Elenco Electronics AM-550CK, Assembly And Instruction Manual

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Figure 23
.707
452kHz 458kHz
455kHz
T2 - IF Coil
(White)
TP4 - Test Point Pin
(see Figure F)
Q3 - 2N3904 Transistor NPN
(see Figure A)
R10 - 470
W
Resistor
(yellow-violet-brown-gold)
R7 - 39k
W
Resistor
(orange-white-orange-gold)
R9 - 10k
W
Resistor
(brown-black-orange-gold)
C7 - .02
m
F or .022
m
F Discap
(marked 203 or 223)
C8 - .02
m
F or .022
m
F 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 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 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
SECTION 3
AM-550CK_REV-A_010416.qxp 1/19/16 9:04 AM Page 25