Due to the different kinds of power supplies available, the designs are different in so many aspects, but the following components never lack, and they are usually the cause of many problems in the TV Power supply section:
1. A power switch, relay, or triac to enable main power.
2. A set of rectifiers - usually in a bridge configuration - to turn the AC into DC. Small ceramic capacitors are normally placed across the diodes to reduce RF interference.
3. One or more large filter capacitors to smooth the unregulated DC voltage.
4. A degauss control circuit usually including a thermistor or Posistor (a combination of a heater disk and Positive Temperature Coefficient (PTC) thermistor in a single package).
The TV experiences Blackouts, either intermittently or permanently
The most immediate cause may be the power wall outlet may be faulty; one should check and confirm that it’s working first. After this has been cleared, work up the cord and check for continuity up to the TV power switch. Check for a faulty power switch. Check the mains transformer and confirm that both sections, primary and secondary, are functional.
Loss of Raster, No sound
This may be the result of a break in the PCB or an open component. Check the switch, then the fuse, power resistors, rectifier diodes and the choke inductor. If these devices are confirmed to give the right output, move to the following sections of the TV for trouble shooting:
- Degauss Posistor, check for shorts
- Horizontal output transistor.
- Power supply regulator (if there is one).
Loss of Raster, Weak Sound
This is usually caused by a leaky or open filter capacitor. If the readings are confirmed in their right output levels, move to the following sections of the TV for Trouble shooting:
- Video Amp Output
- Sound Processing Strip.
Dim, Shrunk or Poor Raster Linearity
This symptom is usually accompanied by a humming sound. It’s caused by a reduction in the supply voltage. Check the rectifier diodes or the filter capacitor for correct readings. Then check the main rectifier diode for any defects. There should be a defective component among these. If not, follow the power supply lines on the PCB and confirm continuity. There could also be a case of a loose component in the power supply section.
Overheating
This is a consequence of excessive current demand and is marked by repeated fuse blowing. This is caused by a shorted component especially in the turns of the power transformer. To find the defective component, it may be visually obvious or one has to isolate it for the circuit for testing. Check all the components of the power supply.
Conclusion
Sometimes the solution is not as obvious as mentioned here, thus other conventional approaches are not disqualified.
Reference:
- Samuel M. Goldwasser, Notes on the Troubleshooting and Repair of Television Sets, Copyright © 1994-2007
- Samuel M. Goldwasser, http://www.repairfaq.org
- Alvin A. Liff, J.A. Sam Wilson, Color and Black and White Television, Theory and Servicing, 3rd Edition©1993
- Prentice Hall Inc. upper
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