Heathkit TA-16 - Schematic

Page 35
POINT-TO-POINT HUM TEST
The following tests are performed by intro-
ducing hum signals into the amplifier circuit.
These (hum) signals are induced into the cir-
cuit with the blade of a small screwdriver which
is held between your fingers; the tip of the
blade is then touched to the base leads of the
transistors in a carefully planned sequence.
Hum should be heard from the speakers when
you touch the base lead of each properly
operating transistor. The hum level should in-
crease as you progress toward the input of the
Amplifier. If no hum is heard, it indicates that
there is a faulty or improperly installed com-
ponent in this area; either the transistor being
touched, or one of the parts in its circuit, or a
component or bad solder connection between
this transistor and the last transistor checked.
Reheat all solder connections in the suspected
area.
If a voltmeter is available, it may help to check
the voltages at the transistors in question. Refer
to the Voltage Chart (fold-out from Page 28) And
Schematic (fold-out from Page 41) for the proper
voltages.
Refer to Figure 2 (fold-out from Page 37) when
making the following checks.
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Remove the amplifier from the cabinet.
Be sure the speaker leads from the am-
plifier are connected to the terminals on the
speaker. Also be sure the shielded cables
from the reverb unit are connected to the
amplifier. Do not connect the foot switch.
Turn all controls fully counterclockwise.
Plug the amplifier line cord into an AC
outlet, and place the On-Off-On switch in
one of its ON positions. Use the ON posi-
tion that gives the loudest hum in the
speakers.
NOTE: The circled numbers preceding the fol-
lowing paragraphs correspond to the circled test
point numbers in Figures 2A and 2B (fold-out
from Page 37). The test procedure in paragraph
0 should be performed at test point 0 in
Figure 2, etc. All the following tests should be
made on the top side of the chassis.
CAUTION: There is line voltage present on the
rear of the LINE REVERSE switch. Therefore,
it is suggested the following tests be made
carefully. Never touch the Amplifier or reverb
chassis with one hand while making tests using
the other hand. It is a good habit to keep your
"free hand" in your pocket or behind you when
making tests, thus avoiding the possibility of
getting a shock.
Power Amplifier Section
o
Touch the tip of the screwdriver to test
point 1 (the base of transistor Q7). A faint
hum or buzzing noise should be heard from
the speakers. If no hum is heard, the diffi-
culty is between stage Q6 and the speakers.
Check transistors Q7, Q8, Q9, and Q10 for
poor solder connections, wiring errors, or
overheated parts.
Touch test point 2 (the base of transistor Q6);
a louder hum than before should be heard.
Normal Channel Preamplifier Section
®
s
Touch test point 3 (the base of transistor Q3)•
again the hum level should increase.
Touch test point 4 (the base of transistor Q1)
and turn up the Normal Channel Volume
control; the hum should increase to an ex-
tremely high level as the Normal Channel
Volume control is rotated clockwise.
Reverb Channel Preamplifier Section
Refer to Figure 2B for the following tests.
®
Touch test point 5 (the base of transistor
Q4); the hum level should be about the
same as that obtained from transistor Q3.
Touch test point 6 (the base of transistor
Q2) and turn up the Reverb Channel Vol-
ume control; the hum should increase to an
extremely high level as the Reverb Channel
Volume control is rotated clockwise.