Heathkit AR-14 - Page 83

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The signal from C 107 is also applied through iso-
lation resistors R181 and R183 to the Tape Out-
put jack.
Transistor Q5 further amplifies the signal.
A small portion of the signal from the collector
of Q5 is applied as feedback through capacitor
Cll 7 and part of the tone control network to the
base of Q5. The output signal from the col-
lector of Q5 is coupled through capacitor C 119
and resistor Rl 49 to the base of transistor
Q7. Transistors Q7 and Q9 are direct-coupled
amplifiers which further amplify the signal.
The amplified signal from the collector
of Q9 is coupled through capacitor C125 to the
base of driver transistor Qll.
The amplified signal from the collector of Ql 1
is applied to the bases of the output tran-
sistors, Q13 and Q15. Diode Dl0l determines
the AB operating point of the output transistors,
eliminates crossover distortion, and also pro-
vides temperature stability.
Transistors Q13 and Q15 are connected as a
push-pull output stage. The output signal from
this stage is applied through capacitor Cl29 and
the switch contacts on the rear of the Bass
control to the speakers. The output signal is also
applied across a voltage divider network made
up of resistors Rl 73 and Rl 75. The voltage di-
vider applies a portion of the signal through
resistor R151 and capacitor C121 as overall
negative feedback to the base of transistor Q7.
If a stereo headphone set is plugged into the
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Phone jack, the signal is applied to the head-
phones. The speaker can be disconnected using
the switch on the Bass control.
POWER SUPPLY
The fused transformer-operated power supply
uses four silicon diodes, D200 through D203, in
a bridge rectifier circuit. Capacitor C201 filters
the supply voltage for the power output stages.
Resistor R201 and capacitor C202 provide the
supply voltage for the stereo indicator lamp.
An electronic filter, consisting of transistor
Ql00, resistors R205, R206, and R207, and ca-
pacitor C205, provide filtering for preamplifier
transistors, Q3, Q4, Q5, and Q6. The final
filtering for high-gain low-noisepreamplifiers
Ql and Q2, is accomplished by resistor R204
and capacitor C204.
The supply voltage for the Tuner section is ob-
tained through resistors R201, R202, and R203.
Capacitors C202 and C203, provide filtering.
The DC voltage for the pilot lamps is obtained
from the junction of diodes D200 and D202.
Dropping resistor R200 lowers the voltage to
the proper lamp operating voltage. The switched
AC socket and the primary of the power trans-
former are activated by the On-Off switch. The
unswitched AC socket is connected directly
across the line and supplies power contin-
uously.