Ex Serviceman Needs Help - Page 1
Forum section: electronics / repair

author: adrianed
author_rank: Newbie
timestamp: April 15, 2018, 01:28:32 am
content: Hi FolksI retired from servicing before the flat panel screens came out here in Australia so I have no experience repairing them.I have decided to learn a bit by repairing my own Samsung LA40C 550 JIF TvIt looks to me that all the back lights have gone offIs it safe for my volt meter to check the voltage supply to the backlights or is there perhaps some reason that this cannot be done ( I presume it is Dc )Also could someone give me an idea how high the volts are in the backlight supply lineIf there is a better way of checking the supply a suggestion would be welcomeIf anyone is experienced with this model It would be handy if they could let me know if there is a common part that breaks down in this model ( It appears that there are several different power supply boards for this modelAdrian

author: duak
author_rank: Super Contributor
timestamp: April 15, 2018, 05:14:07 am
content: I would bet your TV has a Cold Cathode Fluorescent Light (CCFL) backlight. There are a couple of small tubes, on the top and bottom edges of the panel. I don't know what the voltage is on a larger screen but on a laptop or even a 20" monitor it's like 800-1000 VAC so it's a good thing you asked. The HV is generated by a small inverter and connected to the CCFLs with some silicone wires of maybe 1 mm diameter similar to but smaller that those used with a CRT. If you have a 'scope, it can usually pick up the field from the inverter just by getting close to the wires. I believe the frequency is something like 20-40KHz. I wouldn't try to measure the voltage unless I used a HV differential probe that could safely handle KV AC signals.Try looking for Samsung LCD TV backlightBTW, If you've been out of computer and display repair for a while, one thing you'll find is that electrolytic capacitors, especially in power supplies, don't last all that long even for reputable manufacturers like Samsung. Look up bad capacitors.Best o' luck

author: tpowell1830
author_rank: Frequent Contributor
timestamp: April 15, 2018, 05:35:32 am
content: Quote from: adrianed on April 15, 2018, 01:28:32 amYes, check the power supply board for bulging or bad caps. Also, find the the little inverter transformers (depends on size of screen, but there could be multiple or just be one little flat transformer) for the light and ohm them out to see if there is one open. This is the most common failures. Of course, make sure supply is not storing charge before proceeding.

author: DC1MC
author_rank: Super Contributor
timestamp: April 15, 2018, 07:40:09 am
content: Maybe a service manual will help you:http://www.go-gddq.com/upload/2014-01/14012815322983.pdfGood luck,DC1MC

author: adrianed
author_rank: Newbie
timestamp: April 16, 2018, 02:42:10 am
content: Thankyou duak,I did watch a post on YouTube where they said not to touch the backlight power, well of course a serviceman would know not to touch power supply point but they didn't give a reason, so thankyou for the answer to that.A shame about electolytic capacitors they are in everything, at least we know of a weak point in power suppliesAdrian

author: adrianed
author_rank: Newbie
timestamp: April 16, 2018, 02:52:31 am
content: Hi tpowell1830Thankyou for your suggestionsNo bulging capacitors so I shall check them with ESR meter, in any case perhaps change the easy ones, but don't like disturbing the printed board without reasonThere are several inverter transformers on this tv but I assumed that being as all the lights are out I need to look for a more central point IE along the main backlight supply lineAdrian

author: adrianed
author_rank: Newbie
timestamp: April 16, 2018, 02:58:23 am
content: Thankyou DC1MC,Thankyou for that informationFor me it will take some good luck, perhaps I should take a refresher course, life is so full of things we would like to do. perhaps I should just try to buy a new power supply but I will not learn much from thatAdrian

author: LateLesley
author_rank: Frequent Contributor
timestamp: April 16, 2018, 11:24:37 am
content: Maybe give this video a watch -A lot of newer screens have LED backlight's instead of the CCFLs, and when they die, they can take out the whole string of LEDs, depending on if they are wired in series or not. Maybe that video will help in diagnosing the problem.

author: vinceroger
author_rank: Regular Contributor
timestamp: April 17, 2018, 06:17:47 pm
content: this set uses ccfl tubes i would start by doing the basic tests on the power supply and see if any voltages are missing like bl_on pin does this set when you first turn it on give a quick flash of back light on the screen it maybe one or 2 seconds this could mean you have a faulty inverter transformer or ccfl tube

author: LateLesley
author_rank: Frequent Contributor
timestamp: April 18, 2018, 12:56:06 am
content: if it is CCFLs, they i'd agree the inverter could be the culprit. I'd be making a bee-line for the inverter FETs, it's quite common for them to die. I've replaced a few in monitors in my time.

author: adrianed
author_rank: Newbie
timestamp: April 18, 2018, 05:49:48 am
content: Thankyou vinceroger,There is no flash from the backlights when or after switching on.I can see the picture on the dark screen if I point a flashlight to it and the sound is workingThe backlights just failed to come on when the tv was switched on, no half measures & no previous signesJust like to say I have been delayed into doing the checking due to various things I have had to attend to but do thank you and everyone who has responded to my request for help and I shall use all the tips that have been offeredAdrian

author: adrianed
author_rank: Newbie
timestamp: April 18, 2018, 05:55:28 am
content: Hi LateLesley,Thankyou for the YouTube video, very helpful, and the tip about the FetsAdrian