Samsung LE26B455C4W backlight problem

[thth]

I'm repairing a Samsung LE26B455C4W TV for a friend. The fault is no backlight. Sound and everything else is OK, the picture is there (looked with a flashlight). The power supply board is BN44-00259A.

I searched the net and this forum and I found some info where to look but so far I haven't found the problem.

What I have tried so far:

I found a Samsung service note for this model about a diode causing backlight problems. The diode has to be changed to a more realiable type but that part is already the correct type.
http://www.docfoc.com/samsung-lcd-tv-repair-power

Managed to fry the inverter FETs + fuse when measuring voltages, replaced those. Before that, I had already tested the old FETs and they were OK.

After that, however, the backlight came on. Happily, I turned the power off and put the back cover on but the backlight didn't work any more.

When the other board is disconnected from the power board the backlight flashes on very briefly when TV is plugged in.

Checked the DC blocking capacitor (0.33 uF / 400 V) in the inverter circuit. Seems OK.

I found out that in some other models it helps if a jumper is cut disabling a cut-out feature or something. Presumably it doesn't apply with this model?

[Andrew F. Ali]

Re: Samsung LE26B455C4W backlight problem

Are you getting BL-On voltage from the Main Board to the Power Board?? Post clear focused pictures of the boards from the backside of your TV.

[thth]

Re: Samsung LE26B455C4W backlight problem

Here are a couple of pictures. They are not clear or focused, unfortunately. Sorry about that. I'll try to get a better camera next time.

I'll measure the BL-ON voltage and report it here later.

[budm]

Re: Samsung LE26B455C4W backlight problem

BN44-00259A.
Pictures provided by shopjimmy.

[budm]

Re: Samsung LE26B455C4W backlight problem

Are you getting close to 400VDC between the two legs of the main filter cap when TV is turned on?

[thth]

Re: Samsung LE26B455C4W backlight problem

Thanks for the excellent pictures!

Are you getting close to 400VDC between the two legs of the main filter cap when TV is turned on?

It was around 320 volts if I remember correctly. I'll re-measure that too.

[budm]

Re: Samsung LE26B455C4W backlight problem

If you only seeing only 330VDC then the PFC Voltage booster is not working which will cause the Inverter circuit not to work either. You also check the diodes (DP804, 805, 810) in the PFC section again.

[thth]

Re: Samsung LE26B455C4W backlight problem

There is a pin marked inv_on/off on the connector between boards. The voltage on that is 5 volts.

The voltage on the electrolytic cap is 320 volts. I thought it was supposed to be rectified mains voltage (or line voltage) ie. 230 Vac x 1.41 = 324 Vdc ?

[budm]

Re: Samsung LE26B455C4W backlight problem

There is a pin marked inv_on/off on the connector between boards. The voltage on that is 5 volts.

The voltage on the electrolytic cap is 320 volts. I thought it was supposed to be rectified mains voltage (or line voltage) ie. 230 Vac x 1.41 = 324 Vdc ?

It is only true if it does not have PFC Voltage booster circuit, you board has PFC circuit using Q802S, IC U801S, Inductor LP801S, Diodes DP 804, 805, 810.

[dick_barton]

Re: Samsung LE26B455C4W backlight problem

There is a pin marked inv_on/off on the connector between boards. The voltage on that is 5 volts.

The voltage on the electrolytic cap is 320 volts. I thought it was supposed to be rectified mains voltage (or line voltage) ie. 230 Vac x 1.41 = 324 Vdc ?

The PFC circuit Budm mention boost the supply voltage.

[thth]

Re: Samsung LE26B455C4W backlight problem

OK. Thanks for the clarification. I tested Q802S (FET) with a cheap Chinese component analyzer and it checked OK. Should I change it anyway? The original is K8A60DA. I have IRF840 and 2SK3562, would one of these work?

The diodes checked OK with the multimeter diode check function.

[budm]

Re: Samsung LE26B455C4W backlight problem

The MOSFET in the PFC circuit usually failed as shorted circuit and blows the main fuse, so it is either the PFC IC is bad or it is not getting the switched DC to run when the PS-ON is present. You need to get the spec of the PFC IC so you can check the DCV between the GND and the VCC pin when PS-ON is present.

[thth]

Re: Samsung LE26B455C4W backlight problem

There is 18 volts in the VCC pin of U801S (= ICE3BR0665J)

[budm]

Re: Samsung LE26B455C4W backlight problem

There is 18 volts in the VCC pin of U801S (= ICE3BR0665J)

That is not the PFC IC,, the PFC IC is UP801S.

ICE3BR0665J is SMPS IC not PFC IC.

[thth]

Re: Samsung LE26B455C4W backlight problem

OK. I got confused when you said earlier U801S, sorry. I found the correct IC (FAN7530) and the VCC is about half a volt so it is not getting power.

[thth]

Re: Samsung LE26B455C4W backlight problem

I have tried to trace the Vcc for the FAN7530 but I'm not sure where it is supposed to come from. Does anyone have a schematic?

[thth]

Re: Samsung LE26B455C4W backlight problem

Hello again. It's been a while since I worked on the TV but now I'm trying again.

I removed a jumper that gets the Vcc for the FAN7530 (pin 8). The voltage is 14 volts but it goes low when I connect the jumper. Also I heard a click from somewhere.

So the FAN7530 is drawing too much current? Could the fault be after the output of the IC or would the built-in over-current protection handle that?

[thth]

Re: Samsung LE26B455C4W backlight problem

I changed the FAN7530 chip but it didn't help. What can I do next, please help?

[dick_barton]

Re: Samsung LE26B455C4W backlight problem

Manual available from


Check CP809 electrolytic capacitor and it's ESR

[thth]

Re: Samsung LE26B455C4W backlight problem

Thanks for the manual link! I checked CP809. It was OK (ESR was 0.2 ohms). Can you suggest what I should do next?