JVC LT-42DG1 AAW grey blotches on screen

[maxvalutech]

This set came to me with a sound problem which was fixed by service menu reset. I have been testing it for a few days as the owner told me that problem had been fixed before but only lasted 2 days before returning - offhand I think her remote control since lost may have gone wonky.
To test I often put sets together especially as my wife is a big tennis/cricket fan. Last night after set had been on for around 5 hours just it seemed out of nowhere these 2 grey blotches appeared on top rhs. At first I thought just shadows but they stayed in place on channel change etc. This morning turned set on and no sign of these blotches. Doesn't seem like tcon more like ccfl but they tend to be vertical. Any ideas ??

[maxvalutech]

Re: JVC LT-42DG1 AAW grey blotches on screen

Set turned on again and after about an hour first small sign of blotch nearest mid screen. It slowly gets darker and starts to extend down the screen. Due to background often hard to see even on white but other times very evident. Now been on a couple of hours and I think the second one faintly showing. Strange !!

[dskall]

Re: JVC LT-42DG1 AAW grey blotches on screen

Doesn't look like backlight. I would say panel problem.

[maxvalutech]

Re: JVC LT-42DG1 AAW grey blotches on screen

You are likely very right as now I see a trace of similar blotch on lhs coming from near top of screen and extending down about one third of screen - still very faint but it is there.

[freakaftr8]

Re: JVC LT-42DG1 AAW grey blotches on screen

This is called mura. Its what happens to LCD panels when it starts to break down from undue stress to the panel, panel matrix has been overheated alot, or irregularities in production of the panel that start to break down prematurely. I have seen this alot in Sharp LCD panels, when gate drivers fail, and are removed from one side, forcing the remaining side to overwork with the voltages provided.

One assumption is that a gate or a source driver embedded I to the LCD panel used for video production pulls too much current, starving the others so they work undercurrent, causing the LCD panel to have these dark spots that VCOM or AVDD cannot be spread evenly through the LCD panel..

Essentially one area suffers while other areas are fine, requiring less current than supplied to some drivers, known as COF, (chip on film)

Basically the LCD panel is bad in will probably get worse over time until the areas affected are completely black.

[maxvalutech]

Re: JVC LT-42DG1 AAW grey blotches on screen

Great explanation. I have in mind someone with vision problems who needs a bigger set and this may suit him for a number of months at least.

[maxvalutech]

Re: JVC LT-42DG1 AAW grey blotches on screen

Well 5 months and this set is back with me. Backlights OK as is sound and menu , source, no signal panels show on screen but no picture. Could this be that picture power needs result in no picture due to the failing panel as basically set out above. But I do add that a full blue screen is sometimes displayed with no sign of faults.

[maxvalutech]

Re: JVC LT-42DG1 AAW grey blotches on screen

Made enough space in my very limited work area to get av input via DVD to this set and the picture is OK from that source so that answers my question at least as far as this TV is concerned. I do note that previous sound problem of low to very high at mid range has returned - this was fixed in service menu reset of sound range so it does look as if there is a problem on main board with settings and this may extend to rf input as set tunes in OK but does not display rf.