Samsung LN26B360 Flashing Backlight

[devnull]

Hello,

I was given a Samsung LN26B360 that I would like to fix. The issue is that the backlight "flashes". When the power button is pressed the TV plays its startup sound, the sound comes on and video appears. Then the backlight turns off and stays off for about 2 seconds then the backlight turns back on for about one second and turns off. The cycle continues until you turn off the TV. During the cycle you can hear sound and if you do the "flash light" method you can see the image, you can also see the image during the brief time the backlight is working.

I looked at the caps and did not see any that were bulging. I have taken some readings across the 30 pin header as well as on the large capacitor.

Below are my readings:

Large Cap "On": 387V
Large Cap "Off": 164V

Readings from the 30 pin header:
(I have omitted the GND and N.C. Pins)
1 - 1.19V
2 - 1.75
3 - 5.16
7 - 13.08
8 - 13.07
13 - 5V
14 - 5.15V
15 - 5.15V
16 - 5.15V
19 - 13.06V
21 - 13.05V
22 - 13.06V
23 - 5.14V* 0V**
24 - 0V
25 - 3.16V
27 - 4.45V* 0V**

* = Voltage present during the short time the backlight is on.
** = Reading when the backlight is off.


Attached are some pictures of the IPB.

(IPB part # BN44-00259A)

I am unsure which component is bad or how I should go about testing for a faulty component. I was hoping someone could suggest what I should check next.


Thank You.

[cmj21973]

Re: Samsung LN26B360 Flashing Backlight

Here are the manuals for your set.

http://elektrotanya.com/samsung_ln26.../download.html

http://elektrotanya.com/samsung_ln26.../download.html

[retiredcaps]

Re: Samsung LN26B360 Flashing Backlight

I was hoping someone could suggest what I should check next.

1) It sounds like you have 2 seconds to black. Possible causes are:

a) bad caps
b) bad secondary inverter transformer windings
c) bad ccfls
d) bad ccfl wiring (return wiring)

2) All explained in detail starting with post #19 at

https://www.badcaps.net/forum/showthread.php?t=10419

3) I would concentrate on b) first. This isn't your exact transformer, but it is close enough to show which are the secondary pins. Report your findings. See

http://www.lcdparts.net/TransformerD...ProductID=3804

4) Those are excellent photos.

5) After looking at the transfomers on

http://www.lcdparts.net/T.aspx

it looks like LT-HU26 means 26 inch TV. LT-HU32 is 32 inch T, and so on.

6) If you don't have 2 seconds to black, you may have a corrupted eeprom on the main board that is causing the TV to reset?

[PlainBill]

Re: Samsung LN26B360 Flashing Backlight

That may not be a power supply problem, the main board seems to be turning the inverter on and off. Pin 23 is 'Inv_On/OFF' while 27 is 'DET_5V'. 'DET_5V' may be triggering the problem. Unfortunately the picture of the bottom of the board isn't clear enough to trace the signal line.

PlainBill

[retiredcaps]

Re: Samsung LN26B360 Flashing Backlight

Pin 23 is 'Inv_On/OFF' while 27 is 'DET_5V'. 'DET_5V' may be triggering the problem.

Arrgh. I didn't see the legend in the photos. Back to sleep.

[randtek]

Re: Samsung LN26B360 Flashing Backlight

Disconnect the connector from the power board for the cable that goes to the main board. Plug the unit in. If the backlight comes on and stays on, your main is most likely causing the issue. If the backlights still flash, probably a power board issue, could be CCFL.

[tom66]

Re: Samsung LN26B360 Flashing Backlight

Are you in the US, or a place with 115V electricity?

It sounds a lot like the PFC section of the power supply in your monitor is dropping out. It could be a bad main capacitor, a bad current sense resistor, a bad controller IC, bad startup capacitor, or, or...

But I think it is definitely the cause of the problem because I have an LCD TV that will play sound right down to 40Vac but the backlights drop out at 150Vac. (Rated 230Vac.) It had a separate transformer just for the backlight 24V power, I'm guessing the main supply was capable of working on a lower voltage, but the backlight supply wasn't.

[devnull]

Re: Samsung LN26B360 Flashing Backlight

Thank you all for your fast responses.

The readings from the secondary inverter transformer windings:

Transformer 1:
S1 & S2: .979
S3 & S4: .978
(All other combinations resulted in an infinite reading)

Transformer 2:
S1 & S2: .985
S3 & S4: .986
(All other combinations resulted in an infinite reading)

I have attached a zipped pdf of the schematic for the IPB.
I can attempt to take better photos of the bottom of the board to help troubleshoot the issue with DET_5V if the schematic is not enough.

@randtek: I tried your suggestion. The backlight flashes for a split second when I plug in the power cable and then nothing, TV appears to be "off".

@tom66: Yes I am in the US and electricity is 115V.

I am going to continue down the list of possible causes in retiredcaps list.
Starting with the CCFL test. I will have to figure out a way to do it as the IPB "slides" into the all the CCFL connectors at once. I will have to see if I have some high voltage wire.

[retiredcaps]

Re: Samsung LN26B360 Flashing Backlight

The readings from the secondary inverter transformer windings:

Those are good.

I can attempt to take better photos of the bottom of the board to help troubleshoot the issue with DET_5V if the schematic is not enough.

Yes, please.

I am going to continue down the list of possible causes in retiredcaps list.
Starting with the CCFL test.

Before you do that, let's see a better picture for the above area.

[tom66]

Re: Samsung LN26B360 Flashing Backlight

Does the large cap drop out when the lights go out?

[devnull]

Re: Samsung LN26B360 Flashing Backlight

I have attached two photos starting at the 30 pin connector and following the #27 PIN. I followed it until it hit a small IC. I marked the 27th PIN in the first image.

The large cap voltage stays the same the whole time the TV is on. It only drops when I turn the TV off.

[retiredcaps]

Re: Samsung LN26B360 Flashing Backlight

I have attached two photos starting at the 30 pin connector and following the #27 PIN. I followed it until it hit a small IC. .

If that is UI802, can you make out a part number on the chip? The schematic shows 393 in green, but nothing else.

[devnull]

Re: Samsung LN26B360 Flashing Backlight

Yes it is UI802.
From what I can make out:

A393F
906

[devnull]

Re: Samsung LN26B360 Flashing Backlight

I went ahead and did the CCFL test. I tested each and every bulb by itself. I still get the "2 seconds to black". Interestingly, with a single bulb it does not fully illuminate. However when all 4 bulbs are attached they all fully illuminate. Is that normal?

The lcd has 4 bulbs total, at one end of each bulb there is a small black mark. (See attached photo) Could ALL the bulbs be bad?

There is also no return wires, instead there seems to be a metal "cap" at the end of each bulb which snaps into a holder.

I am open to suggestions on what to test next.

[retiredcaps]

Re: Samsung LN26B360 Flashing Backlight

I went ahead and did the CCFL test. I tested each and every bulb by itself. I still get the "2 seconds to black". Interestingly, with a single bulb it does not fully illuminate. However when all 4 bulbs are attached they all fully illuminate. Is that normal?

1) Testing a single ccfl will cause the pwm inverter chip to shutdown backlights. The pwm inverter chip, in lay terms, works as an "all or nothing". Either all the ccfls work or no backlights.

2) Testing a single ccfl one at a time is designed to see if you have a completely broken ccfl. If one ccfl didn't light then you found the culprit.

3) A single ccfl may not light up all the way because the pwm inverter chip is in the process of shutting it down.

The lcd has 4 bulbs total, at one end of each bulb there is a small black mark. (See attached photo) Could ALL the bulbs be bad?

Unlikely.

There is also no return wires, instead there seems to be a metal "cap" at the end of each bulb which snaps into a holder.

The guide was written with computer lcd monitors in mind and is mostly applicable to lcd TVs as well.

[retiredcaps]

Re: Samsung LN26B360 Flashing Backlight

A393F

I had no luck in finding anything with that part number that is an IC.

[devnull]

Re: Samsung LN26B360 Flashing Backlight

I did some searching and I found this:
http://www.datasheetcatalog.com/data.../KIA393F.shtml

The PINs in the schematic appear to match up to UI802 as it appears in the IPB schematic.

Are there any other tests to narrow it down to either CCFL or IPB?

[retiredcaps]

Re: Samsung LN26B360 Flashing Backlight

I am open to suggestions on what to test next.

Long shot, but try resoldering all the inverter transformer pins with fresh solder?

[retiredcaps]

Re: Samsung LN26B360 Flashing Backlight

I just realized we don't have a picture of your main/logic board.

[retiredcaps]

Re: Samsung LN26B360 Flashing Backlight

Readings from the 30 pin header:
1 - 1.19V

Pin 1 is power on/off. If this is equivalent to PS_ON, I would have expected to see either 0.0 or 3.0V.