Diagnose PS BN44-0467A

[edugimeno]

Re: Diagnose PS BN44-0467A

According to the schematic QM851 (the large TO220 mosfet) drives B13V, not B5V. Also tracing the mosfet's Source pin on the real board I get into pins 1-3-5-7 on CN802 which corresponds to B13V
Im doing all these test with all wires connected to PS (PS to MAIN and PS to BL)
Resistance on mosfet legs while OFF the board:
G to D: 14 Ohm
D to S: 11 Ohm
G to S: 4 Ohm

How does removing a mosfet that's almost a shortcut not affect the BL status?

[R_J]

Re: Diagnose PS BN44-0467A

If you look at the schematic (or the board) without B13V QM853 will not be turned on so NO B5V as well
The mosfet seems to be bad, D~S should be almost open with 11Ω this "switch" never turns off.

If CM865 (CM867 schematic) was stll charged up when QM851 was removed it could turn on QM853 via RM867 (RM805 schematic)) so there could still be B5V

As for the backlight, that is a more complicated circuit, It must be seeing a high on the ENable pin from soewhere. most Samsung boards have pullup resistors on the power supply for PS_ON and BL_ON so without a main board connected the power supply turns on full and the backlights turn on as well.

Is ICD851 a sem2106 or something else?

[edugimeno]

Re: Diagnose PS BN44-0467A

The main board was connected all the time for these tests so the signals were driven by it, and I was turning the unit on and off to get these signals during on and off state
I can't figure out the model for the BL driver IC, I'm attaching a picture of the best I've able to take from it as it may look similar to some IC you know

[lotas]

Re: Diagnose PS BN44-0467A

The main board was connected all the time for these tests so the signals were driven by it, and I was turning the unit on and off to get these signals during on and off state
I can't figure out the model for the BL driver IC, I'm attaching a picture of the best I've able to take from it as it may look similar to some IC you know

OZ9998BGN

https://aliexpress.ru/item/100500179...00017589890855

[edugimeno]

Re: Diagnose PS BN44-0467A

Ok thanks to the good sight of LOTAS I've found the PDF for this driver IC.
It takes ENA signal at pin 5, which, after taking several pictures and drawing lines on my screen to follow tracks, seems to go through CD861/RD862, and then RD861/RD863 goes down to the opposite area of the board directly to B_ON in the CN to MAIN


I will probably check the voltage directly on the ENA pin in driver IC while ON/OFF and see what I get

Thanks

[edugimeno]

Re: Diagnose PS BN44-0467A

Ok I've spent some time on this TV today. The worst this of this repair is not being able to find an schematic for this PS. This makes this all extremely difficult for me.

I had mosfet QM851 (STP652F) out of the board, as it had a short. Still the BL was ON all the time with ths mosfet OUT.

I tried to check voltages while TV ON/OFF at the ENA pin in the BL driver IC (OZ9998BGN), and it didn't show any valid voltage. I suspected it needed the (still shorted) main mosfet back on the board. I soldered it and checked again. My suspicion was right, actually with the mosfet OUT the TV wasn't fully turning on but cycling on-off all time. So I put it back in.

With QM851 back on the board I was still getting BL ON all the time (while TV off and on), but when TV was on I could see 5v into the ENA pin of the BL driver IC, and while the TV was off I saw 0,5V. At this time BL was still lit. I shorted ENA pin to GND to get a true 0,00 v and still BL was lit

Why is the BL ON when the ENA pin is LOW and it actually follows the signal from the B_ON pin from the main-to-ps plug?

I'm tempted to buy a replacement for the main mosfet to see if it changes but if having it on the board (shorted) and having it off the board makes no difference....makes me think this is not the problem. But it may be something indirectly related... can you guess...???

Thanks!

[R_J]

Re: Diagnose PS BN44-0467A

The OZ9998BGN ENable pin is pin 9 NOT pin 5

[lotas]

Re: Diagnose PS BN44-0467A

And here on the diagram it is 5pin. (ENA).

[edugimeno]

Re: Diagnose PS BN44-0467A

The OZ9998BGN ENable pin is pin 9 NOT pin 5

Like LOTAS pointed out, I found that schematic showing pin 5 as ENA, which does make total sense with my board, it has just some caps and voltage dividers at the signal taken from B_ON from main-to-ps connector

The schematic you may have found, like me, showing pin 9 as ENA, states OZ9998B (not BGN), which seems to be different. My board reflects the one LOTAS found

[R_J]

Re: Diagnose PS BN44-0467A

Like LOTAS pointed out, I found that schematic showing pin 5 as ENA, which does make total sense with my board, it has just some caps and voltage dividers at the signal taken from B_ON from main-to-ps connector

The schematic you may have found, like me, showing pin 9 as ENA, states OZ9998B (not BGN), which seems to be different. My board reflects the one LOTAS found

I was just going by the datasheet, which did not seem to match the board.

[edugimeno]

Re: Diagnose PS BN44-0467A

Yes you probably found this one Im attaching, page 16

Anyway do you have any hint on what could be holding BL on all the time, even with EAN pin LOW? Does the shorted mosfet relate to that?

THANKS!

[R_J]

Re: Diagnose PS BN44-0467A

Without the proper datasheet, it is a guess as to what the enable pin should be, It would seem that having a low on the pin, enables the ic, so it could be that it requires a logic high to disable it.
From what I can trace RD861 is connected between the enable line and the B5V line, so without the B5V voltage this might be why the backlights are on.

Off the ENAble line there is RD862,CD861 to ground, and RD863 which connects to the B_ON pin, Off this line is RD861 which goes all the way up the board to JP814 (B5V)

[edugimeno]

Re: Diagnose PS BN44-0467A

But I measured 5v while the TV was on and 0.5v while not was off, both situations while BL was kept ON. So there's been a high condition that didn't turn the BL off

[R_J]

Re: Diagnose PS BN44-0467A

So the voltage on pin 5 changes from +5v to 0v I suspect the ic is bad.

[edugimeno]

Re: Diagnose PS BN44-0467A

So the voltage on pin 5 changes from +5v to 0v I suspect the ic is bad.

Yes. Why? This is the B_on signal from the main board that gets to this ENA pin. To me it sounds like it's doing what it should, turn off (0v) when it wants the BL off

[R_J]

Re: Diagnose PS BN44-0467A

Yes. Why? This is the B_on signal from the main board that gets to this ENA pin. To me it sounds like it's doing what it should, turn off (0v) when it wants the BL off

If it is doing what it should then what's the problem?

[edugimeno]

Re: Diagnose PS BN44-0467A

Ah ok I understand you now. I need to test again but it looked to me like the dimming function was working. I will check again and see if the menu setting to change BL level works. Would this mean the driver IC chip works fine?

I still don't understand how having a shorted mosfet does not affect this behaviuor

Thanks

[edugimeno]

Re: Diagnose PS BN44-0467A

I put the board back in. I tested backlight settings in the menu and it does affect in a very low range. Like sliding the menu range from 0 to 20 it changes what you would expect from 18 to 20.

So somehow the BL driver IC does something but not all. Not sure if this happens due to a reduced power from the main mosfet, that's shorted but it actually shows 7 ohm I believe

Also when you turn the TV off once, it won't turn back on again until unplugged. Probably related to the bad mosfet too. I will probably find a replacement mosfet but if I need to replace the BL driver I will let this TV go as this is a small set and it's not worth much money neither much (more) time

Thanks

PD: Im attaching a couple videos of how the BL (/slightly) changes with the settings, and how the BL stays on full lit when you turn it off. Note that the phone camera is also adjusting light level so it can be barely seen
Too large to upload, sharing via web here:
https://file.io/SShrlQp1C0Kh

[R_J]

Re: Diagnose PS BN44-0467A

Inside the backlight ic are a 1000 transistors, I suspect the one that is off pin 5 that controls it is damaged, so the backlight controller stays working, all the time. this would not effect the dim function only the enable part is defective.

[edugimeno]

Re: Diagnose PS BN44-0467A

Yes, that could be... I will probably take this TV to the dump...