Samsung LE40A756 video defect

[Davi.p]

Hi, some days ago i've searched for an hour in to the forum without luck, i've used search terms like "streaks" "stripes" "lines", maybe due to my poor english i have miss other terms, the defect is as the pictures, When one first power up it shows no inteligible image but only vertical bars with some fading and moving as the video source (image 1), sometimes it shows like image 2-3, when one switches between video sources the images repair itself much and i can see like image 4-5. I have sprayed with freezer air the tcon and no change (also the little ISL gamma buffer chip) and the mainboard (but without desoldering the CPUs shield), my first doubt was CPU soldering, the mainboard also is keep bent in the back by a metallic sponge so is suspicious. Do you recognize this defect?
Thanks.

[dick_barton]

Re: Samsung LE40A756 video defect

Looks very much like a panel fault.

[Davi.p]

Re: Samsung LE40A756 video defect

Switching it on shows always image 1, but if i detach left 2 ribbons from tcon to panel or right 2, the image shows up (half) but with vertical stripes from lighter items like images 4-5.

[budm]

Re: Samsung LE40A756 video defect

Is the T-Con board mounted on the bottom center of the LCD panel?

[Davi.p]

Re: Samsung LE40A756 video defect

No is the upper center.

[dskall]

Re: Samsung LE40A756 video defect

I would be looking at tab bonds. I have one with smearing on osd. Applied thermal pads on outer tabs and is working so far.

[Davi.p]

Re: Samsung LE40A756 video defect

Maybe mine is different, i've watched 1 video on YTube of the EEVblog series (in the title "Samsung..dumpster dive") and tab bond don't affect, also a technician in an Italian forum with the same model same defect says: Tcon changed, tab bonds touched: no change, , ripple measured:OK.... If anyone remebers how to do a self test mode on the panel i thank he.

[dick_barton]

Re: Samsung LE40A756 video defect

The self test uses the tcon to test the panel so would only eliminate the mainboard if the picture was clear of faults and not the Tcon.

Go to page 29 of the attached pdf

[diif]

Re: Samsung LE40A756 video defect

Often touching the tab bonds does nothing, only when you strip the panel to reveal all of the tabs is it clear that part has come away from the screen.

[dick_barton]

Re: Samsung LE40A756 video defect

I've never successfully sorted out a panel that smears lines down the screen similar to the last photo in post #1, however I'm willing to learn.

[Davi.p]

Re: Samsung LE40A756 video defect

OK thanks i will check the pdf, now i need a pause, too much tests today...

[Davi.p]

Re: Samsung LE40A756 video defect

The test for the LG seemed a bit unconfortable than the one i've done, i have at home another Samsung 46" lcd and i have tested the screen by connecting its power+mainboard, and the defect is the same.. so i think is the panel but is so strange that powering up the tv to the digital tv it shows only stripes, if i swap between inputs or i put in Analog TV the image come back (with little defect).. who knows why?

[Davi.p]

Re: Samsung LE40A756 video defect

Hi i don't think anyone can help me but i say.. I have leaved the screen bezels and tested all the components (only resistors and ceramic caps) on the 2 boards on the top under the frame, nothing strange, i don't have test all the ultra tiny 100ohm resistors because i don't think them centers to the problem and i was not comfortable, then i have done the self tests for the FRC-tcon video output and the mainboard video out that are explained in the service manual, they played very well apart for a fixed big halo that involves some of the colors bars in the upper right area. Th next movement is to separate panel and foils to see if there is some substance inside, but on the top pcb nothing dirt apart a little dust.. BYE.

[Davi.p]

Re: Samsung LE40A756 video defect

Some photos
http://bpssec.altervista.org/Samsung...6-difetto.html
i can notice that this halo is present in the test screens only when there are graphical elements in horizontal..

[Davi.p]

Re: Samsung LE40A756 video defect

I have tested the bigger testpoints on the side tabs of the screen, on each there was 6 bigger than others, but only 2 nudes, the top right tab previously i had noticed warmed a little more than the others (cools), on this and on the next two i have scracthed also the other 4 TP, on all TP (scratched and not) i have measured about 0,7v referred to chassis gnd. Is this normal? I was hoping for a broken trace but not.. are we sure tcon is good? If i don't fix the fault i want to sell all the boards..so..

[Davi.p]

Re: Samsung LE40A756 video defect

E-ehm, i apologize for my poor english :-G....
on the panel's pcb's i have tested all the testpoints, the tensions are repeated near all the tab bonds, apart from two TPs near the rightmost tab bond, one is lower than the others , 0,3v against approx 1v, the very last TP is Vcom34 (as i remember) and it measures 8,7v against 6,3v of all others Vcom TPs, is this a problem? The vcom voltages comes out from 3 buffer chips i7814hs on the tcon, i have changed two of them with 2 i7514hs that i had , usually theese chips are on the wide pcb's on the panel, anyway no changes... bye..