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Sound,
Raster, Picture, Vertical Roll. When a condition
like that shown in Figure 4 is encountered, the first step
will be to determine whether the trouble is in the vertical
oscillator circuit, or in the sync circuits. If vertical
sync pulses are not available at the Input of the oscillator,
the picture will be unstable, and the setting of the vertical
hold control will be critical. If the trouble is in the
vertical oscillator circuit, usually it will be impossible
to adjust the vertical hold control so that a normal picture
is obtained even momentarily.
Fig. 4. Vertical rolling. The motion may
be up or down, fast or slow, depending
upon the setting of the vertical hold coltrol.
To
determine whether the vertical roll is caused by a defect
in the vertical oscillator circuit, or a defect in the sync
circcult, carefully adjust the vertical hold control and
notice whether or not you can make the pleteure roll up,
and then roll down.
If you cannot adjust the hold control so that the picture
will stop momentarily, the defect is probably in the vertical
oscillator circuit itself; if you can adjust the oscillator
to the proper frequencey, but the picture will not hold,
the defect is probably in the sync circuit.
By referring to our lay-out diagram, we find that a 12AT7
is used as the horizontal and vertical sync amplifier, and
a 12AX7 is used as the horizontal and vertical sync clipper.
In this particular set, one-half of the 12AT7 is used to
amplify the vertical sync pulses, and failure of that section
of the tube could cause vertical roll. Also, half of the
12AX7 is used as the vertical sync separator, and failure
of that half of the 12AX7 could cause vertical roll. Therefore,
the serviceman should try replacing each of these tubes.
Very few of the receivers built today use one separator
for the vertical sync pulses and another separator for the
horizontal sync pulses; instead one tube is used for both
functions.
When this type of circuit is used, a defect in the sync
separator-amplifier stage will cause the picture to roll
both vertically and horizontally. Therefore, that complaint
usually indicates a defect in one of the sync circuits,
and tests should be made in those circuits.
Raster, Picture, But No Sound. This complaint
indicates a defect in a circuit that is used only for the
sound.
With the set turned on and the volume control set at its
mid-position, remove the audio output tube from its socket.
A sharp click will be heard in the speaker if the output
transformer and the output tube are in good condition. Then
remove and replace the 6T8 ratio-detector-audio amplifier.
If that stage is operating, a click will be heard. This
same testing procedure can be used with the second sound
i-f tube, and the first sound i-f tube, etc.
By using this procedure, you will be able to locate the
defective stage. If replacing the tube used in the circuit
does not correct the complaint, voltmeter tests and ohmmeter
tests will be necessary to locate the defective part.
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