This video I got from the seller. I suspected a main board problem, it did not look like the panel was faulty.
When the image was visible, it presented very interesting patterns.
Let's analyse this quickly:
- There are random pixels scattered across bounded areas
- Some pixels are at the wrong place (the letters in the first image)
- The panel loses its complete image and goes dark. Some supply voltage might be dropping regularly.
This wasn't a panel fault case as there were no stationary faults. The faults varied with the image structure. The menu generated different distortions than the static from the missing TV signal. Such noise can only be produced by the main board or the TCON.
As it was a lot easier to swap the main and I had one working spare at hand, I tried that first. No change. Then I swapped the also easily accessible dimming board, through which the signal is routed to the TCON. No change either.
Damned, so I really had to go through the tedious work of uncovering the TCON. It is buried under the sub-frame on which all other boards live. My standard routine to make sure that the panel is not to blame is to unplug each one of the flat cables to the panel. That did not show any static faults in the panel like it would do if address or source drivers were broken. And it still flickered.
The TCON was faulty! I checked all the voltage testpoints I could find, including the gamma voltages of the infamous AS15 chip. The VCC_S and VCC_M where unstable. They should be 3.3V but they swung about 400mV down in pretty much the rhythm that the panel was going blank. I knew I was onto something here. The TCON has two big chips, which are supplied identically. I did not assume that both chips were faulty, instead there had to be a common source of the problem.
I think VCC_S/M are provided by the big chips. The actual 3.3V regulator was absolutely stable.
And I found it. Surprisingly it was the 1.8V LDO regulator for the big chip's core, which was swinging in the same cadence. I think it rebooted the chips all the time and crashed the RAMs, too.
This little sucker is an EC50177 B. It is impossible to get in the TO-252 packaging, not even from Aliexpress. The alternatives in TO-252 I found did not deliver the required 1A.
But before I ordered some spare regulators, I wanted to make sure that this was the only fault. I removed the chip (and broke it to pieces while doing that...) and attached my lab supply. Hooray, the VCC line was rock solid again! The chips pulled around 0.622 amps without glitches.
Now I just had to see the TCON working with the panel attached.
And look at that! A flawless picture :-)
The only 1.8V fixed voltage reg I found from a local source (not Aliexpress again, please!) were LD1086 in the TO-220 package. Fair enough, there was plenty of free space and they are slim enough. And they can deliver 1.5A current. I'd work something out. So I ordered 3pcs from eBay for 7€.
The TO-220 package needed some modifications. I snipped off the tab and bent the legs. With some care it fit perfectly well. I also put a cooling pad on top of it so that it has contact to the cover.
TV working fine. Hallo Frau Green!
This model is built like a tank. It is very heavy, almost impossible to topple over, and the screen is protected by an extra layer of plastic. The ideal device for families with children where things sometimes fly around in the living room
