Heathkit TA-16 - Page 42

Page 38
CIRCUIT BOARD X-RAY VIEW
(VIEWED FROM FOIL SIDE)
PRIMARY
CIRCUIT
BREAKER
R101 R102 C101
CIRCUIT BREAKER
NOTE: Transformer T1 shown wired
for 120 V AC.
R105 R106
SECONDARY
CIRCUIT
BREAKER
R114 R116 RI15 R113 R112 R111 R109
CHASSIS PHOTOGRAPH5
C102 R107 R108
CIRCUIT DESCRIPTION
The Guitar Amplifier circuit consists of two
preamplifier channels, a power amplifier, and a
power supply. Each preamplifier channel has
two inputs plus Volume, Bass, and Treble con-
trols. The normal preamplifier channel is gen-
erally used for the accompaniment guitar, or
some other instrument, and a microphone; the
reverb channel, which features variable re-
verb and tremolo, is generally used for the
lead guitars.
Each section of the Guitar Amplifier circuit
will be described separately. As you read the
description, refer to the Block Diagram (fold-
out from Page 37) for signal flow, and refer
to the Schematic (fold-out from Page 41) for
circuit details.
NORMAL CHANNEL PREAMPLIFIER
Signals from the Normal input jacks are coupled
through isolation resistors R101 and R102. These
signals are mixed at the junction of R101
and R102; the resulting signal is coupled through
capacitor Cl to the base of first preamplifier
transistor Ql, a special low-noise transistor.
The amplified signal from the collector of Q1
is coupled through capacitor C2 to the Volume
control. From the arm of the Volume control,
the signal is coupled through resistor R104,
the Bass control, and capacitor C3, and through
the Treble control and capacitor C4 to the base
of second preamplifier transistor Q3.
The signal is amplified and inverted in Q3,
therefore, the signal at the collector is out of
phase with the signal that was applied to the base.
This out-of-phase signal is applied back to the
Bass control through capacitor C5 and resistor
R6, and back to the Treble control through re-
sistor R7.
With the input signal at one end of the Bass
control and an out-of-phase signal at the other
end, there will be a point near the center of the
control where bass response will be essentially
the same as that of the input signal. As the arm
of the control is moved counterclockwise, more
of the out-of-phase signal is applied to the base
of Q3 and a cut in bass response results; as the
arm of the control is moved clockwise, more of
the input signal is applied to the base of Q3 and
a boost in bass response results.
Capacitor C101, across the Bass control, by-
passes higher frequencies, limiting the effect
of the control to the lower frequencies. Capac-
itor C3, between the arm of the control and the
base of Q3, has a high value to pass the low
frequencies with very little attenuation.
The Treble control works the same way as the
Bass control. The main difference between the
bass and treble circuits is that the treble
coupling capacitor, C4, has a low value to block
low frequencies while passing high frequencies
with very little attenuation.
The tone compensated signals present at the
collector of Q3 are coupled through capacitor
C6 and resistor R27 to the base of mixer ampli-
fier transistor Q6.
REVERB CHANNEL PREAMPLIFIER
The circuits of transistors Q2 and Q4 and the
reverb channel tone control network are identical
to the circuits in the normal channel, and oper-
ate in the same way. The output signal from the
collector of second preamplifier transistor Q4
follows two paths: One path is through capacitor
C14, resistors R24 and R25, and capacitor C15
to the base of mixer transistor Q6; the other
path is through resistor R43 and capacitor C23
to reverb driver transistor Q11.
THE REVERB DELAY CIRCUITS
This reverb channel signal is amplified by tran-
sistor Q11 and then is coupled through capac-
itor C25 to the reverberation unit, which
consists of two transducers and two delay
springs. The input transducer changes the elec-
trical signal into physical motion, which is de-
layed slightly as it travels down the springs to
the output transducer. The output transducer
changes the physical motion back into an elec-
trical signal, which is coupled to the Reverb
control. The signal at this point is essentially
the same as the input signal, except that it is
delayed slightly.