EEVblog #794 - Samsung Dumpster LCD Repair Connector Followup - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: September 15, 2015, 12:22:19 pm
content: After repairing the melted connectors on the processor board, will the dumpster dive Samsung LCD TV work?A quick follow-up video to this disaster:

author: firewalker
author_rank: Super Contributor
timestamp: September 15, 2015, 12:35:39 pm
content: Let it RIP.Alexander.

author: smashIt
author_rank: Contributor
timestamp: September 15, 2015, 05:25:35 pm
content: somehow it looks like there is some charge building up on the vertical electrodesbut this should be impossible in a plasmascreenmaybe there is something wrong with the column-drivers

author: open loop
author_rank: Regular Contributor
timestamp: September 15, 2015, 07:34:59 pm
content: I had a similar experience with an iMac 27" where a replacement panel was like $$$$ just waited and watched on eBay for a smashed screen to come up. Bagged a replacement smashed panel but with a good T-con spelling? board for labout 15 pounds or so. Should be easy to find a broken TV (smashed screen) on eBay which uses the same panel. You may even get the remote you don't have. With a remote you can often get into diagnostic modes with a "secret" key sequence. Tried downloading the service manual? Apologies for not checking other threads on the forum.

author: GNU_Ninja
author_rank: Regular Contributor
timestamp: September 16, 2015, 11:08:05 am
content: Kudos to David2 on sorting that connector; I wouldn't have the patience

author: Godzil
author_rank: Frequent Contributor
timestamp: September 16, 2015, 04:08:41 pm
content: Poor (little) thingStupid idea, as there is a software running on theses things, could it be just a stupid software issue?Something that got corrupted that is about configuring the pannel frequency or equivalent

author: DanielS
author_rank: Frequent Contributor
timestamp: September 16, 2015, 08:15:44 pm
content: I'm thinking corrupt firmware - a flipped bit or two in the image processing firmware could explain most of the issues.Too bad there probably is no easy way to re-image the chips with known-good firmwares to test that out.

author: bktemp
author_rank: Super Contributor
timestamp: September 16, 2015, 08:38:48 pm
content: Quote from: DanielS on September 16, 2015, 08:15:44 pmHow would a bad firmware explain the accumulation of intense colour values?As already said, there are several facts pointing to an analogue problem on the TCON board (VCOM/gamma voltages or maybe even too low VGH/VGL).Those opamps tend to fail quite often, because they run hot on large screens. A new TCON board will probably fix it, but that would be both expensive and boring. If it is one of the VCOM/GAMMA buffers it will be a cheap an easy repair.I had a similar problem: Bought a new gamma buffer ic on ebay for 5€, replaced the old one and the problem was solved.

author: retiredcaps
author_rank: Super Contributor
timestamp: September 16, 2015, 09:10:11 pm
content: Quote from: bktemp on September 16, 2015, 08:38:48 pmWas it the AS-15 by any chance?

author: DanielS
author_rank: Frequent Contributor
timestamp: September 16, 2015, 11:49:57 pm
content: Quote from: bktemp on September 16, 2015, 08:38:48 pmHow about a software error in the DSP-based image processing/scaling? For example: not resetting the state of a line filter buffer or a missing buffer flip so you end up accumulating indefinitely in one buffer until it saturates.Quote from: bktemp on September 16, 2015, 08:38:48 pmThe main issue I have with this theory is that the color intensity seems to affect text exclusively. The window borders and other non-text graphical elements do not appear to produce any visible streaking. If it was a timing, drive or other general issue, I would expect it to affect everything on-screen in a similar fashion.If it was an analog heat-related issue, I would also have expected a reaction from Dave's earlier freeze-spraying rounds but he got absolutely no change whatsoever from that.

author: VL
author_rank: Newbie
timestamp: September 17, 2015, 01:39:43 am
content: It would be interesting to see what happens when the TV gets a proper 1920x1080 60Hz input signal over HDMI and the OSD disappears... Would that make the screen problems go away? If not, displaying some patterns and moving colored/black/white boxes around, would those cause different error patterns to show up, or would they clear away that red pattern where the OSD bled over the screen?

author: Tek_TDS220
author_rank: Regular Contributor
timestamp: September 17, 2015, 02:43:07 am
content: Regarding the brittle connectors, they were likely brittle before the heat treatment. Why? Since it melted, it must have been a cheap thermoplastic like PBT. Manufacturers load up the thermoplastic with fillers to drive down the cost even more and raise the heat distortion temperature. Too much filler makes the plastic brittle.I haven't been impressed with the quality of Samsung equipment. I paid more for a Samsung TV because I thought it would be more reliable than a Won Hunglo (sp?). It turns out they share a lot of the same parts.

author: bktemp
author_rank: Super Contributor
timestamp: September 17, 2015, 05:33:21 am
content: Quote from: retiredcaps on September 16, 2015, 09:10:11 pmYes. But I have also seen other gamma buffers failing. Most manufacturers seem to underestimate the required capacitve current this buffers have to deliver.Quote from: DanielS on September 16, 2015, 11:49:57 pmLook at the first video, at the end is a test image:Every bright pixel generates this effect.

author: EEVblog
author_rank: Unknown
timestamp: September 17, 2015, 06:15:28 am
content: Quote from: bktemp on September 16, 2015, 08:38:48 pmI suspect it's something like that too.I'd like a schematic before I went changing chips willy-nilly (even if I can get them).I haven't had a really good sit-down with the TCON board though and investigated it at the circuit level. IIRC I checked all the labelled voltages.

author: bktemp
author_rank: Super Contributor
timestamp: September 17, 2015, 06:48:11 am
content: I would start by trying to identify the analogue part of the TCON board, ignoring everything digital:There are some boost converters for generating all the analogue supply rails. You have already measured one in the first video.There should be at least 3 voltages:- Vanalogue (+9 - +16V, this generates the power for all analogue parts of the display and TCON board)- VGH (+15 - +25V, only for switching the fet gates on)- VGL (-5 - -16V, only for switching the fet gates off)But there are also opamps for all the gamma voltages. There are usually 9-20 voltages generated either using a resistive divider chain or using DACs embedded in the gamma buffer.Depending on the panel, either some or all channels are buffered.Since the TCON is part of the LCD panel it is usually not included in service manuals for TVs. But this is not a big problem as long as you are able to find datasheets for the parts:If you have identified the buffers, you can compare the input and output voltage. If they differ, the buffer or its supply has failed.

author: amyk
author_rank: Super Contributor
timestamp: September 17, 2015, 10:51:45 am
content: I would suspect the gamma voltages if the lines moved around depending on the image displayed - you can use gradient test patterns to determine this.If the lines stay in the same place, it's probably localised to the column/row drivers.Interesting thread on column/row drivers in Russian repair forum:http://monitor.net.ru/forum/topic375748-96.html

author: Psi
author_rank: Super Contributor
timestamp: September 17, 2015, 10:57:05 am
content: The fact that it only happens on columns that have OSD text makes me pretty sure its a problem with a digital section.

author: bktemp
author_rank: Super Contributor
timestamp: September 17, 2015, 11:04:40 am
content: Quote from: Psi on September 17, 2015, 10:57:05 amLook at Dave's previous video. It is on all pixels that exceed a certain brightness level.

author: bktemp
author_rank: Super Contributor
timestamp: September 17, 2015, 11:27:32 am
content: Quote from: amyk on September 17, 2015, 10:51:45 amYou are probably right.But I am still pretty certain it is an analogue voltage problem:For all who do not know how a TFT LCD works a short explanation:The source driver generates a voltage according to the desired brightness of the pixel. This is done in parallel for all subpixels in each row. Yes, there are virtually 5760 DACs inside the source drivers!This analogue voltage is stored into a tiny capacitor in each pixel. The row is selected by N channel FETs driven by the row drivers. The image data is fed into the source pin (therefore they are called source driver). When there is a positive voltage on the gate, this voltage charges the capacitor. A negative voltage is applied to all other rows to hold the value in the capacitor. All 1080 lines are written this way in each frame.Since a LCD gets destroyed by a DC signal, the voltage generated by the source drivers is inverted in each frame. The liquid crystals are beeing effected only by the absolute value of the voltage.A possible explanation for the visible effect:If the negative gate voltage is missing, a high enough negative voltage on the source pin might switch the fets on and the source voltage leaks into each pixel on the same column. Since this only happens for the negative polarity, a DC offset is created. This leads to image sticking and will eventually damage the LCD when applied for a long time.

author: ornea
author_rank: Unknown
timestamp: September 17, 2015, 01:37:54 pm
content: You may not have had success with this one but it gave me the confidence to open a dead plasmaand have a crack.Replaced 1 dead 22ohm 5w resistor for $0.40 now working like a bought one. Owner very happy.Thanks Dave and David.

author: retiredcaps
author_rank: Super Contributor
timestamp: September 17, 2015, 04:46:35 pm
content: Quote from: ornea on September 17, 2015, 01:37:54 pmLet me guess, some sort of LG 50PK550 series or like?

author: Rasz
author_rank: Super Contributor
timestamp: September 17, 2015, 07:25:09 pm
content: Quote from: bktemp on September 16, 2015, 08:38:48 pmthis! either driver/buffer or broken voltage supply connection on the ribbons (locate test points on flex near COF chips and buzz them out to the tcon)Quote from: EEVblog on September 17, 2015, 06:15:28 amYou wont find one, Tcon is considered a non serviceable part + it is usually designed and build by the manufacturer of the panel. No tv/monitor manufacturer provides diagrams for them. Whats more chips used on them are also almost always custom, or very specialized and heavy NDAed with no public datasheets, most are from small niche Taiwan companies like himax, parade, novatek, richtek.Quote from: EEVblog on September 17, 2015, 06:15:28 amWould be great if you could use your vast nerd connections and find someone who does TCON/LCD panels for a living, and talk him into AmpHour interview. Its very niche knowledge, so I suspect manufacturers train people from the grounds up.

author: JustSomeGuy
author_rank: Newbie
timestamp: September 17, 2015, 09:31:22 pm
content: The Daves makes a point of saying the streaking does not start appearing until after running a while. Specifically, the TV is turned on around 3:38 and Dave2 kick-points at the screen around 3:50 in the video. That's 12s worth of video before failure. I would like to suggest this is an overheating problem.The streaks remind me of a video card I once had that would work find until it warmed up. Once warmed up streaks just like these appeared. Properly affixing the heat sink on the card resolved the issue permanently.I can not help but wonder if a "Freeze Test" would be indicated as a next step. Similar to what he did in video #565.

author: Godzil
author_rank: Frequent Contributor
timestamp: September 18, 2015, 07:05:36 am
content: Dave already done a freeze test without any luck

author: monkeysuncle
author_rank: Contributor
timestamp: September 18, 2015, 12:53:32 pm
content: My recollection of the previous videos in this series are that Dave focused on the column drivers. Could vertical stripes like this be caused by leaking row drivers? Like to know what happens if you physically probe or freeze-spray the chips and connections related to the row drivers.

author: bktemp
author_rank: Super Contributor
timestamp: September 18, 2015, 01:08:08 pm
content: Quote from: monkeysuncle on September 18, 2015, 12:53:32 pmIt looks so. Since the whole panel is affected all row driver ics beeing bad is unlikely. So I don't think the actual panel or the row or column drivers are the problem.It could be a bad connection somewhere between the TCON and the panel or a failed voltage on the TCON board.@ DaveLook for a negative voltage on the TCON board or on the LCD panel. Maybe it is labeled VGL or GOFF.It is usually generated using a charge pump near the main boost converter on the TCON board, because they tap of the signal from the switching node.

author: repairfanatic
author_rank: Newbie
timestamp: September 21, 2015, 11:21:51 am
content: Hi mr. dave jones.i saw your video about the samsung with vertical lines and bars showing up after turning the tv on. the problem is the lcd screen itself. but the tv can still be fixed without replacing the panel or the lcd screen.we had similar walk in units with the same problem and managed to fix them. what we do is we increase the Voff voltage to -16 volts. with some bigger sized TVs we increase it to -18 volts. that negative voltage is responsible for the TURN OFF STATE of the TFTs. there is a feedback resistor for the Voff regulation mr jones. lessen its value and you will notice the Voff voltage will go up.