Small RCA TV white screen

[BustinCaps]

I am trying to fix a small RCA TV for a friend. It holds sentimental value for his father. Initially it had what looked like some burned out resistors and would not turn on. I changed out the resistors and now it turns on but I only get a white screen. I have no experience fixing TVs but hope someone can point me in the right direction on what to check or how to proceed. Thanks. Attached are some pictures.

[dskall]

I guess you can start by testing voltages and posting model.

[nomoresonys]

Could be a bad capacitor/s the electrolytic caps are usually the ones that go first, can be bad and not be bulging.

[nomoresonys]

and check all the solder joints: https://www.google.com/search?q=bad+...ih=607&dpr=1.5

[R_J]

The plug in picture four looks crooked, make sure it is seated properly. (the flat cable does not just pull out and get pushed back in, the latch must be released) What resistors did you replace that made it turn on, Resistors don't usually burn unless something else is bad.

I guess the redit bot couldn't help 3 months ago

[BustinCaps]

Originally Posted by nomoresonys

Could be a bad capacitor/s the electrolytic caps are usually the ones that go first, can be bad and not be bulging.

I've checked all the caps and they all look good.

[BustinCaps]

Originally Posted by R_J

The plug in picture four looks crooked, make sure it is seated properly. (the flat cable does not just pull out and get pushed back in, the latch must be released) What resistors did you replace that made it turn on, Resistors don't usually burn unless something else is bad.

I guess the redit bot couldn't help 3 months ago

Yeah the cable has a brown latch that slides out then you slip the cable in and slide it back in to lock it. The connection should be good. The resistors are the ones in picture 4 right below transistor. The thing is I replaced the top transistor too but maybe it was the wrong one. The markings on the original transistor were 339 d. The transistor I replaced it with is a N channel. Maybe it should be a P channel?

[BustinCaps]

Originally Posted by dskall

I guess you can start by testing voltages and posting model.

Thanks for the reply. It's an old RCA that was made for Radio Shack. Model # 16-3066. I could not find schematics anywhere.

[R_J]

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

[dskall]

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

[BustinCaps]

Originally Posted by R_J

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.

[BustinCaps]

Originally Posted by dskall

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.

IMG_2146.jpg

IMG_2148.jpg

[R_J]

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.

[dskall]

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]

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]

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

[BustinCaps]

Originally Posted by R_J

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.
IMG_2150.jpg

IMG_2152.jpg

[nomoresonys]

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

[R_J]

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

[BustinCaps]

Originally Posted by nomoresonys

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

Thanks. I'll check it out.