Samsung LN40B540P8F LCD Backlight Problems

[h_smith]

I have the symptom where the backlight is cycling. I have read other threads about various problems with transformers and MOSFETs and have a few questions.

A bit of background on the TV is that it was used very little and that is why I was considering trying to repair it. It was plugged in and not being used when a nearby lightning strike took out a 60" plasma, cable box, receiver, and other small devices (+ $2K in lost electronics). I suspect that this strike also damaged this TV but I cannot determine how badly. It was several days later that the backlight started cycling and it was not immediately after the strike. I have heard there can be a delay between the strike and apparent damage so that is a possibility. I also realize there is a possibility that other boards are damaged and that will not be discovered until the inverter/backlights are addressed.

The TV does turn on, the backlight cycles on and off. I can hit the menu button on the remote and see that the menu is displayed. At this point it seems to be an inverter/backlight only issue (maybe).

I have little knowledge in debugging electronics so I am looking for a few pointers to debug.

A few of the questions that I have are:

1) When I disconnect the Inverter Board (BN44-00264B) from the logic board, should the backlight come on and stay on? It does come on briefly but then cycles on and off thereafter.

2) Reading other posts on the forum It seems that transformers can be bad. There are no visible signs over overheating. I would like to know if I can test them with a multimeter while they are in-circuit or do they need to be desoldered and removed to test each one? I have the same question for the MOSFETs.

3) I have also read on other posts that it is possible that there could be problems with the backlight bulbs where they are drawing too much current and this is what causes the inverter board to go into an overcurrent mode. Is there and easy way to rule out the bulbs with a meter/power supply or is this a visual inspection only?

I have visually inspected all the capacitors on the inverter board and they look OK. I also tested all of the diodes and they are also all OK. The two fuses are Ok.

Based on the above, and given the age of the TV and symptoms, would it be best to:

1) Attempt to debug the inverter board and/or bulbs?

2) Roll the dice and buy a refurbished inverter for $50 off of eBay and hope that the bulbs are not the problem?

3) Take the TV to the electronics recycler and call it a day?

I am a hobbyist interesting in debugging but I can see that pulling all of the devices (transformers, MOSFETs, capacitors) would be quite an investment of time (since I rarely do it). If the symptoms suggest that buying the refurb inverter board is the best course to take to save time please let me know.

Any advice would be appreciated.

Thanks.

Heath

[neilc6]

Re: Samsung LN40B540P8F LCD Backlight Problems

Disconnect the cable between the main board and power supply board. The backlights should turn on.

[neilc6]

Re: Samsung LN40B540P8F LCD Backlight Problems

That transformers on that board tend to burn up or fail. Normally there is a burnt spot on it. Deadly high voltages in the inverter section which is why it is covered.

https://www.badcaps.net/forum/showthread.php?t=70871

[h_smith]

Re: Samsung LN40B540P8F LCD Backlight Problems

That transformers on that board tend to burn up or fail. Normally there is a burnt spot on it. Deadly high voltages in the inverter section which is why it is covered.

https://www.badcaps.net/forum/showthread.php?t=70871

Must they be removed to check the resistance in the primary and secondary coils?

Thanks.

[tvtimmy]

Re: Samsung LN40B540P8F LCD Backlight Problems

No, but they should read + or - 3% of each other. If you find one that reads differently, it's probably the one.

[h_smith]

Re: Samsung LN40B540P8F LCD Backlight Problems

I pulled the transformer that had a reading of 950 ohms on one of the secondary coils and ordered a new one.

Before installing the new one I checked the resistance on the primary and it was 10.6 ohms. The secondary coils were at about 850 ohms.

My understanding is that the primary should be around 13 ohms and the secondaries at 1.1K ohms.

Do you suppose that I received a defective unit?

Should I ask for another one to be sent?

This is the one that I purchased from ShopJimmy via eBay:

https://www.ebay.com/itm/QGAH02095-O...B/273987590975

Thanks.

[tvtimmy]

Re: Samsung LN40B540P8F LCD Backlight Problems

What were the readings of the other 2 white ones?

[h_smith]

Re: Samsung LN40B540P8F LCD Backlight Problems

Interesting enough I said to heck with it and installed the part even though I was not seeing 13 ohms (saw 10.6 ohms) on the primary or 1.1K on the secondaries (850 ohm on both) and the damned thing worked!

I am not an electrical guy so it is a mystery. I told ShopJimmy about my observations that the resistances did not seem right and asked for some specification of what they were supposed to be. They said they did not have any. I am a bit mystified by the whole TV repair business. I do not understand how you can fix these things without having proper schematics and testable specifications for the parts?

Anyhow, all is well that ends well. I can't believe the thing is working again.

Moved on to repair my LG 60" Plasma. Throwing the dice that a nearby lightning strike took out the same logic board as the first nearby strike did. This time I am not attempting board repair but found one at Electroparts Online that was reasonably priced (compared to eBay listings with no guarantees that the boards were or whether they were known to be working) with free shipping. If I get both my sets back in to operation it will be a miracle!

P.S. - When using an ohm meter on the transformers while in circuit I was reading 3.5 ohms on the primary. Once removed I was seeing the expected 13 ohms. (testing the old part). The secondaries can be checked in-circuit and the one was a 1.1K and other and 950 ohms. No clue why the disparity between the resistance on the secondaries had the effect that it did. I only replaced this part and it worked.

[tvtimmy]

Re: Samsung LN40B540P8F LCD Backlight Problems

Good fix. I bought a 6 pack of those transformers a few years ago. Used 2, and can't find any more to repair. I'll keep looking........

[Unspun01]

Re: Samsung LN40B540P8F LCD Backlight Problems

P.S. - When using an ohm meter on the transformers while in circuit I was reading 3.5 ohms on the primary. Once removed I was seeing the expected 13 ohms. (testing the old part). The secondaries can be checked in-circuit and the one was a 1.1K and other and 950 ohms. No clue why the disparity between the resistance on the secondaries had the effect that it did. I only replaced this part and it worked.

I've repaired this style inverter board a few times.

On one TV it was the white transformer that failed. On another it was the blue one.

The primaries are usually ok as they use a thicker wire. The secondaries have many more turns than the primary and hence use a much thinner wire. As a result of ccfl lamps aging, they draw more current to light and remain lit and this stresses the thinner gauge of the secondary winding until it runs hot, and fails the coating on the wire and then adjacent turns of the secondary wire fuse together or burn up. The resistance of the winding can go up or down (also affecting the secondary current) as a result and the protection will operate and shut down the backlight.

You were correct when you noticed the difference in the coil resistance on the secondaries from one transformer to another. The one that is not like the others is the bad one. if I recall, the white transformers have two secondaries to measure, while the blue transformer only has one secondary...

If the bad transformer was running hot for a while, it will usually have a noticeable burned spot in the white or blue tape on the top of the transformer. If not, you can sometimes see evidence of heating/discoloration on the underside of the board when it is removed. This is the transformer to replace. If it failed quickly, you may not see burn marks and you need to rely on resistance testing.

One TV I fixed had 3 bad transformers on it... The tape was charred black on 2 of them. I didn't suspect the 3rd one until I did resistance tests.

The parts are not too expensive from Shopjimmy or Aliexpress.

white: QGAH02095
blue: QGAH02098

[h_smith]

Re: Samsung LN40B540P8F LCD Backlight Problems

Just a quick note, the bad transformer was a white one. There was no evidence of discoloring or overheating. There was only a difference in the resistance readings on the secondary coils, 1.1K versus 950.

Time will tell if there is a bad bulb that was the root cause of the failure. From what I have read this is a weak component that does not hold up over time. The set that I have is very lightly used.