Elenco AM Radio Kit Manuel d'utilisateur télécharger pdf (Page 5)

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INTRODUCTION

MIXER

LOCAL

OSCILLATOR

FIRST

IF AMPLIFIER

SECOND

IF AMPLIFIER

DETECTOR

TRANSISTOR

AUDIO

AMPLIFIER

AGC

Figure 1

Speaker

Antenna

Section 1Section 2

Section 3

Section 4Section 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

1 2 3 4 5 6 7 8 9 10 ... 35 36

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Elenco AM Radio Kit Manuel d'utilisateur Page 5

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