Heathkit AR-14 - Circuit Description

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Page 67
CIRCUIT DESCRIPTION
Each major section of the Receiver will be
described separately in the following Circuit
Description. For ease of explanation, the Source
switch will be described in the FM position.
Follow the circuit on the Block Diagram (fold-
out
from Page 66) and on the Schematic (fold-
out
from Page 79) while reading the Circuit
Description. The letter-number designations
(R4, Cll5, R212) for all resistors, capacitors,
and diodes have been placed into the following
groups to make them easier to locate on the
chassis and Schematic.
1 - 99
100 - 199
200 - 299
FM TUNING UNIT
FM tuner section.
Amplifier section.
Power supply section.
The FM signal from the antenna is applied to the
primary of balanced input transformer Tl in the
FM tuning unit. The secondary of transformer Tl
forms a tuned circuit with trimmer capacitor Cl
and capacitors C2 and ClA (antenna section of
tuning
capacitor). The signal selected by this
tuned circuit is coupled through capacitor C3 to
RF amplifier transistor Ql.
The signal is amplified by transistor Ql. The RF
tuned circuit of Ql selects the desired signal
and couples it through capacitor C8 to the base
of mixer transistor Q2. The RF tuned circuit con-
sists of coil L2, trimmer capacitor C6, and ca-
pacitors C7 and C6A (RFportionoftuningcapac-
itor).
The local oscillator transistor, Q3, operates ata
frequency that is 10. 7 me higher than the received
FM signal. The oscillator frequency is de-
termined by a tuned circuit composed of coil L4,
trimmer capacitor C14, and capacitors C13 and
C14A (oscillatorportionoftuningcapacitor). The
output signal from this oscillator is coupled
through capacitor Cl0 to the base of mixer
transistor Q2.
A small DC voltage is coupled from the ratio de-
tector circuit through resistor R5 to diode Dl
in the collector circuit of the oscillator. This
DC voltage, which changes as the tuning is
changed, is u~ed as an AFC (automatic fre-
quency control) voltage to lock-in the local
oscillator frequency with the station being
tuned in.
The oscillator frequency is locked-in by the AFC
voltage in the following manner: The capacitance
between the elements of diode Dl changes when
the AFC voltage that is applied to it changes.
This capacitance is connected in series with
capacitor Cl9, and these two capacitances are
connected in parallel with part of coil L4. Thus,
when the capacitance of diode Dl is changed by
the AFC voltage, the total capacitance across
coil L4 is changed. This change in the tuned
circuit capacitance changes the frequency of the
oscillator in such a way as to maintain proper
tuning.
The oscillator and the received FM signals are
mixed in transistor Q2 to produce a 10. 7 me IF
(intermediate frequency) signal, that is coupled
through transformer T2 and capacitor C21 to
the first IF amplifier transistor, Q4. The
amplified IF signal from the collector of tran-
sistor Q4 is coupled through transformer T3
to
the base of second IF amplifier transistor
Q5. This IF signal is again amplified by tran-
sistor Q5, coupled through transformer T4,
and amplified by transistor Q6. From Q6 the
signal is coupled through transformer T5 and
amplified by the fourth IF amplifier transistor
Q7.
A portion of the signal voltage is taken from the
collector of transistor Q6 and rectified by diode
D2 to produce an AGC (automatic gain control)
voltage. This AGC voltage, which increases and
decreases with the strength of the received FM
signal, is coupled through resistors Rl4 and
Rl2 to the base of transistor Q4, where it
automatically controls the gain of the IF signal.
The AGC voltage is then coupled fr.om the emitter
of transistor Q4, through resistor Rl0, to the base
of RF amplifier transistor Ql. This automat-
ically controls the gain of the FM signal in
the tuner section.