From Trash to TV

Repair log of a LCD TV resurrected from trash for a second life

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Last week I found a LCD TV in trash at my apartment. I got curious after checking the LCD panel was still intact and hoped I could salvage some components. I took apart the TV and discovered the power supply and CFL inverter boards with blown transistors and some key components were missing (desoldered) . I presume after a power surge or something the SMPS and inverter got toast and it was sent off to some service center and they could not get it working or find the right parts and the owner decided to dump it in the bin.

The logic board looks physically fine, no burn marks or weird smell, components looks alright and I got a hunch that it may be just the power supply and CFL inverter gone bust and the TV can still be made functional. The inputs were RCA, component, HDMI and VGA and I can definitely use this as a spare monitor or a Raspi Kodi TV for my guest room - only if I can get this working. Two challenges, Step 1 - To verify if the Logic board still works and Step 2 - If logic and screen works then replace the CFL backlight with LEDs.

Blown Transistors on Inverter board

Missing mains filter capacitor and mosfet

Step 1: The power and LCD connector pin designations were silk screened on the main PCB... Sweet! The LCD driver board is a generic TV board using Mediatek MTK8227 SoC. The TV is assembled by a reputed manufacturer, but I guess as with all other manufacturers, they source these generic boards from China and dump their logo in ROM and have custom UI and that's about it now a days!

A quick search and you can find a lot of these cheap LCD driver boards based on similar SoC on AliExpress or similar Chinese ecommerce sites. The semiconductors on these boards are mostly obscure components, most of them have no online datasheet or have minimal documentation. After poring over internet for few hours, I got hold of some incomplete datasheet and also a schematic of a different model TV from another manufacturer using same SoC. This was a huge help as there is a pinout of MTK8227 and signal descriptions and now I can trace and probe signals to various peripherals.

The power requirement of the SoC board is simple, all it needs is a 5V, 12V and Gnd. I connected the power headers to a computer PSU, crossed my fingers and switched on. No magic smoke and after a while I checked the ICs and regulators and they were just warm to touch. Seems ok and the red power LED on the front of the panel was ON. Since I did not get the remote along with the TV, I tried the power button on the side of the TV. Nothing happened. I had temporarily illuminated the back of the panel with a LED flashlight and the panel seems blank, no activity, pushed all the buttons and there was nothing! The SoC itself was warm to touch, so I knew it was working, but somewhere something was broken.

As Dave from EEVblog always says that the first rule of debugging is “thou shall test thy voltages.”, I started with voltage regulators on board, all seems ok except the Vcore 1.2V was reading 0.4V to 0.5V. .... BAM! I found the offending component and It was a 8 pin switching regulator IT7833 1.2V@3A.

VCore Regulator 1.2V@3A

I could not find much information on this regulator, but the one page incomplete datasheet helped me decode the regulator. I didn't had any 1.2V switching buck regulator in my junk box, but I did find my trusty old LM317 regulator and turned the output all the way down to 1.25V and hooked it up to Vcore after desoldering IT7833. Turned ON the power supply and I could see flickers on the dimly lit screen, then the manufacturer logo and finally a 'no Signal' animation on the LCD panel. YAAY - The panel works!!!

A very faint 'no signal' animation. working!!!

Step 2: This was easy. I had few feet of left over 12V white LED strip, which I cut in eight pieces, stick them inside the white plastic enclosure (for CFL) and wired them all in parallel. The LEDs face the white diffuser stuck to the back of LCD glass.

Warning: Be very careful while handling the CFL as they are very fragile and contains Mercury. Dispose them off in accordance with your local waste disposal laws.

replacing CFL with white LED strips

Testing LED Strips

Testing: Connected all the power cables and hooked up my Raspberry Pi to HDMI input and the 1080 HD LCD fired up flawless, looks beautiful .. Awesome :)

I managed to get almost uniform illumination across the screen. I feel I could get much more illumination if I use better and brighter LEDs, for now, this works. Played a movie, works fine. I also programmed a universal remote control with manufacturer code and have a working remote for all basic functions in no time.

There seems to be an issue with audio, could not get anything out of the speakers,...

Read more »

  • Audio and Power Fix

    sidsingh04/04/2016 at 00:41 0 comments

    Power Section:

    I was working on TV this weekend to iron out the remaining issues. The cheap ebay LM2596 DC-DC switching converters arrived and would not work out of the box! Output voltage seemed fixed at 11.2V. I ended up spending quite sometime to debug the LM2596 board and after comparing the application circuit in datasheet found that programming resistor R1 was randomly selected to 330 ohms. R1 should by 1K 1%, Replaced R1 with correct value and was able to vary the output voltage. The precision pots on these boards were also no-name low quality ones, so ended up replacing them as well. Also, these boards have insanely bright blue LED at the output, I disconnected them. Cheap DC-DC converter modules were not cheap after all!

    TV logic PCB needs 5V and 12V to operate. I decided to use a 12V,4A power supply and since Vcore 1.2V regulator was also fried, needed two of these LM2596 modules to step down (12 V to 5V) and (12V to 1.2V). The LED back-light also needs 12V which can be tapped directly off the input supply.

    Switching regulators were mounted on a heat sink and I soldered everything together on a general purpose PCB. Added screw terminals for input and output voltages, LED backlight, added capacitors at input and output and also added a MOSFET to control LED backlight. Powered up and things were working ok. Output voltage seems stable and heatsink gets a little warm after some usage. I was also able to power the Raspberry- pi directly off the USB port of TV, this means less clutter and one less power adapter.

    Fixing the Audio:

    Audio is handled by ST micro STA559BW. This is a dedicated digital sound processor and has stereo Class D power amp integrated. The STA559BW datasheet is very detailed and in-depth and I don't think I will understand in a short time how this chip is implemented for this application. For now, I concentrated on checking the supply voltage, bias voltages, signal levels at various pins.

    The power supply on all pins seems good and measure up to their respective voltage. I checked various digital signals using an oscilloscope and all signals were present - I2C SDA and SCL were good and SPI data input to the chip also seems active. Chip Enable was high, which was good. Chip RESET was held at -- Umm ... at 0.4V!!!

    Datasheet says it should be at digital Vdd of 3.3 V. RESET pin is tied to 3.3V with a 10K resistor and 0.1uF capacitor to ground, which delays the reset rise at power ON.

    There were no other components tied to reset pin - only a resistor and a capacitor and both of them tested good - so why the reset is held low at 0.4V? I desoldered the resistor and capacitor and connected a wire to the reset and added discrete 10K resistor and a 0.1 capacitor on a breadboard and I still saw the reset is hovering around 0.3 - 0.4V! replaced 10K resistor with 1K and still no good. I directly tied the reset pin to 3.3V Vdd and recycled the power - and I GOT SOUND --- YAAY!!

    I still do not understand what may be the issue, my best guess is this chip is partially damaged and still works if reset is tied to Vdd. So my final fix was to solder a thin wire permanently between Vdd and Reset. works everytime and all the audio functions - Vol up/down, presets and equalizer works flawless.

    This repair is complete and I am glad, I was able to get this working and a free TV for my Guest room :).

    Thanks for reading.

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Bibhu wrote 09/19/2017 at 21:08 point

Hi sir, Great job, many many thanks to you. i like this type of uncommon solution. it's save my time. thanks once again.

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Mike Maluk wrote 04/25/2016 at 03:04 point

Great log, very descriptive! 

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danjovic wrote 04/25/2016 at 02:07 point

Nice Job! Congratulations!

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sidsingh wrote 03/29/2016 at 08:35 point

Thank You Michael!

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Michael Vowles wrote 03/29/2016 at 05:23 point

Great read, I always like repair logs like these!

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