Elenco Electronics AM-550TK Instruction Manual download pdf (Page 20)

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SECTION 3

Figure 15

.707

452kHz 458kHz

455kHz

TP4 - Test Point Pin

(see Figure A)

T2 - IF Coil

(White)

Q3 - 2N3904 Transistor NPN

(see Figure C)

R10 - 470Ω Resistor

(yellow-violet-brown-gold)

R7 - 39kΩ Resistor

(orange-white-orange-gold)

R9 - 10kΩ Resistor

(brown-black-orange-gold)

C7 - .02μF or .022μF Discap

(marked 203 or 223)

C8 - .02μF or .022μF Discap

(marked 203 or 223)

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.

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 to separate the radio stations.

The gain at 455kHz in the second IF amplifier is fixed

by the AC impedance of the primary side of transformer

T3, and the DC current in Q3. The current in Q3 is set

by resistors R7, R9 and R10. Both C7 and C8 bypass

the 455kHz signal to ground, making Q3 a common

emitter amplifier. The signal is coupled from the first IF

amplifier to the second IF amplifier through transformer

T2. The IF transformers not only supply coupling and

selectivity, they also provide an impedance match

between the collector of one stage and the base of the

next stage. This match allows maximum power to

transfer from one stage to the next.

SECOND IF AMPLIFIER

THEORY OF OPERATION

ASSEMBLY INSTRUCTIONS - SECOND IF AMPLIFIER

1 2 ... 15 16 17 18 19 20 21 22 23 24 25 ... 31 32

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