Samsung UE37D5720 permanent reboot, 3.3V ok but MCU hangs up

[yoritomo]

Samsung UE37D5720
Chassis: U66A
Main board: BN94-05068V
Power Supply: BN44-00422A

Hello all, this TV set is lighting on and permanently rebooting after the Samsung logo, the STB 5V coming from the power supply is correct, the mainboard is producing the 3.3V at the regulator IC1203 , this one is correct as well and it is the only permanent voltage i could find on this main-board.

When rebooting all other voltages are shutting down and when controlling the oscillation at the quartz of the micro-controller i could see with the oscilloscope that it disappears when rebooting, that means the MCU is hanging up.
I could not find any schematics for this mainboard to find the reset pin and power input of the MCU and i can't peal off the solid square heat-sink from the MCU to see the number of it.

If you have any ideas thanks a lot again.

[diif]

Re: Samsung UE37D5720 permanent reboot, 3.3V ok but MCU hangs up

You can try a reset of the EEPROM by putting a wire between RESET and SDA.
This video will give a pointer although it's in German there are English subs. https://www.youtube.com/watch?v=DDzOSdfA41s
I believe they also suffer from corrupted/failed NAND that needs replacing.

[yoritomo]

Re: Samsung UE37D5720 permanent reboot, 3.3V ok but MCU hangs up

Thank you very much for this link, i tried many time that hard reset, doing exactly as he said in the video but no success. Very useful tip for a next issue anyway.

From the main-board i can hear a high pitch hissing when on, it perhaps means it has a short somewhere ?
What are the main powers absolutely necessary for the MCU to work properly ? I would expect the main 3.3V , 1.5V ram power and the core 1.2V right ? if any is missing the MCU is down, am i right ?

[diif]

Re: Samsung UE37D5720 permanent reboot, 3.3V ok but MCU hangs up

It depends on the noise, it could just be coil whine. Hissing could also be a capacitor venting (but I think doubtful)
Are there any markings next to the connector from the power supply to main board ?
They should be marked with voltages.
I believe the D series Samsungs and possibly others have the EEPROM load information from the NAND. If the NAND becomes corrupted then this isn't possible and results in a boot loop.

[yoritomo]

Re: Samsung UE37D5720 permanent reboot, 3.3V ok but MCU hangs up

Hello i finally solved the problem by a surprising way.

I froze the nand with a spray, then the tv could start normally, long time enough to enter in the service menu and to apply the factory reset, i let it run for a long test period, cold or warm, it still be always working now since yesterday.
This is not a BGA nand chip, just a normal one, then i don't expect any fails after from it, anyway i don't neglect that another factor could cause the rom corruption, then let's see after a while.
Anyway i will set the post as Solved.