Repairing Your TV EDID

[Andreas] found that his home theater PC would not boot one day. Oddly, if he disconnected his HDMI cable from his TV, it would boot fine.  While most of us would have dug into the PC, he realized that it was a signal from the TV that was incorrect.  Luckily, LG had included a full schematic with the TV.  What he was able to figure out, using a home made snooper was that the EDID eeproms had somehow become corrupted. Not to worry, [Andreas] slapped together a full blown I2C interface and prepared to reprogram them with the correct data.  He noticed, however, that the eeproms were write protected.  On a whim, he decided to write to them any way and found that it was successful. He has some theories as to why they were writable, but says that he doesn’t want to pull the TV back apart to confirm.

[thanks evlapix]

31 thoughts on “Repairing Your TV EDID

  1. A modern, consumer TV that includes schematics??!!!

    Color me astonished.

    If any company were to do this with their current models, I know I’d be replacing all my TVs with those. Even if just to show them that it’s the right thing.

  2. Yeah I’m with ‘Whoever’, I used to love that devices half the time came with schematics and used it with great success to mod them, but then they stopped doing that once everything went SMD, and I guess a law changed maybe? Anyway I haven’t seen a schematic included for years, but it would be damn cool if they started doing that again.

  3. No, if you read it, it looks like the EEPROMs were corrupted due to faulty write protection circuitry.

    I’m very surprised they included schematics on such a recent piece of equipment. Nicely done, LG!

  4. Neat, I wonder if he will rebuild the EDID info?

    I think there are some apps that can build you a custom one, or you can read it from another of the same TV, possible even just one with similar specs.

    If you don’t want to write the Edid chip can you pull that pin from a cable and use that cable with the offending display?

    I know with VGA monitors you can do this (for various reasons I don’t recall at the moment.)

  5. I wonder too if he could have simply cut the DDC wires or are they used for other things than DDC in the HDMI standard? Or are they even a requirement and won’t it work at all then?

    With VGA they had to allow it since BNC connected monitors don’t use data channels, and BNC used to be a standard for high-end use, but with HDMI they might have gone nasty and decided to make datachannels enforced.

  6. LG included a schematic ? Something very odd about that. Maybe LG has realized the access to their service centers sucks.

    He used the parallel port but you could also use the bus pirate boards and do it a lot quicker.

  7. @whatnot

    About 15 years ago I was the owner of a warranty repair center for GE. We got everything from tv to cordless phones , they paid $45 labor to repair a $20 phone , something about customer must get back the same item. Anyway, we had thick notebooks holding about 1000 pages each with schematics and service manuals to all the devices. The shelves holding the manuals took up a entire wall, there was about 25 of them . Later updated to cdrom and could pull them up on a pc.

    If something came in we didn’t have the manual for we had to pay $30 to get it from sams photofact. They still exist today online

    For me now the problem with devices is not the schematic it is identifying the chips themselves. They have started doing one off chip production runs where they only mark the chips with batch numbers . Sometimes you get lucky and they have the company logo of who made the chip but sometimes even that doesn’t help. Often the service manuals will show the connections to the chip and what voltages should be present but still leave out what the internals of the actual chip are.

    To make it worse companies like broadcom, oh how I hate thee, are so paranoid about their designs you can’t even get datasheets unless you get a lawyer and have wads of cash to throw at them.

  8. I one had a a repair man replace the lcd on my samsung 47″ – the guy looked like a plumber turned tv repairman, my 9 yr old daughter could have done the job (aside from the lifting). There was only about 4 wires inside the whole tv aside from the speaker connecs and the power…

    He accessed some type of manager mode using a thumb drive and some button code, I tried to get it out of him, but he wouldn’t budge.. Maybe I should have bribed him with a 12 pack or a $20?

  9. what i’m understanding here is that you can write malware to screw people’s TV HDMI info and botch their sets

    basically this makes for another problem that bewilders most tech people as much as grandma is bewildered by gmail. fun times!

  10. The EEPROM was not write protected. Pin 7 is the Write Control line and it is active low. Pulling it to ground allows the EEPROM to be written to. Pulling it high would write protect it.

  11. I don’t get why he couldn’t have done the writes over the PC’s SM bus instead of hacking up an HDMI cable. There is a Linux userspace lib for it, just as there is with the parport.

    I think he’s actually read out a page from the EDDC data rather than the EDID. That could be why the preamble doesn’t look right. There’s another register in the TVs EDID PROM to switch between pages. Maybe that got poked by something running on his PC and it just got left in that state.

  12. @nubie – It was only two bytes that was corrupted in the EEPROM. After fixing the header, the EDID checksum matched.

    @James – This is the biggest mystery of all. Perhaps it’s just a bug in the GPU BIOS. I don’t suspect these type of erros happen that often

    @nes – I didn’t know that the driver exposed the i2c bus until after I rewrote the EEPROM. However, it seems the nVidia driver does not allow write access to the EDID ROM. I did not investigate why.

  13. Man, that is worrying.
    I’m thinking of replacing my dead old Packard Bell EHR2080 DVR with a home theatre PC.
    I dont want to plug a GPU into my brand new 40″ Sammy only to have it corrupt the TV’s firmware (if thats how it happened)!
    I’m into electronics and home repairs myself (Tried to repair the DVR to no avail, the PSU outputted correct voltages on all rails, problem was on the mainboard with those scary big SMD chips!) but I wouldn’t even know where to begin to fix my TV!
    And no, Samsung did NOT provide schematics.
    Great work there Andreas!

  14. @Andreas re. E-DDC pages, there’s a rough description on WP:
    HDMI sets have to have it, and unless I’m mistaken that’s a 256 byte EEPROM in your set, meaning you could have one extra page of EDDC. E-DDC ROMs are meant to reset to reading the first page if anything goes wrong so that older equipment (e.g. your BIOS maybe) can still recognize the display. However some I2C EEPROMs don’t implement this and will maintain their state even if power cycled.

    Just saying that /could/ be what had happened here.

  15. Don’t worry about a home pc corrupting eprom data on your tv. This hack is an example of how NOT to design a display. Most manufacturers are aware that the EDID data does not need to be accessed by anyone except service personnel and will block write access to the chip unless the display is in service mode.

  16. Site’s back.
    Interesting concept. The site he referenced had more interesting information, and an interesting hack regarding what can be done to confuse a system running Windows from the point of view of a system running Linux.

  17. This might be a way to insert custom resolutions into a stock monitor, I noticed this “feature” on my Samsung lappy.
    The panel actually supports 3D via alternate frame but as there is no way to output the sync signals to glasses I’d need to use the webcam LED or something creative.

    Another related hack, modify a cheap tablet to do 3-D by adding a simple LM1881 on the sync output to send the required IR pulse.

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