The most likely cause is a dried up main filter capacitor. Once the
effective capacitance drops low enough, 120 Hz (or 100 Hz in countries with
50 Hz power) ripple will make its way into the regulated DC supply
(assuming full wave rectification).
|NotTaR of Television Sets : Reduced width picture and/or hum bars in..
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Another likely cause of similar symptoms is a defective low voltage
regulator allowing excessive ripple. The regulator IC could be bad
or filter capacitor following the IC could be dried up.
Either of these faults may cause:
- A pair of wiggles and/or hum bars in the picture which will float up
the screen. For NTSC where the power line is 60 Hz but the frame rate
is 59.94 Hz, it will take about 8 seconds for each bar to pass a given
point on the screen. (On some sets, a half wave recitifier is used
resulting in a single wiggle or hum bar).
- Hum in the sound. This may or may not be noticeable with the volume
- Possible regulation problems resulting in HV or total shutdown or power
cycling on and off.
The best approach to testing the capacitors is to clip a good capacitor of
approximately the same uF rating and at least the same voltage rating across
the suspect capacitor (with the power off). A capacitor meter can also
be used but the capacitor may need to be removed from the circuit.
Once the capacitors have been confirmed to be good, voltage measurements
on the regulator should be able to narrow down the problem to a bad IC
or other component.
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