Badcaps.net Forum
Go Back   Badcaps Forums > Troubleshooting Hardware & Devices and Electronics Theory > Troubleshooting TVs and Video Sources
Register FAQ Calendar Search Today's Posts Mark Forums Read

 
Thread Tools Display Modes
Old 06-29-2022, 01:07 PM   #1
BustinCaps
Member
 
BustinCaps's Avatar
 
Join Date: Mar 2016
City & State: California
My Country: United States
I'm a: Knowledge Seeker
Posts: 33
Default Small RCA TV white screen

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.
Attached Images
File Type: jpg IMG_2127.jpg (522.2 KB, 22 views)
File Type: jpg IMG_2126.jpg (625.4 KB, 22 views)
File Type: jpg IMG_2125.jpg (563.7 KB, 17 views)
File Type: jpg IMG_2128.jpg (421.4 KB, 20 views)
File Type: jpg RCA-SMD.jpg (699.0 KB, 20 views)
BustinCaps is offline   Reply With Quote
Old 06-29-2022, 02:17 PM   #2
dskall
Badcaps Veteran
 
dskall's Avatar
 
Join Date: Oct 2016
City & State: Bakersfield ca.
My Country: usa
Line Voltage: 120VAC 60Hz
I'm a: Knowledge Seeker
Posts: 2,809
Default Re: Small RCA TV white screen

I guess you can start by testing voltages and posting model.
__________________
I assume no responsibility for any stupid suggestions I might post.
dskall is offline   Reply With Quote
Old 06-29-2022, 03:19 PM   #3
nomoresonys
Badcaps Veteran
 
Join Date: Jan 2013
City & State: midwest
My Country: U.S.
I'm a: Knowledge Seeker
Posts: 8,191
Default Re: Small RCA TV white screen

Could be a bad capacitor/s the electrolytic caps are usually the ones that go first, can be bad and not be bulging.
nomoresonys is offline   Reply With Quote
Old 06-29-2022, 03:22 PM   #4
nomoresonys
Badcaps Veteran
 
Join Date: Jan 2013
City & State: midwest
My Country: U.S.
I'm a: Knowledge Seeker
Posts: 8,191
Default Re: Small RCA TV white screen

and check all the solder joints: https://www.google.com/search?q=bad+...ih=607&dpr=1.5
__________________
(Signature) One of the first and least expensive things you can do when you have an issue is take the powerboard out of the tv, flip it over and look the solder joints over with a magnifying glass if needed, add a little flux to make the solder flow well and add solder to any joints that look even a little suspect. You can practice on a scrap pcb board if needed, careful not to bridge any joints, and always let the board sit for quite a while or discharge the big caps with an appropriate resister before working on it (safety first) to try to keep from getting shocked, it doesn't take 1000 volts to KILL, a lot less can kill, so be careful, it's only a tv. THANKS FOR THE NO TIP POLICY....WELL NO TIP NO WORK.
nomoresonys is offline   Reply With Quote
Old 06-29-2022, 03:24 PM   #5
R_J
Badcaps Veteran
 
Join Date: Jun 2012
City & State: ☻
My Country: Canada
I'm a: Professional Tech
Posts: 8,161
Default Re: Small RCA TV white screen

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

Last edited by R_J; 06-29-2022 at 03:51 PM..
R_J is offline   Reply With Quote
Old 06-29-2022, 07:23 PM   #6
BustinCaps
Member
 
BustinCaps's Avatar
 
Join Date: Mar 2016
City & State: California
My Country: United States
I'm a: Knowledge Seeker
Posts: 33
Default Re: Small RCA TV white screen

Quote:
Originally Posted by nomoresonys View Post
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 is offline   Reply With Quote
Old 06-29-2022, 07:28 PM   #7
BustinCaps
Member
 
BustinCaps's Avatar
 
Join Date: Mar 2016
City & State: California
My Country: United States
I'm a: Knowledge Seeker
Posts: 33
Default Re: Small RCA TV white screen

Quote:
Originally Posted by R_J View Post
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 is offline   Reply With Quote
Old 06-29-2022, 07:47 PM   #8
BustinCaps
Member
 
BustinCaps's Avatar
 
Join Date: Mar 2016
City & State: California
My Country: United States
I'm a: Knowledge Seeker
Posts: 33
Default Re: Small RCA TV white screen

Quote:
Originally Posted by dskall View Post
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.
BustinCaps is offline   Reply With Quote
Old 06-29-2022, 07:52 PM   #9
R_J
Badcaps Veteran
 
Join Date: Jun 2012
City & State: ☻
My Country: Canada
I'm a: Professional Tech
Posts: 8,161
Default 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

Last edited by R_J; 06-29-2022 at 08:46 PM..
R_J is offline   Reply With Quote
Old 06-29-2022, 08:37 PM   #10
dskall
Badcaps Veteran
 
dskall's Avatar
 
Join Date: Oct 2016
City & State: Bakersfield ca.
My Country: usa
Line Voltage: 120VAC 60Hz
I'm a: Knowledge Seeker
Posts: 2,809
Default Re: Small RCA TV white screen

Good clear pictures of mainboard and t-con would help. Do you have voltage to t-con?
dskall is offline   Reply With Quote
Old 06-30-2022, 05:36 PM   #11
BustinCaps
Member
 
BustinCaps's Avatar
 
Join Date: Mar 2016
City & State: California
My Country: United States
I'm a: Knowledge Seeker
Posts: 33
Default Re: Small RCA TV white screen

Quote:
Originally Posted by R_J View Post
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.
Attached Images
File Type: png 3843B pwm.PNG (85.3 KB, 5 views)

Last edited by BustinCaps; 06-30-2022 at 05:54 PM.. Reason: C402 does show voltage
BustinCaps is offline   Reply With Quote
Old 06-30-2022, 06:35 PM   #12
BustinCaps
Member
 
BustinCaps's Avatar
 
Join Date: Mar 2016
City & State: California
My Country: United States
I'm a: Knowledge Seeker
Posts: 33
Default Re: Small RCA TV white screen

Quote:
Originally Posted by dskall View Post
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
BustinCaps is offline   Reply With Quote
Old 06-30-2022, 06:36 PM   #13
R_J
Badcaps Veteran
 
Join Date: Jun 2012
City & State: ☻
My Country: Canada
I'm a: Professional Tech
Posts: 8,161
Default 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.

Last edited by R_J; 06-30-2022 at 06:56 PM..
R_J is offline   Reply With Quote
Old 06-30-2022, 08:10 PM   #14
dskall
Badcaps Veteran
 
dskall's Avatar
 
Join Date: Oct 2016
City & State: Bakersfield ca.
My Country: usa
Line Voltage: 120VAC 60Hz
I'm a: Knowledge Seeker
Posts: 2,809
Default 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

Last edited by dskall; 06-30-2022 at 08:14 PM..
dskall is offline   Reply With Quote
Old 06-30-2022, 09:12 PM   #15
R_J
Badcaps Veteran
 
Join Date: Jun 2012
City & State: ☻
My Country: Canada
I'm a: Professional Tech
Posts: 8,161
Default 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
R_J is offline   Reply With Quote
Old 06-30-2022, 09:48 PM   #16
dskall
Badcaps Veteran
 
dskall's Avatar
 
Join Date: Oct 2016
City & State: Bakersfield ca.
My Country: usa
Line Voltage: 120VAC 60Hz
I'm a: Knowledge Seeker
Posts: 2,809
Default 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.
dskall is offline   Reply With Quote
Old 07-01-2022, 06:48 PM   #17
BustinCaps
Member
 
BustinCaps's Avatar
 
Join Date: Mar 2016
City & State: California
My Country: United States
I'm a: Knowledge Seeker
Posts: 33
Default Re: Small RCA TV white screen

Quote:
Originally Posted by R_J View Post
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
BustinCaps is offline   Reply With Quote
Old 07-01-2022, 07:33 PM   #18
nomoresonys
Badcaps Veteran
 
Join Date: Jan 2013
City & State: midwest
My Country: U.S.
I'm a: Knowledge Seeker
Posts: 8,191
Default Re: Small RCA TV white screen

looks like this one will work: https://www.digikey.com/en/products/...AO3420/1855956
nomoresonys is offline   Reply With Quote
Old 07-01-2022, 07:40 PM   #19
R_J
Badcaps Veteran
 
Join Date: Jun 2012
City & State: ☻
My Country: Canada
I'm a: Professional Tech
Posts: 8,161
Default 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.
R_J is offline   Reply With Quote
Old 07-01-2022, 07:46 PM   #20
BustinCaps
Member
 
BustinCaps's Avatar
 
Join Date: Mar 2016
City & State: California
My Country: United States
I'm a: Knowledge Seeker
Posts: 33
Default Re: Small RCA TV white screen

Quote:
Originally Posted by nomoresonys View Post
Thanks. I'll check it out.
BustinCaps is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump



Badcaps.net Technical Forums 2003 - 2022
Powered by vBulletin ®
Copyright ©2000 - 2022, Jelsoft Enterprises Ltd.
All times are GMT -6. The time now is 07:29 AM.
Did you find this forum helpful?