST IF AMPLIFIER The operation of the first IF amplifier is the same as for the second IF amplifier with one important difference The gain of the first IF amplifier decreases after...

Страница 28: ...Figure 27 This short prevents the AGC from lowering the gain of the first IF ampifier Set the RF generator to 455kHz no modulation and minimum amplitude output Set the oscilloscope for a vertical sen...

Страница 29: ...he selectivity of the antenna Q factor is high the image will be reduced sufficiently The oscillator circuit must also change when the radio is tuned in order to remain 455kHz above the tuning of the...

Страница 30: ...white to the hole marked 1 4 Wire Type Antenna Solder the 4 colored wires to the PC board r Wire A green to the hole marked 3 r Wire B red and black twisted together to the hole marked 2 r Wire C whi...

Страница 31: ...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...

Страница 32: ...he power ON Increase the amplitude of the RF signal until the oscilloscope registers 0 5Vpp Align transformers T3 T2 and T1 for the maximum AC reading on the oscilloscope Decrease the amplitude of the...

Страница 33: ...na coil may have to be on the end of the ferrite rod as shown in Figure 33 Change the frequency of the RF generator to 1400kHz and adjust C1 until a 400Hz tone can be heard coming from the speaker Car...

Страница 34: ...the station is at its loudest and reduce the volume if necessary Retune the radio for another weak station and repeat this procedure until there is no more improvement noticed on the weakest possible...

Страница 35: ...result is a speaker less efficient and distorted To eliminate the low frequency cancellation a speaker is placed inside an enclosure Now the front sound waves are prevented from traveling to the back...

Страница 36: ...SCHEMATIC DIAGRAM ELENCO 150 Carpenter Avenue Wheeling IL 60090 847 541 3800 Website www elenco com e mail elenco elenco com...

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