Vestel 17MB130P short circuit on 3.3V

[Retrocomputer]

Hi everybody!
I have a 55" LED TV made by Vestel. All of a sudden it would not turn on anymore, no signs of life whatsoever. I opened it up and found the power supply gone, no voltages on the output connector. Swapped the board with a known functioning one, now all supply voltages are there, but still no signs of life, except for the red LED in the SPDIF port. A rapid measuring of the test point voltages shows all the rails working except for 3.3V (both 3V3_STBY and 3V3_VCC) and all the related ones, such as 1V_VCC_CPU or 1V_VCC_CORE. The 3.3V stepdown regulator becomes rapidly very hot (as much as 85-90°C) and testing with the multimeter shows a direct short (0 ohms) from 3V3_STBY and 3V_VCC to GND. Imaging the board with a thermal camera, though, shows nothing getting warm except for the regulator, due to the high current flowing through it. Already tried desoldering the regulator and it is not shorted, as the rails still show 0 ohms to ground. I'm a bit stuck as nothing else gets warm so I can't really understand where the short lies.

Here's the schematic diagram:
https://1drv.ms/b/s!Aiwky_BwJ7fkpA3u...pD8B0?e=UjNOiq

Thank you in advance
Davide

[senndogg]

Maybe you should inject a 3V from a more powerful power supply (or battery) to this rail.
If you are lucky, a smd capacitor will blow up, and the short will clear.

[Retrocomputer]

I thought about that but I’m worried it might blow up something else too … I’d try with something more methodical first but I’m a bit stuck

[dick_barton]

Looking at your circuit it looks as though 3V3_STB and 3V3_VCC may be connected together.
Have you checked D11, C331 and C332. The smd caps tend to go short.
Also if D140 was short it would also ground those supplies.

You mention the 3V3 stepdown regulator but there are two are there? U112 and U124

[Retrocomputer]

I surfed a bit through the schematics, it seems that almost all of the nodes in the 3v3 rails are lifted from ground via one or more resistors. There are only a couple of places (namely the ones with C331/2, D117, D140 and C333 and all the caps networks there). Unfortunately though if have no idea on how to identify the caps since they’re not marked on the board. D117 and D140 already removed, not shorted.

Thank you!!

[arizanotlari]

Board view : https://yadi.sk/i/SZ7T8vvxWGs4rA

[Retrocomputer]

THANK YOU SO MUCH!

Thanks to the boardview, I managed to find where the short lies: as soon as I lifted F128 and F130 the short on the 3.3v_stby line went away. Unfortunately the board is still acting up. Now there's still a short on the 3v3_vcc line to ground, and when the board is supplied with voltage, the CPU (U114), the 3v3_vcc and the 1V_vcc_core regulators (U100 and U103 respectively) get all very hot quite fast. 3v3_stby is now present and regulated. I'm starting to fear the CPU is shorted internally... What do you think? What else can I do?

Thank you!!
Davide

[arizanotlari]

all supplies has fuse or 0 ohm resistor.

desolder one by one ; f149, f124,

[Retrocomputer]

Now the 3v3_vcc short is gone, lifting F136.
Unfortunately, the 1V_VCC_CORE rail is still shorted to ground. It goes in just a few places on the board, and only 3 has direct connections to GND possible. Two protected via fuses (F115 and F116) and an array of 14 caps unfused. I removed them but the short is still there... I removed U103 too and the two caps C148 and C154, no change. It seems the processor is gone for good... What do you think?

thank you!
Davide


Thank you!!

[arizanotlari]

it is obvious...cpu is dead.