Small RCA TV white screen

[BustinCaps]

Re: Small RCA TV white screen

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?

[BustinCaps]

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?

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.

[R_J]

Re: Small RCA TV white screen

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

[nomoresonys]

Re: Small RCA TV white screen

wondering the same, I think he had a bad mosfet too.

[dskall]

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?

[BustinCaps]

Re: Small RCA TV white screen

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.

[R_J]

Re: Small RCA TV white screen

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.

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

[BustinCaps]

Re: Small RCA TV white screen

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.

[BustinCaps]

Re: Small RCA TV white screen

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.

[BustinCaps]

Re: Small RCA TV white screen

looks like this one will work: https://www.digikey.com/en/products/...AO3420/1855956

Thanks. I'll check it out.

[R_J]

Re: Small RCA TV white screen

Digikey has the AO3420 which is a 6 amp version that should be ok. They also have a RUR040N02TL 4 amp version.

[nomoresonys]

Re: Small RCA TV white screen

looks like this one will work: https://www.digikey.com/en/products/...AO3420/1855956

[BustinCaps]

Re: Small RCA TV white screen

The circuit is not likely to work when you replace a FDN339AN mosfet with a RN1406 transistor (Marked XF)!
The smps ic operates at a min freq. of 50khz and up

So I put back the old mosfet transistor and resistors. Turned it on and immediately got the magic smoke from the transistor. That transistor is toast. I then put back the XF transistor and get the below grainy pictures. There is a red or green line that appears vertically when I switch from VHF to UHF. I'm guessing I need FDN339AN mosfet for it to work properly. This mosfet looks like it's going to be hard to find. It's out of stock at most places.

[dskall]

Re: Small RCA TV white screen

I didn't say the circuit is good. Just looks like controller isn't damaged. And I did misread frequency on datasheet.

[R_J]

Re: Small RCA TV white screen

The circuit is not likely to work when you replace a FDN339AN mosfet with a RN1406 transistor (Marked XF)!
The smps ic operates at a min freq. of 50khz and up

[dskall]

Re: Small RCA TV white screen

Your controller chip is probably ok. The resistors you replaced are probably current sense and are a different problem. If you ohmed the originals and they were 1.5 ohms then I would put them back in. And you will need to read secondary voltages after the diode. Not at output of transformer. You may be able to check controller output if your meter reads frequencies up to 5k

[R_J]

Re: Small RCA TV white screen

Why suspect the transformer? the transistor you installed is wrong, the source resistors should be 1.5Ω not 10Ω and circuit is not going to work with wrong parts installed, and could cause even more damage. I suspect there was nothing wrong with that circuit and there was likely a fault in the secondary causing this smps to be loaded down.
C323 is the filter capacitor on a negative supply, that is why the positive is connected to ground. It might be possible the smps circuit is trying to work.

Where are you checking the secondary voltages that you get "zero or -.02v and -.03 v coming out on the secondary"

To work on these circuits properly you need a scope to see if the ic is working and generating pulses.

[BustinCaps]

Re: Small RCA TV white screen

Good clear pictures of mainboard and t-con would help. Do you have voltage to t-con?

Sorry about the bad pics. Here are some other pics.

[BustinCaps]

Re: Small RCA TV white screen

If it was a sot23-3 marked 339, (in picture 5) it was a FDN339AN N-Channel mosfet. How did you find the correct values of the resistors, if they were burnt? What values did you use? They seem to be the mosfet source resistors, and are likely around 1Ω.
If the mosfet was bad, the drive ic301 may also have been damaged.

This is a SMPS, IC301 is the drive ic for Q305 mosfet that drives the transformer T301. I would check for shorted diodes on T301 secondary like D302, D303, D305, D306 etc.
You are likely missing all the voltages created by this smps circuit

I wasn't sure about the resistors so I got some 10 ohms resistors (10R0). When I check the old resistors that I removed, they are 1.5 ohms.

IC301 appears to be 3843B pwm controller (see attached image). The below values are what I see for the pins 1-8 on IC:
1. 6.7v
2. 1.6v
3. .15v
4. 2.3v
5. 0 v
6. 1.3v
7. 12.2v
8. 5v

T301 shows 12.5v going in on the primary at two inputs but zero or -.02v and -.03 v coming out on the secondary. Could transformer be bad?

Also, capacitor C323 shows negative voltage on the ground side of -12v and nothing on the positive side. Strange??


Any ideas? Thanks.

[dskall]

Re: Small RCA TV white screen

Good clear pictures of mainboard and t-con would help. Do you have voltage to t-con?