Question about Sanyo DP42746 42 in. Plasma Television

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The tv display has no image but audio is present the backlight of the tv is not showing at all and even if you put a flashlight on a angle in the display it will not reveal any image being produced, what can be the cause of this can it be the main board?

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Does the backlight come on when you first power up the tv and then go out? This would indicate a backlight issue. There are usually several backlight inverters which all stop when one is broken depending on which one. You cannot check the voltage with a typical multimeter because it is very high, but you can disconnect one at a time until you get a display... it will be dim, but this will tell that it is a backlight issue and which inverter is the problem. It may be a CPU issue also... Honestly, for that tv, you're better off buying a new one. Repairing it isn't cheap or easy.

Posted on Dec 31, 2013

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Posted on Jan 02, 2017

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I'm afraid that while their intention was good, that most the posted solutions miss the mark. Let me guide you thru the diagnosis and possible cure. This is detailed and lengthy, since these points appear often in LCD TV/Monitor repair. You may wish to warm up your soldering iron and skip to Step (11) for the repair procedure.

(1) We know that much of the Main Power Supply is working since many of the audio, video and digital processing sections appear to be working (sound, remote control, tuner, and video out).

(2) We know that the Backlight Inverter is working. Even thought the screen appears to be dark, light from the backlight tubes can clearly be observed. Further, even thought the screen is dark, you *can* observe a small amount of the backlight thru the screen.

(3) When working on LCD TVs and Monitors, the typical problem is backlight failure. (Fortunately this is NOT the case here.) Backlight failure is most often due to Backlight Inverter failure, or Main Power Supply failure. Rarely do the bulbs fail outright (although occasionally thru abuse a CCFL tube is cracked or shattered).

(4) The typical failure mechanism on LCD TV/Monitors that have many in-service hours on them is CCFL tube aging. As the CCFL tubes get older they require increasingly higher voltages to maintain the proper regulated current thru them which in turn creates the correct light output. The higher voltage is often beyond the design limit of the Backlight Inverter leading to component failure. Typically discharge breakdown arcing on the output transformers (that drive the CCFL lamps), or drive transistor failure in the Backlight Inverter power supply. Alternatively, the Main Power Supply may fail. Typically the Main Power Supply supplies +12V, or +15V, or a higher voltage (+24V?) to the Backlight Inverter subsystem. Once again, as the lamps age, more current is drawn, and if the Backlight Inverter doesn't fail outright, it draws excessive current from the Main Power Supply leading to either Main Power Supply failure. Alternatively, the Main Power Supply protects itself by shutting down once the excessive current is detected. This can often be seen as "monitor cycling" where by a LCD Monitor will initially power up, but subsequently cycles every few seconds as the Backlight Inverter draws too much current causing the Main Power Supply to shut down. Then, after a brief recovery period, the cycle repeats.

(5) Another failure mechanism, that should not be overlooked, is Electrolytic Capacitor failure. The Internet has many sites documenting either manufacturing defects in Electrolytic Capacitors used in any of the subsystems (Backlight Inverter, Main Power Supply, Digital Signal Processor, etc.). However, more often than not, Capacitor failure is due to component stress-due to the high ripple currents present in inadequately designed switching power supply subsystems. Additionally, many inferior Electrolytic Capacitors of Chinese origin are inadequately designed-they lack trace chemicals in the Electrolyte necessary to assure long service life.

(6) If you suspect any of the mechanisms described which cause an absence of CCFL backlight, you can often use a small pocket flashlight, and observe the screen image *is* in fact present on the LCD panel, but in the absence of sufficient backlight, you mistakenly believed the LCD panel to be dead.

(7) This is not the case here. We can clearly see that the CCFL backlight *is* lit, but we also do *not* observe any image on the LCD panel, even with an external flashlight.

(8) Thru this diagnosis of exclusion we assume that either the LCD panel itself is defective, or that the LCD panel is not receiving the correct drive signals from the Digital Signal Processor subsystem.

(9) A cursory examination reveals that the Digital Signal Processor board is producing output activity, which-even if malfunctioning-would likely produce some sort of LCD display activity.

(10) The LCD panel is totally dark (backlight is on, but no image at all). This failure is so absolute, it leads one to suspect power related problems as opposed to logic or drive problems.

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