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When
the trouble has been isolated to the horizontal oscillator-amplifier
section of the receiver, tests should be made to determine
which stage is not functioning-the horizontal oscillator
or the horizontal amplifier. To do this, connect a dc voltmeter
across the grid resistor of the horizontal amplifier. If
the oscillator is operating, voltage will exist across that
resistor having a polarity such that the grid end of the
resistor will be negative. Then briefly stop the horizontal
oscillator by shorting its grid to its cathode and again
check. If the voltage which is available across the grid
resistor disappears when the oscillator is stopped, the
oscillator Is operating; if no voltage is available at first,
or no change is noted when the tube grid is shorted to the
cathode, the oscillator is not operating properly.
If the oscillator is not operating, you should obtain the
service information for the receiver, and check the voltages
in that circuit. If any voltage Is incorrect, check that
portion of the circuit to locate the defect.
If the oscillator is operating, then it is necessary that
you make voltage checks in the horizontal output circuit.
Again, the service information will be useful. (Be sure
to check the screen voltage of the horizontal amplifier
tube. One of the most frequency causes for failure of the
horizontal amplifier stage is a shorted screen bypass condenser).
Also, it would be well for you to check the output of the
B supply for the receiver. If the B voltage is low, decreased
voltage is supplied to the horizontal sweep section. This
decreases the horizontal sweep signal and the high voltage
available. Occasionally the high voltage will decrease sufficiently
to produce the complaint-"no raster."
The Westinghouse Model H-207 receiver uses one section of
the dual-triode 12AU7 for the horizontal discharge section
of the horizontal oscillator. This means that failure of
the 12AU7 dual-triode tube used as a dc restorer and horizontal
discharge tube could cause the oscillator to stop operating.
However, most modern sets use only one tube in the oscillator
circuit.
Sound, No Vertical Deflection. When normal
sound output is obtained, and a thin, bright, horizontal
line appears on the screen of the picture tube, the condition
would be described as, "Sound, raster, no vertical
deflection."
A trouble like this indicates a defect somewhere in the
vertical oscillator-amplifier circuit. A check with Figure
3 shows that a 12AU7 dualtriode tube is used as a vertical
oscillator and a vertical output tube; try a new tube in
this circuit.
If replacing the tube does not correct the complaint, further
circuit tests will be necessary to locate the defective
part. Obtain the service information and make voltage tests.
If any voltage is incorrect, check that circuit for cause.
Sound, Raster, No Horizontal Sync. A condition
like the one shown in Fig. 2 indicates that the horizontal
oscillator is not operating at the correct frequency. Therefore,
the trouble must be in the horizontal sync "chain."
Rotate the horizontal hold control through its range and
notice whether you can obtain a picture that holds still
momentarily and then flops over. If you can pass through
the point of proper sync, but the picture will not lock-in,
it Indicates that the trouble is in one of the circuits
used to supply synchronizing information to the horizontal
oscillator. If you cannot reach the point of proper sync
by adjusting the horizontal sync control, the oscillator
frequency has shifted or a
defect in one of the horizontal sync stages is "pulling"
the oscillator away from the proper frequency.
If rotating the horizontal hold control to one end of its
range will produce a pattern having only three or four slanting
lines, try adjusting the horizontal lock-in range control.
In addition, some sets have a horizontal frequency control
which must be adjusted in certain cases.
If adjusting these two controls will not correct the complaint,
obtain the service information for the set and check the
voltages in the horizontal oscillator circuit. If all of
the voltages in that circuit are correct, check the voltages
in the horizontal sync stages. You are certain to find the
defect In one of these two sections. In making tests of
this type, a complete understanding of the particular circuit
and its operation is an asset.
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