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Old 07-01-2022, 07:47 PM   #21
BustinCaps
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Default Re: Small RCA TV white screen

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Originally Posted by R_J View Post
Digikey has the AO3420 which is a 6 amp version that should be ok. They also have a RUR040N02TL 4 amp version.
Thanks. Someone else also suggested this.
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Old 07-01-2022, 08:13 PM   #22
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Default Re: Small RCA TV white screen

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Originally Posted by R_J View Post
Digikey has the AO3420 which is a 6 amp version that should be ok. They also have a RUR040N02TL 4 amp version.
I believe RUR040N02TL 4 amp version is the one I have in the TV right now. The one's that are marked XF. Attached is what I bought from DigiKey.

RCA Transistor.PNG
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Old 07-01-2022, 08:33 PM   #23
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Default Re: Small RCA TV white screen

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Originally Posted by BustinCaps View Post
I believe RUR040N02TL 4 amp version is the one I have in the TV right now. The one's that are marked XF. Attached is what I bought from DigiKey.

Attachment 267674
That will work, You should have mentioned you got the mosfet from Digikey earlier as there is also a 100ma transistor with the marking XF, which would not work.

So good to here you have it repaired. The red/green line is the R.F. channel indicator
Attached Files
File Type: pdf RN1406_datasheet_en_20210830.pdf (879.7 KB, 2 views)

Last edited by R_J; 07-01-2022 at 08:49 PM..
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Old 07-01-2022, 11:21 PM   #24
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Default Re: Small RCA TV white screen

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Originally Posted by R_J View Post
That will work, You should have mentioned you got the mosfet from Digikey earlier as there is also a 100ma transistor with the marking XF, which would not work.

So good to here you have it repaired. The red/green line is the R.F. channel indicator
Yes it is working. Thanks for your help. Much appreciated. It appears the whole problem was the wrong resistors. I was hoping you could explain how the transformer works. Is it upscaling or downscaling the power and is it converting DC to DC or DC to AC? I want to learn what was going on there.
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Old 07-02-2022, 11:16 AM   #25
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Default Re: Small RCA TV white screen

In your case power supply is 12v. So to run backlight's and such the transformer is boosting voltage.
I'm confused. Resistors where bad? You then replaced with wrong value. Then put old ones back in, or new ones of correct value?
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Old 07-02-2022, 12:47 PM   #26
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Default Re: Small RCA TV white screen

wondering the same, I think he had a bad mosfet too.
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Old 07-02-2022, 01:08 PM   #27
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Default Re: Small RCA TV white screen

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Originally Posted by BustinCaps View Post
Yes it is working. Thanks for your help. Much appreciated. It appears the whole problem was the wrong resistors. I was hoping you could explain how the transformer works. Is it upscaling or downscaling the power and is it converting DC to DC or DC to AC? I want to learn what was going on there.
Transformers only work with AC they do not pass or convert DC (the diodes then convert the ac to dc)
The mosfet you replaced switches the raw 12volt input across the pimary of the transformer at about 50khz, this induces a current in the secondary windings of the transformer, there are likely 3 or more secondary supplies in this unit, one of them is a negative voltage whose diode is likely D303 and C323 is the filter cap.
D306, D305, D302, likely supply other voltages for the tv.

I suspect the mosfet shorted and you replaced it with a proper replacement, BUT you also replaced the source resistors at the same time. the values you used for those resistors were too high a value (2 10Ω in parallel=5Ω) The controller monitors the voltage drop across these resistors and was likely sensing a fault and shutting down. When you reinstalled the correct resistors (2 1.5Ω in parallel= 0.75Ω) the circuit operated as it should.

The inverter circuit for the ccfl uses a different transformer (T401) and drive circuit

Last edited by R_J; 07-02-2022 at 01:21 PM..
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Old 07-02-2022, 01:10 PM   #28
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Default Re: Small RCA TV white screen

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Originally Posted by dskall View Post
In your case power supply is 12v. So to run backlight's and such the transformer is boosting voltage.
I'm confused. Resistors where bad? You then replaced with wrong value. Then put old ones back in, or new ones of correct value?
So I replaced mosfet and resistors at the same time. I thought both were bad. Only the mosfet was bad and the old resistors (1 ohm) where good. The new resistors I used were 10 ohms so that was the problem.

I think there is a separate transformer that runs the backlight power to the lcd screen.
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Old 07-02-2022, 01:27 PM   #29
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Default Re: Small RCA TV white screen

Quote:
Originally Posted by R_J View Post
Transformers only work with AC they do not pass or convert DC (the diodes then convert the ac to dc)
The mosfet you replaced switches the raw 12volt input across the pimary of the transformer at about 50khz, this induces a current in the secondary windings of the transformer, there are likely 3 or more secondary supplies in this unit, one of them is a negative voltage whose diode is likely D303 and C323 is the filter cap.
D306, D305, D302, likely supply other voltages for the tv.

I suspect the mosfet shorted and you replaced it with a proper replacement, BUT you also replaced the source resistors at the same time. the values you used for those resistors were too high a value (2 10Ω in parallel=5Ω) The controller monitors the voltage drop across these resistors and was likely sensing a fault and shutting down. When you reinstalled the correct resistors (2 1.5Ω in parallel= 0.75Ω) the circuit operated as it should.

The inverter circuit for the ccfl uses a different transformer (T401) and drive circuit
That's exactly what happened. Forgive my lack of knowledge on electrical circuits but is the mosfet creating the ac by switching at 50khz? Is the mosfet creating the ac from the 12v input?
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