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XBR-75X940D array backlight works but LCD panel shows no picture

With a video input, I can see the array backlight move with the picture, but the LCD screen itself is black. Even on-screen menus don’t appear - the LCD is not functional.

There are three boards in the signal path - the main board (BM1), the DPS board, and the Timing Control board. The fact that the backlight is active with video input suggests that the main board is fine, though the speakers don’t seem to work which suggests that it may not be.

The DPS board sits between the main board and both the Timing Control board and the array backlight control board. I can find very little information on how to diagnose it.

The Timing Control board has 12.6V at its power connector and the on-board fuse is fine. I have confirmed that the panel is not shorted.

The links between each board are high speed digital, and I don’t have a logic analyzer. I’m not sure how to go about narrowing it down to which of the three boards warrants replacement.

Répondre à cette question J'ai le même problème

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I have:

1) Verified supply rails to each board are correct and stable

2) Re-seated and inspected all connectors

3) Replaced main (BM1) board, DPS board, and T-con board

At this point, it's looking likely (though uncommon?) that the panel has completely and totally failed on a 4 year old TV. Disappointing.

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Hi @hikeonpast

The service manual (your model number seen on p.2) seems to indicate that the cable to the tcon board, the mainboard or the panel itself may be the problem

Here’s the no picture troubleshooting flowchart from the service manual which may help.

Having no audio also seems to put it back to the mainboard as it is not just a problem with the video section of the TV

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Thanks. I've already reviewed the service manual in detail. It seems to presume a service tech that has one of each component as a spare and focuses on the optimal order to do trial replacements rather than showing how to definitively determine which part has failed.

I've reseated all of the FFCs (flex cables) - the service manual suggests one is the most likely to be the problem. In my experience, FFCs don't just fail without physical or thermal damage.

I've ruled out the panel itself being the issue by testing each half separately. That points to either main board or T-con board, though it's strange that the manual doesn't include the possibility of a defective DPS board also, which sits between main board and T-con board.

Agreed that audio may be the smoking gun here. The odd thing is that I do get audio feedback (bing!) from menu interaction (even though I can't see it), so the audio issue seems limited to external sources (only tested HDMI so far).

par

@hikeonpast

It may be that in tracing the no audio that it may help in finding the video problem perhaps as far as checking voltages on the boards. If a voltage is missing for an audio circuit it may imply that the supply from which it is being derived is missing as well which may be being fed to the video section.

Just what I'd try anyway.

par

Thanks; good call. The PSU is 12V only, and all other rails are produced locally on each board. I've confirmed that the 12V is solid at the input to each board, and have confirmed all the 5V and 3.3V rails that I've been able to identify. It's clear that Sony is not a fan of built-in test points for field service. Electrolytic caps are fine (it's only a 4 year old TV).

Chasing the audio is tricky. Menu-generated audio works while HDMI audio does not. Audio goes over I2S from the main board to the DPS board, and through the DPS SOC to a D/A and amplifier on the DPS board. I'm guessing that the main board is the one that runs android, hosts the UI, and generates menu sounds. If true, then the DPS board is fine and the main board is suspect in that it generates menu sounds but does not pass video sounds.

The haunting thought that I had last night: Since I use an AVR and not the TV speakers, did I disable the TV speakers in the config when I first installed the set. Can't confirm without video...

par

@hikeonpast

Try an analogue signal input type i.e. Component video + audio and see if there is any audio.

par

That's a good suggestion, but my concerns are:

1) It would be a non-deterministic test because I can't see which input the TV is assigned to. It would be stabbing in the dark to try to change to an analog input so I couldn't really trust the lack of audio via analog input as being conclusive.

2) I can't rule out that I disabled the TV speakers when I did the initial install. I also can't confirm that setting because I can't see the menus and there is no RS232 port or other means to change configs. To me that discounts the value of chasing missing audio as a clue to the LCD issue.

I really appreciate your suggestions; please keep them coming.

par

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@hikeonpast this “though the speakers don’t seem to work “ combined with your no video situation leads me to make the main board the number one suspect. Of course, you will have to check every video input source i.e. component, coax etc. and see if that makes a difference. Also, make sure that you disconnect any cable or sat boxes and try a local source like a game console/BR/ DVD etc.

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Thanks, and agree that missing audio seems like the smoking gun pointing to the main board. That said, it seems odd to me that a failed main board would allow the array backlight to receive enough chroma signal to behave normally.

As I mentioned, the LCD doesn't show a picture even with menus - i.e. video generated within the TV itself, so while it may indeed be the main board, it's not input-specific as far as I can tell.

par

which is more readily available or cheaper of the suspect boards? try replacing that board first and then to the next....

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Hi there,

I have exactly the same issue, and have replaced both the t-con and main boards. I get the system sound (Sony music background) but no image. The backlight is working though. I read on a post that the power board could be an issue, but after reading your comments, it seems less probable.

Update (09/06/2021)

Here’s the picture of the power connector to the T-con board.

Block Image

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Hi JP,

Sorry to hear that your set is having issues. I replaced each of the logic boards in sequence: Main board, T-CONs, DPS board. I verified that the power supplies were stable, but did not replace either supply board.

My conclusion was that the panel itself failed, and that the TV was unfixable. Other than replacing the board-board interconnect cables (which were all re-seated) I'm not sure what else I could have swapped. I spent dozens of hours trying to get the TV to work, with zero results.

I reached out to Sony, asking for a discount on a replacement given the price of the 940D and failure in less than 4 years. I was told no way.

After a bunch of (begrudging) research into new TVs, I bought a new LG OLED. I couldn't afford an OLED when I bought the 940D, but they have come down in price significantly. While I'm not happy that my 4-year old Sony died, and was bummed that Sony wouldn't stand behind it, I couldn't be happier with my new LG OLED.

par

When in board swapping mode, I was surprised how many replacement parts were available on ebay - I had no problem finding multiple vendors with each of the boards that I intended to replace.

Only after I gave up on repairing the TV did it dawn on me why there were so many spare boards in circulation: The panel is the weak link in this TV, so everybody with a dead TV parts it out and sells the boards as spare parts. Makes perfect sense in hindsight.

par

Hi Hikeonpast,

Thanks for getting back to me on this. I am still in process of swapping a couple of boards, but since you mention the Power supply boards, I took a second look at the power sources and found that the power cable to the T-con board had blemishes inside (I would like to show you a picture). Power boards a pretty inexpensive, so I’ll try that before junking my set. By the way, I am in between the XR77A80J and the C1 for a replacement.

Thanks again for you prompt response.

Best,

JP

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