Elenco Electronics AM-550TK Instruction Manual download pdf (Page 5)

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INTRODUCTION

MIXER

LOCAL

OSCILLATOR

FIRST

IF AMPLIFIER

SECOND

IF AMPLIFIER

DETECTOR

AUDIO

AMPLIFIER

AGC

Figure 1

Speaker

Antenna

Section 1Section 2Section 3Section 4Section 5

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. 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

Superhet 550T’s reception capabilities.

The Superhet 550T 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. 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

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Elenco Electronics AM-550TK Instruction Manual Page 5

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