Samsung LN-T375HA will not turn on, power light stays red.

[bluebuda]

Hi, I have been fixing electronics for about 2 and a half to three years.

I am posting this for two reasons.

First, because I have already diagnosed the problem and fixed it and I am hoping to help others who might have this problem in the future by saving them some cash. XD

Second, because I am what I would call a noob, I know guys who have been doing this for years, so I could use a little help myself, it seems all good, I help others and get help myself. =)

So to start, if you have a Samsung TV and it wont turn on and the power light stays red it's a real good chance it's your power board, I will provide a picture of this models power board tomorrow along with the problem I found on mine, you could also google the power board model number for visual identification, google is your friend. =D

This TV is a LN-T375HA and the power board is a BN44-00157A, if you take the back off the TV it will be a tan color board in the very middle of the TV.

A quick run down on trouble shooting it, look to see if any capacitors are blown, if you don't know how google it, it's the same for all electronics, check to make sure the fuse is good, if it is blown the bridge rectifier might be bad, once again google can help explain how to deal with bridge rectifiers, check for cracked resistors, if you have a cracked resistor this could indicate the main IC has blown, something for a tech to repair and not a beginner.

In my case none of these things were the case, and I'm posting this in case anyone else comes across this problem, instead of buying a new power supply board find PC802S on the power supply board, it will be a small 4 legged square black part, to remove it you will need a solder sucker and a soldering pen, they are not hard to remove, for those who are more familiar with electronics repair this is where it gets a bit harry for me, LOL, I believe it is an optocoupler but I'm not 100% sure the part is found on the web as 817B, and if I were to read the entire chip it reads as L0744 817B X.

I replaced it with the same chip as I just happened to have one, but here is my question, I know some of you very experienced techs know the answer to this, let's say the chip blows out again, and I don't have that exact chip to replace it with again, what chips could I cross it with as a suitable replacement?

[budm]

Re: Samsung LN-T375HA will not turn on, power light stays red.

That is an Optoisolator, one side has LED (pin 1 and 2), the receiving side has photo transistor. This opto that is damaged, is bad in the LED side or in the transistor side, It is used for the feedback for voltage regulation. Is this opto for the 5v standby or for the 12v/24v POWER supply section.
When LED is on, you should have about 1.2~1.4Vdc on pin 1 & 2. The transistor side should read open when LED is OFF and about 0.1V if the LED current is 20mA.

[bluebuda]

Re: Samsung LN-T375HA will not turn on, power light stays red.

Thanks for the quick reply and the info. =D

I'll look into it and get back with you tomorrow when I post the pics of the power board.

[bluebuda]

Re: Samsung LN-T375HA will not turn on, power light stays red.

Alright, so real quick here is a picture of the Power board, supposing my legs are labeled correct as I tend to be a bit dyslexic from time to time this is what I can tell you.

I first became aware that the chip was functioning incorrectly but what I would call sure dumb luck, LOL, and I just happened to be right.

I board has 2 other 817B Optoisolators, so I did some probing on them, the chip I replaced had a much different reading from the other two, although when I replaced it the reading was still different, so i would just say I was lucky. XD

Anyways, when I measured legs 3 and 4 on the bad chip with my multi meter in ohms mode it read 211 Ω, after replacing the chip it would read either 21 MΩ or 3.8 MΩ.

I can also tell you this, when the TV is off, bad chip or good chip it reads like this in DC mode if you ground the negative probe to the set frame and test the legs with the positive probe they read as follows, legs 4 and 3 will both read 37v, and leg 2 will read about 4v while leg 1 reads 5v. When a good chip is in and the TV turns on Legs 1 and two remain the same and legs 3 and 4 will drop down into the mv range.

As far as testing the legs with the negative and positive probes on legs 1 and 2 the reading bounces all over the place and reads 1.2v briefly at random intervals.

When the positive and negative probes are used to test legs 3 and 4 with the TV on it reads 8.6v and when the TV is off it drops down into the mv range.

As far as if the Optoisolator is in the 5v or 12v/24v range I would not know that right off hand by looking at the board, I would have to trace the leads to see where it is going, perhaps you can tell by looking at the picture or the information I have provided, but without tracing it it a bit above my head at the moment, I will have time later this week to trace it and see, or perhaps you can school me in how to tell without tracing the board. XD

Also, is any 4 legged chip like this with 817 on it a suitable replacement for it, or am I going in the wrong direction with my thoughts on this chip?

[budm]

Re: Samsung LN-T375HA will not turn on, power light stays red.

"I measured legs 3 and 4 on the bad chip with my multi meter in ohms mode it read 211 Ω" that means the transistor inside is shorted out. It should read really high resistance.
Since you have the spec of the original Opto, you just have to match the spec, but you will be better off get the same one since it is part of the feedback circuit so you do not want to use some other part that may not have the same spec, especially the current transfer ratio spec. When you work on the primary side of the circuit, the negative leg of the large DC filter cap will be your circuit ground reference point, not the chassis ground.
The secondary side you can use the chassis as the circuit ground point for reference point.

[bluebuda]

Re: Samsung LN-T375HA will not turn on, power light stays red.

Thanks for you help! XD It is much appreciated!

[tibimakai]

Re: Samsung LN-T375HA will not turn on, power light stays red.

I have one of these sitting in my garage with half a picture. I understood that I have a shorted panel.

[bluebuda]

Re: Samsung LN-T375HA will not turn on, power light stays red.

Have you tried to disconnect and reconnect the ribbon cables on the Tcon board?

Half a picture would automatically make me suspicious of the Tcon board, does the half a picture that shows up look good or is it bad, as in the picture is the wrong colors, has artifacts, or lines running through it?

[budm]

Re: Samsung LN-T375HA will not turn on, power light stays red.

@blue, here is the diagram for the BN-00157A:
http://www.tgohome.com/ServiceManual...g_BN44-00157A_[SCH].pdf

[tibimakai]

Re: Samsung LN-T375HA will not turn on, power light stays red.

Have you tried to disconnect and reconnect the ribbon cables on the Tcon board?

Half a picture would automatically make me suspicious of the Tcon board, does the half a picture that shows up look good or is it bad, as in the picture is the wrong colors, has artifacts, or lines running through it?

TV doesn't start up at all, just blinks the LED. If I disconnect the ribbon on that bad side, the TV starts up.

[bluebuda]

Re: Samsung LN-T375HA will not turn on, power light stays red.

Hmmm. I have never experienced that symptom, all I can say is that a bad panel has never stopped any TV from turning on that I have worked on, I have fixed maybe around 40 to 50. All I can say is it's strange behavior, if you wanted to spend a little money checking into it you might be able to buy the Tcon from Shop Jimmy, if it doesn't work you could get a refund minus a restocking fee and return shipping.

Hopefully someone with more experience than me can offer their input on this.

[tibimakai]

Re: Samsung LN-T375HA will not turn on, power light stays red.

I have purchased a T-con from Ebay. The old one had a blown chip(tom66 said that it's the gamma chip and if that blows that is fatal for the panel).

[bluebuda]

Re: Samsung LN-T375HA will not turn on, power light stays red.

Interesting, I know nothing about gamma chips so I can't help you in that way, I was simply taught by a television repair shop owner as I wanted a bit of knowledge for myself on how electronics work, obviously I did not learn even close to everything in 2 and 1/2 years and I would say that a gamma chip is something I have never even heard of and so it is way over my head, I would love to read all about them though if anyone has any references to it, I googled it but was unable to find it.

Anyways, sorry I could not be of more help, perhaps someone else with more experience than me will be able to help.

I can tell you from experience if the panel was bad at the shop we would scrap the set, pull all the working parts off and sell them because the panels are often impractical to replace considering the price they sell for is much, much, more than a new set.