Samsung UN40J5200 Main board failure

[jason123]

I got this 40" smartv for next to nothing t repair. It was simply not turning on and front red LED continually blinking. No backlight. Backlight came in when main board is disconnected.

Monitoring the voltage at PWR-ON pin on the power supply board, i was getting 6.5v to ground (fluctuating) - explains the blinking front LED)

I had a UN43J5200 parts laying around and sure enough, the main board fixes the problem. Tv works great.

Questions:
1. Anyway to change the TV model when i look at "contact Samsung support"... still shows UN43J5200 though i flashed the firmware with UN40J5200? Is there a service menu to access?

2. Anyone has seen this issue with this mainboard? Fluctuating 6.5 V instead of steady (13v i am guessing) voltage? Anyone with schematic? Thank you

[jason123]

Re: Samsung UN40J5200 Main board failure

Ok, I was able to fix the Model Number and Serial Number to match actual TV in the Service Menu (Mute 1 8 2 PWR)

Now.. only question # 2 is out there and I appreciate any input to troubleshoot the main board.
Thank you

[jason123]

Re: Samsung UN40J5200 Main board failure

Does anyone know what else needs to come to the main board to turn the supplies on? I have 13V coming into the power board (external out of set testing) but the only rail that's coming on is the 5V

[Diah]

Re: Samsung UN40J5200 Main board failure

i would first things checking the audio section as it share with 13 V.

[jason123]

Re: Samsung UN40J5200 Main board failure

Does power on require an external pull-up?? it's reading 0.4 V now with board having 13V and GND coming to it. Nothing else is connected. UD pin 2 has 2.9 V on it.

[Diah]

Re: Samsung UN40J5200 Main board failure

what... are you feeding the MB stand alone with external 13V...LOL how it come this idea to you.. this is TV not transformer..

[jason123]

Re: Samsung UN40J5200 Main board failure

Same idea.. Applying 13V to a board should act the same. I think I'm missing some handshaking / pullups..
I've already "fixed" the TV and it's in operation.. trying to troubleshoot the board standalone.. I've done it before.. on LG boards.. this one is a bit different

[Diah]

Re: Samsung UN40J5200 Main board failure

you can not do it in this way... you can first start with yours DM and compare resistance on the 13v pin on MB the default one and the working one. if you got low res. on the fault boards go head to audio section and DC to DC circuit.. if the res.. same or close to be same. then your fault Main IC .. of course the working one should be out of boards to compare.

[jason123]

Re: Samsung UN40J5200 Main board failure

Would you or anyone happen to know what the pull up / pull-down resistor value on the power supply for PWR_ON??
I would love to understand your reasononing so I can learn. Why can't I test standalone (Youtube is full of technicians troubleshooting boards out of a system) - Can you please explain??
This board is not shorted. I can apply 13 V and it's drawing only few hundred milliamps. Some DC-DC Supplies are coming on, and others are not.

[nomoresonys]

Re: Samsung UN40J5200 Main board failure

Most of the stand alone board testing is done on the power board, the powerboard has the standby, mainboard gets the signal to turn on sends signals to powerboard, tv turns on.

[jason123]

Re: Samsung UN40J5200 Main board failure

Great.. i have a power button connected and i have 13v connected externally. Just need to know what loading to put on the PWR_ON pin

[nomoresonys]

Re: Samsung UN40J5200 Main board failure

Do you have a link to one of those youtube videos?

[budm]

Re: Samsung UN40J5200 Main board failure

Great.. i have a power button connected and i have 13v connected externally. Just need to know what loading to put on the PWR_ON pin

PWR-ON pin goes to the outside world, I.E. the power supply board, to turn on the rest of the power supply on the power supply board. no need to do anything on the PWR-ON pin of the connector that goes to the power supply board.
The power switch of the TV sends the on signal to the MCU, then MCU sends out the PWR-ON, BL-ON, DIM, and the switched 12V on the main board to run the T-CON board, and also turns on the rest of the power supplies on the main board. So check and see if the 12VDC is present on the LVDS connector when you hit the power switch.
BTW, when TV is off the 13V will be around 8V as printed on the power supply board, enough to keep the main board alive and draw less power, when PWR-ON is received then it will kick up to 13V.
You may have to to have T-CON board connected too if this main board requires to have T-CON board connected.
What Voltages testing have you done on the working main board so you can compare the readings of the good and bad main board when you apply 13V to test the main board only?

[jason123]

Re: Samsung UN40J5200 Main board failure

So with power supply connected and power button - Power button doesn't seem to have any effect on rails - as if the transistor connected to it is not working..

When I apply ~13 V, I get the 4.95 V and 3.2 V come up. On the connector to the Power supply, I have 13 V on all such labeled, GND, and the PWR_ON is 12.96 V. No other voltages at this point. PWR_ON is not changing when pressing the power button.

Near the dual fets in the Photo - F201 is good, but doean't have voltages on either side. F202 below has 13V on both sides (good and conducting)

Gates and Sources of the Dual FET have about 13.x V on them. Drains are 0 V (2 FETS Seem to be connected in parallel)

[budm]

Re: Samsung UN40J5200 Main board failure

So with power supply connected and power button - Power button doesn't seem to have any effect on rails - as if the transistor connected to it is not working..

When I apply ~13 V, I get the 4.95 V and 3.2 V come up. On the connector to the Power supply, I have 13 V on all such labeled, GND, and the PWR_ON is 12.96 V. No other voltages at this point. PWR_ON is not changing when pressing the power button.

Near the dual fets in the Photo - F201 is good, but doean't have voltages on either side. F202 below has 13V on both sides (good and conducting)

Gates and Sources of the Dual FET have about 13.x V on them. Drains are 0 V (2 FETS Seem to be connected in parallel)

Fuse F202 is the 12V source feeding the MOSFET S1 pin 3 and S2 pin 1.
Q206 MOSFET #1 G1/S1/D1 is for switched 12V B-PWR, it is not being turned on right now by the MCU VIA Q207, for Q207 to be on the B_POWER_EN has to be about 2V or more to turn on Q207 which in turn turns on Q206 G1/S1/D1, it is P-CH MOSFET so for it to be on the GATE 1 pin 4 Voltage has to be at least 1V less than SOURCE pin 3 Voltage to have 12V on D1 pin 5&6.
Fuse F201 is fed from Q2016 G2/S2/D2, for the MOSFET #2 to be on GATE 2 pin 2 has to be at least 1V less than S2, since you do not have Voltage on F201, It means that the MOSFET #2 is not being turned on either.
So the MCU is not sending out many control signals, it can be software problem or hardware problem.
BTW, The MOSFET's are not connected in parallel, they are setup as two switches.

[jason123]

Re: Samsung UN40J5200 Main board failure

Thanks Budm... All makes a lot of sense. I looked at the heatsink on top of the MCU and it looke a bit bowed not flat. It came off very easily, as it was only connected on the edge of the MCU due to bowing. See photos. I think the MCU was exposed to excessive heating, likely for too long. I'll attempt a reflow - though I know it's not going to really "Fix" long-term.
Thank you again