3.5
3.6
obtain a marker indication when the trap is
properly aligned. This increase in marker sig-
nal amplitude can cause errors in i-f align-
ment by causing overload of the i-f stages in
the receiver.
This basic coverage of the operation of the
sweep, marker and pattern circuits should be
well understood because these are the basic
building blocks for the VIDEO, IF, CH 4 and
CH IO functions of the Model 415.
PIX OSCILLATOR AND CHROMA FUNCTION
The primary purpose of Pix Oscillator Q402 is
to furnish a pix carrier signal at the television
i-f frequency. This is added to the output of
the 44 MHz sweep oscillator. The use of this
frequency will be outlined in the later sections
on MODEL 415 functions.
CHANNEL 4 AND CHANNEL 10 MIXING
OSCILLATORS Q301 AND Q303
Channel 4 Mixing Oscillator Q301 produces a
crystal-controlled signal frequency at 113 MHz.
The difference between this frequency and the
i-f pix carrier frequency (45.75 MHz) is 67.25
MHz, which is the r-f pix carrier frequency for
Channel 4. The difference between
ll3 MHz
and the i-f sound carrier (41.25 MHz) is 71.75
MHz, which is the r-f sound carrier frequency
for Channel 4. This procedure can be used to
find the Channel 4 r-f equivalent of each i-f
marker reference frequency. This is further
explained in Section 3.8. Also, by subtracting
the i-f frequency spectrum (38 MHz to 48 MHz)
from 113 MHz the Channel 4 frequency spec-
trum (75 MHz to 65 MHz) is obtained. Note
that in the subtraction process the
low end of
the i-f sweep range corresponds to the
high
end of the Channel 4 frequency range and
vice versa.
In television receiver tuners, the local oscil-
lator operates
above the incoming signal by
an amount equal to the i-f frequency. When
exactly adjusted by the fine tuning adjust-
ment, the oscillator is running at 113 MHz.
Therefore, when the 75 MHz to 65 MHz sweep
range of Channel 4 is fed into the tuner,
it is
converted down to the original 38 MHz to 48
MHz sweep range.
These principles are used in the Model 415 to
generate a Channel 4 signal. When the FUNC-
TION Switch of the Model 415 is in the CH4
position, the output of Channel 4 Oscillator
Q301 is mixed with the output of the 44 MHz
sweep oscillator at Mixer Amplifier Q302.
This frequency combination is coupled through
the FUNCTION switch and the attenuator to
the RF-IF-VIDEO OUT jack and into the anten-
na terminals of the receiver under test. Be-
cause the Channel 4 mixing frequency is
crystal controlled, all the i-f markers described
in Par. 3.2 have the same frequency relation-
ship to the television receiver i-f response as
if the i-f sweep signal had been injected at
the mixer grid (or base, if transistorized).
The same description of Channel 4 signal
generation applies to the Channel IO function.
In this case, Channel 10 Oscillator Q303 gen-
erates a 239 MHz signal which mixes with the
38 MHz to 48 MHz i-f sweep from the 44 MHz
sweep oscillator to produce the 201
MBz to
191 MHz Channel IO sweep signal. This is
converted in the television receiver tuner to
the 38 MHz to 48 MHz sweep. Again because
of crystal control of the Channel IO oscillator,
the internal i-f post-injection markers have the
same relationship to the i-f response curve as
if the 38 MHz to 48 MHz sweep had been in-
jected at the mixer.
25
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