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Application
General
Eliminating
AGe Action
Because the agc voltage in a TV receiver varies in
accordance with the signal fed through the if amplifier
to the agc rectifier, a varying dc voltage is present on
the agc bus. This voltage is applied to the grids of
some or all of the if-amplifier tubes and may also be
fed to the grid of the
rf
amplifier tube. If this voltage
is not rendered inoperative during alignment, difficulty
may be experienced in shaping the response curve, and
final results may be misleading.
This difficulty may be avoided by either disconnect-
ing the age system or rendering it inoperative through
application of an external amount of fixed bias which
is
the same as that provided during normal operation
of the agc system. The RCA WG-307B TV Bias Supply
(See Figure 5) is recommended. This supply furnishes
three output voltages continuously
adjustable
from 0
to -15
volts, as well as -100
volts (fixed) for use in
color-TV receivers.
If the RCA WR-69A Television/FM
Sweep Genera-
tor is employed in the alignment setup, bias may be
taken from the bias-supply terminals on the generator.
In addition to being a valuable aid during if alignment,
the external bias voltages are highly useful in tracking
down trouble in agc circuits.
Alignment Test Setups
In sweep-frequency
alignment, the sweep generator
is tuned to sweep the band of frequencies
normally
passed by the wide-band circuits in the TV receiver,
and a trace representing the response characteristics
of
the circuits will be displayed on the oscilloscope.
The
WR-99A is used to provide calibrated
markers along
the response curve for checking the frequency settings
Figure
S.
RCA WG-307B
TV Bias Supply.
of traps, adjustment
of capacitors
and coils, and for
measuring overall bandwidth
of the receiver.
When the marker signal from the WR-99A is coupled
into the test circuit, a vertical "pip" or marker will
appear on the curve. When the WR-99A is tuned to a
frequency within the pass band accepted by the re-
ceiver, the marker will indicate the position of that
frequency
on the sweep trace.
The technician
then
adjusts the circuit components
to obtain the desired
waveshape, using the different frequency
markers as
check points.
The order in which various sections of the television
receiver should be aligned may differ between different
models of receivers.
In all cases, the alignment order
given by the manufacturer
in his service notes should
be followed.
It is not possible, therefore, to recommend a single
alignment procedure which can be applied with equal
success to all television receivers.
Instead, the applica-
tion data given in the following pages are designed for
use in conjunction
with the manufacturer's
service
notes to aid the technician in aligning a receiver cor-
rectly and efficiently.
Receiver
alignment
requires,
in addition
to the
WR-99A Calibrator,
a sweep generator
having essen-
tially fiat output and good sweep linearity, a cathode-
ray oscilloscope,
and a vacuum-tube
voltmeter.
An
RCA WR-69A or WR-59-series Sweep Generator,
an
RCA WO-9lA Oscilloscope, and an RCA VoltOhmyst*,
such as the WV-77E, WV-87B, or WV-98B are recom-
mended. A marker-adder
unit, such as the RCA WR-
70A RF
IIF
/VF Marker Adder, can be used to advan-
tage in the alignment setup.
Tuner Alignment
To clear up any misconception
that a tuner is a
complicated
device, consider the tuner when stripped
to its essentials.
Except for the switching arrangement
and the usual high and low-pass filters, it is about as
simple as the input of a broadcast receiver.
This fact
should be remembered when trouble-shooting
problems
arise which are common to all channels. In these cases,
it is good practice to work with the tuner set to only
one channel position
until the trouble
is corrected.
Afterwards,
other channel positions can be compared
·TMK Reg. U.S. Pat. Off.
• 10 •
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