Need help repairing my old crt tv with no power - Page 1
Forum section: electronics / repair

author: silviasol
author_rank: Regular Contributor
timestamp: May 29, 2021, 12:06:39 pm
content: I have my old 13" tv from 1994 I have wanted to fix for awhile. Finally got around to opening it. It currently will only power on if you leave it unplugged for a few hours, then you can plug it in and power it on. Power it off you have to unplug it and then wait the few fours before plugging it back in and powering it on.I have some pictures here, I think it is the big cap in the second picture as it is bulged at the top. Going to desolder it and order a new one. Any other advice or something I could do so long as I have it open?This has sat for years so I figure is ok to handle, no voltage at all in the cap. It is a 200v rated cap. Any advice for when I put a new cap in and try powering it on, how to decharge the 200v in the cap?

author: BrokenYugo
author_rank: Super Contributor
timestamp: May 29, 2021, 12:50:35 pm
content: Sounds more like the standby/start up circuit. Do you have a schematic? Model number would be good too.

author: SeanB
author_rank: Super Contributor
timestamp: May 29, 2021, 12:56:20 pm
content: Capacitor top has a plastic cover, bulging there is normal, just if it is pushed in gently and does not move will it be faulty. Would check high value resistors in that area, along with all the solder joints around the area, as well around T601 at the front, as that is the standby circuitry. Very commo for resistors to go high in value, and for the solder joints to get brittle and crack, especially with heavy parts like the LOPT, heatsinks and power resistors.You can also get the service manual, by searching for the Samsung model number on the case or the board.

author: RayRay
author_rank: Frequent Contributor
timestamp: May 29, 2021, 12:59:59 pm
content: Could you maybe clean up the big cap with a brush and take another pic? Not really sure what I'm looking at.Anyhow, the problem you're describing does sound bad cap related (and electrolytic ones are more prone to issues than others) that said, you have to keep in mind that things aren't always as they seem (and even a capacitor that looks physically fine can be faulty!) so I'd suggest you do things properly (instead of just hoping that replacing that one cap would suffice). Do you have a capacitance meter (or a multimeter with a decent capacitance range?). The big cap should be discharged automatically within a few mins after the power cord is disconnected (but you should definitely measure it with a multimeter to be on the safe side). You could also do a manual discharge with a 10K resistor. Also, do keep in mind that any replacement caps you put on this board have to be the low ESR (also known as low impdenance) type, and preferably of high lifetime (5000 hours or above) and of a reputable brand, such as Rubycon/Nichicon/NCC (Nippon Chemi-Con)/Panasonic/Ymin. It might also be a good idea to measure the physical size of em with a digital caliper (or alternatively, look up the datasheet of the particular capacitor series to get it's dimensions)

author: RayRay
author_rank: Frequent Contributor
timestamp: May 29, 2021, 01:02:04 pm
content: Quote from: SeanB on May 29, 2021, 12:56:20 pmUh? A capacitor's top should never be bulgy! If it's not flat, then it's definitely faulty.

author: SeanB
author_rank: Super Contributor
timestamp: May 29, 2021, 01:36:20 pm
content: There is a plastic cap on those Samwha capacitors, and it always bulges out, with the aluminium case under them being flat still. You need to gently press down on the plastic, and it generally goes flat, but if it does not go down the capacitor is failing, though those pretty much all will spew their guts out the bottom first, as the seal is generally going to fail there before the burst pattern does. Blowing the top is only for catastrophic failure, like having 300VDC applied to a 200VDC capacitor, they typically last a long time, as they run under a lot less stress than the smaller lower voltage ones.

author: silviasol
author_rank: Regular Contributor
timestamp: May 30, 2021, 01:10:38 pm
content: Quote from: BrokenYugo on May 29, 2021, 12:50:35 pmThe model number of the tv is ttb1340 and the only number on the motherboard I see is 36029-0048-000

author: silviasol
author_rank: Regular Contributor
timestamp: May 30, 2021, 01:18:14 pm
content: Quote from: SeanB on May 29, 2021, 01:36:20 pmIt does press down, so does not seem to be bulged just the plastic cover is warped.Here is a pic of the back of the motherboard. I figure clean it up and maybe resolder all these area's with what looks to be old flux would be a good start? I am seeing this creep up onto the leads of some components so maybe those have cracking solder.What is the dangers of high voltage on this motherboard. I though I read that crt tv's have components that hold high voltage, what should I look out for. Or how long should I wait after powering it on before I can safely work on it again? I have it on a spot I can just leave it so got all the time in the world.Also here is a pic of my multimeter also.

author: YetAnotherTechie
author_rank: Regular Contributor
timestamp: May 30, 2021, 01:45:51 pm
content: It's the CRT that holds the most dangerous charge for longer(Kvolts for days), there must be a video somewhere, but basically it's like shorting a capacitor at the pins. A common method is to attach a wire to the ground straps around the crt, and the other end of the wire to a long thin screwdriver, which you hold by it's INSULATED handle, KEEPING ONE HAND BEHIND YOUR BACK, and slide the screwdriver under the suction cup at the crt, it will discharge with a big noisy spark, and you do it a couple of times until you can keep it no longer sparks.Do so at your own risk!

author: xavier60
author_rank: Super Contributor
timestamp: May 30, 2021, 01:55:47 pm
content: There is no need to discharge the CRT's anode unless the EHT cap needs to be disconnected, which it doesn't. Leave it alone.Expect hazardous voltages anywhere on the main board and CRT neck board.Avoid bumping or stressing the neck board. The CRT's glass sealing tip inside of the pin base can be easily broken.

author: xavier60
author_rank: Super Contributor
timestamp: May 30, 2021, 02:06:01 pm
content: One of the mains phases connects directly to the TV's ground. All metal parts should be treated as mains live.The TV should be powered with an isolation transformer to make voltage measurements safer to do.

author: xavier60
author_rank: Super Contributor
timestamp: May 30, 2021, 02:30:54 pm
content: Some unpowered testing for now should be in the area of Q901 and Q902 regulator transistors.There are a few hot running resistors that are likely to have cooked solder joints as well as on the transistors.First confirm that C851 isn't charged.

author: xavier60
author_rank: Super Contributor
timestamp: May 30, 2021, 02:47:58 pm
content: C950 is a likely fault suspect.

author: silviasol
author_rank: Regular Contributor
timestamp: May 30, 2021, 05:27:01 pm
content: Quote from: xavier60 on May 30, 2021, 02:47:58 pmAre you seeing this from my pictures or did you find a repair manual?

author: james_s
author_rank: Super Contributor
timestamp: May 30, 2021, 06:57:26 pm
content: Quote from: RayRay on May 29, 2021, 01:02:04 pmThe *metal* should never be bulgy, but the pictured capacitor has a plastic disc over the metal that is held by the outer shrink wrap, it's perfectly normal for them to look bulged, I have seen capacitors like that before. The bulge is soft, if you poke at it you can feel that it's just the outer plastic cover.

author: james_s
author_rank: Super Contributor
timestamp: May 30, 2021, 07:00:20 pm
content: Quote from: YetAnotherTechie on May 30, 2021, 01:45:51 pmNot really. The CRT anode uses a high voltage, but the current is in microamps. Speaking from experience, getting zapped by a CRT doesn't feel very good at all, but it's much less likely to kill you than the "low" voltage B+ which is around 180V on most TVs. Also a color CRT has a focus divider fed by the anode voltage in the flyback and that resistor divider will self discharge the anode usually within a few minutes. It is still not a bad idea to discharge the anode if you are going to disconnect it though.

author: fzabkar
author_rank: Super Contributor
timestamp: May 30, 2021, 07:01:58 pm
content: Service manuals (they seem to be missing the PSU section):https://elektrotanya.com/showresult?what=ttb1340&kategoria=&kat2=all

author: xavier60
author_rank: Super Contributor
timestamp: May 30, 2021, 09:02:14 pm
content: Quote from: silviasol on May 30, 2021, 05:27:01 pmFrom the manual,https://diagramas.diagramasde.com/otros/SAMSUNG%20MODELO%20TTB1340.pdfFrom the photos, the solder joints in the power section don't seem so bad.

author: silviasol
author_rank: Regular Contributor
timestamp: May 31, 2021, 02:59:58 pm
content: It would seem to me some kind of component that would always receive power. Since when it is plugged in it will not power on after powering off, unless you leave it sit for a few hours to discharge whatever component. Would that narrow anything down to what it may be?Thinking more about this the problem occurred in about 2007, that is 13 years of use since new. It has sat unused since like 2008. So a different time line then if it happened recently if that also would rule out anything.

author: james_s
author_rank: Super Contributor
timestamp: May 31, 2021, 05:52:52 pm
content: I would bet there's a bad capacitor or possibly a bad resistor in the standby PSU. Can you find a test point labeled something like 5V_stby? Maybe look for the power pin on the main microcontroller or on the IR receiver, that will need to be active at all times for the remote to be able to turn on the TV.

author: silviasol
author_rank: Regular Contributor
timestamp: June 01, 2021, 07:47:19 pm
content: Does this look like cracked solder? Can't seem to see the pins move when I try and budge them but solder looks off.

author: BrokenYugo
author_rank: Super Contributor
timestamp: June 01, 2021, 08:04:00 pm
content: Textbook cracked joints.

author: andy2000
author_rank: Frequent Contributor
timestamp: June 01, 2021, 08:32:22 pm
content: Check or replace any electrolytic capacitors that are located in hot parts of the board. You can see the hot areas because they are discolored.

author: james_s
author_rank: Super Contributor
timestamp: June 01, 2021, 09:02:23 pm
content: Quote from: silviasol on June 01, 2021, 07:47:19 pmYes, absolutely, some of those are definitely cracked. It has to get *really* bad before you can move it visibly.

author: YetAnotherTechie
author_rank: Regular Contributor
timestamp: June 01, 2021, 09:06:42 pm
content: You could maybe, take a brush and a vacuum cleaner to it? It would make your life easier.Then check all the solder joints of all power devices. I recommend removing the old solder and applying new one, not just adding flux or new solder on top.

author: silviasol
author_rank: Regular Contributor
timestamp: June 03, 2021, 01:50:09 pm
content: Quote from: YetAnotherTechie on June 01, 2021, 09:06:42 pmLOL. I tried a vacuum, my attachments did not work too well. The board can not be removed completely from the tube part, wires are permanently connected other wise I would bring it out and hose it off with my air compressor. Probably find a board and carry it out.I know well enough to put new solder on. My ocd will not allow such a thing.

author: james_s
author_rank: Super Contributor
timestamp: June 03, 2021, 05:43:46 pm
content: You don't need to remove the board anyway to clean it off with compressed air. You can even wash the whole thing with soapy water and rinse it off but I don't recommend this to people who do not know exactly what they're doing.