Ua40f7500 samsung tv dark picture

**aashiknaik** · 09-28-2018, 03:55 AM

Re: Ua40f7500 samsung tv dark picture

I have received IC RT6905 GQW .
CAN anybody tell me that is there any difference between RT6905B & RT6905 ?
Hope somebody knows.Pl. give me feedback.
Thanks

**Andrew F. Ali** · 09-28-2018, 07:11 AM

Re: Ua40f7500 samsung tv dark picture

From what I read in your 1st post you said: " Im facing problem of dark picture with my SAMSUNG UA40F7500 TV.
The T con board is BN41-01939B.
CN 98BN950 0963AL229 DS40892.
Backlight working.
Sound ok.
If room lights are off then can see the dark image and able to select source from menu, all commands visible but too dark."

If the backlight are working and you can't see a picture BUT in a darkened room you can see menu and select etc, then the T-CON Bd. is working.
This does not maker sense to me. UNLESS the backlights are very dim.
Could you elaborate on what your symptoms are?
Try to carry up the Brightness and Contrast settings and also the Backlights setting on the main menu.

**Davi.p** · 09-28-2018, 07:33 AM

Re: Ua40f7500 samsung tv dark picture

With those voltages surely tcon is not working properly, i suggest you not to change the IC but inspect why you have an HVIN so low, it feeds current from tcon input supply voltage to the regulators, but between HVIN and VIN can be something wrong, for example a power switch like some i have seen, that switch feeds some panel voltages while VCore for processor and Vcc for memories are always present.

**budm** · 09-28-2018, 09:58 AM

Re: Ua40f7500 samsung tv dark picture

This how RT6905 is being used, as you can see the HVIN is fed from switch MOSFET to supply 12V to run the IC.
BTW, can we see the screen shot of what the pictures look like?
Please also provide the pictures of the whole back side of the TV showing all the boards.

Attached Files

RT6905 sch.pdf (63.2 KB, 65 views)

**aashiknaik** · 10-05-2018, 02:22 AM

Re: Ua40f7500 samsung tv dark picture

Thanks for your reply,
I had tried with brightness control and contrast setting.but not much difference.
Backlight setting unable to operate.

**aashiknaik** · 10-05-2018, 02:30 AM

Re: Ua40f7500 samsung tv dark picture

Originally posted by Davi.p

With those voltages surely tcon is not working properly, i suggest you not to change the IC but inspect why you have an HVIN so low, it feeds current from tcon input supply voltage to the regulators, but between HVIN and VIN can be something wrong, for example a power switch like some i have seen, that switch feeds some panel voltages while VCore for processor and Vcc for memories are always present.

Thanks dear Davi.P,
I have removed IC.so, let me fix new ic and again i will go thro' and come to you.
But thanks a lot for the prompt reply.
Will post reply once i fix IC and find why HVIN is low.

**aashiknaik** · 10-05-2018, 02:34 AM

Re: Ua40f7500 samsung tv dark picture

Originally posted by budm

This how RT6905 is being used, as you can see the HVIN is fed from switch MOSFET to supply 12V to run the IC.
BTW, can we see the screen shot of what the pictures look like?
Please also provide the pictures of the whole back side of the TV showing all the boards.

Dear sir,
Thanks for the prompt reply,
Actually i didn't expect prompt replies from tech buddies but you all are really the most helping hands or let me say helping minds.Really appreciate all.
Meanwhile i had removed ic so, let me fix ic and then will study all voltages and then again come with all pics and details to you.
Thanks a lot again,
Aashiknaik

**aashiknaik** · 10-09-2018, 05:18 AM

Re: Ua40f7500 samsung tv dark picture

Originally posted by budm

This how RT6905 is being used, as you can see the HVIN is fed from switch MOSFET to supply 12V to run the IC.
BTW, can we see the screen shot of what the pictures look like?
Please also provide the pictures of the whole back side of the TV showing all the boards.

Dear sir,
yes, you are right. MOSFET output comes to 10.43V @ 22u coil shown in RT9605 diagram which you have sent.
I have removed IC RT6905 before i show these posts.But it seems that the problem is with MOSFET or surrounding circuit.
AT source S it shows 12.6 V but at D it shows 10.43V.
At G it does not show any voltage. there is 1 resistor & 1 capacitor in series It doesn't show any voltage at resistor or gate but at capacitor input 12.5 v available.
WHAT VOLTAGES WE shall get @ gate as per opinion?
I will fix IC once i get 12v at HVIN.
Thanks,
AashikNaik

**Davi.p** · 10-09-2018, 08:02 AM

Re: Ua40f7500 samsung tv dark picture

The voltage on gate terminal has to be negative respect to the drain. So change the mosfet or test it apart. I warn you that when tcon will be fixed you could have a bad surprise because even with a wrong tcon double image it's not normal..

**aashiknaik** · 10-10-2018, 12:55 AM

Re: Ua40f7500 samsung tv dark picture

Originally posted by Davi.p

The voltage on gate terminal has to be negative respect to the drain. So change the mosfet or test it apart. I warn you that when tcon will be fixed you could have a bad surprise because even with a wrong tcon double image it's not normal..

I tried one more t con of same pcb but of different model and got 2 images up and down section.But it was not dark as found in original t con.That T CON is suitable for UA40F6400.A very small capacitor is there between S and opposite terminal of 6 pin MOSFET.
The other T CON WHICH is suitable for UA40F6400 Model is giving 12.5v @hvin.
I tried to use MOSFET of that T CON but found no voltage at Drain.so, i replaced with original one. With original one i again got 10.43V.
Original MOSFET - D5EV. & the other one having MOSFET D52V.
How to get this MOSFET? Can i replace the original with the other one?
Thanks for your replies

Attached Files

**budm** · 10-10-2018, 01:21 AM

Re: Ua40f7500 samsung tv dark picture

So what is the designator of the MOSFET for the swithced HV12V, Q???
Can I see the pictures of that area where that MOSFET is?
The GATE of that MOSFET should be driven by the RT IC via GD pin as shown in the diagram that you are using as the guide, Q109 is the reference as shown in the diagram.

**aashiknaik** · 10-10-2018, 03:41 AM

Re: Ua40f7500 samsung tv dark picture

Originally posted by budm

So what is the designator of the MOSFET for the swithced HV12V, Q???
Can I see the pictures of that area where that MOSFET is?
The GATE of that MOSFET should be driven by the RT IC via GD pin as shown in the diagram that you are using as the guide, Q109 is the reference as shown in the diagram.

As i searched for D5 in s-manuals it shows PNP & NPN Digital transistor.
The images attached but the IC is removed.

Attached Files

**aashiknaik** · 10-10-2018, 04:03 AM

Re: Ua40f7500 samsung tv dark picture

Originally posted by aashiknaik

As i searched for D5 in s-manuals it shows PNP & NPN Digital transistor.
The images attached but the IC is removed.

marked what i assumed.
As per your post Gate voltages shall be supplied by RT6905 GD point i think first i should fix IC

Attached Files

**chrismurphy** · 10-10-2018, 04:12 AM

Re: Ua40f7500 samsung tv dark picture

Cut the PWM dimmer wire from the main board

**Davi.p** · 10-10-2018, 11:54 AM

Re: Ua40f7500 samsung tv dark picture

I said to you not ot desolder IC!!

**aashiknaik** · 10-10-2018, 11:30 PM

Re: Ua40f7500 samsung tv dark picture

Originally posted by Davi.p

I said to you not ot desolder IC!!

yes, but before i noticed i had already desoldered it. sorry.
Now i will fix it & check again.

Ua40f7500 samsung tv dark picture

Ua40f7500 samsung tv dark picture

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