Samsung TV - LE46M87BDX: no backlight

[Bicurico]

Hello,

My Samsung TV - model LE46M87BDX - is broken, with no backlight.

I purchased the TV second hand and it was in my country house. Due to 3 month of continuous rain and cold, the stone house was unusually humid, which might have contributed to the failure.

Anyway, the TV was working fine and switched on for hours (without anyone watching, just as background noise) and suddenly we noticed that the screen was blank but the sound was still working.

Upon inspection with a flashlight, I verified that the screen is working except for the backlight.

I opened the TV and noticed that 4 caps were swollen - as expected in this series, as it is infamous for bad caps.

Yesterday I exchange the 4 caps and noticed that the previous owner had already exchanged another pair of caps.

But this did not fix the problem, the backlight is still not working.

This TV uses CCFL backlight, as it is prior to the use of LED strips.

Possible faults are:

1) Some other component in the PSU board
2) Fault in the inverter boards (apparently it takes just one of many transformers to get broken for all of the backlight to fail)
3) Fault in the CCFL tubes (not sure if it takes one broken to fail the whole background light)

My problem is that I know nothing about repairing this PSU, apart from the fact that high voltages are present. Also, I don't have any spare components (transformers and MOSFETs) nor do I own a CCFL tester.

With a second hand PSU being sold at eBay at around 50 Euro (or more) I am faced with this reality: for 100-150 Euro I can buy a working second hand TV, for 200-250 Euro I can buy a brand new TV with 2 years of warranty.

It seems the wisest to just get rid of this TV (eventually try to sell it as is for parts for 50 Euro) and get a new TV.

On the other hand, it could be a 10 cent transistor.

Can anyone help suggesting a route for a possible repair or confirm that it is not worth the effort?

Thanks,
Vitor

[fzabkar]

Have you checked the service manual?

https://elektrotanya.com/showresult?what=LE46M87BDX&kategoria=All&kat2=All

These manuals usually don't have circuit diagrams for the backlight inverter, but it's still worth a look.

[shakalnokturn]

Back when I used to repair these quite often bad electrolytics on the 24V line in the PSU would cause more ripple current  towards those on the inverter often resulting in a blown fuse on inverter board.

Second most common cause was an open secondary winding on a transformer.

Last bad IC's and MOSFETs.

A single bad CCFL or transformer will shut the lot down.

[Bicurico]

Back when I used to repair these quite often bad electrolytics on the 24V line in the PSU would cause more ripple current  towards those on the inverter often resulting in a blown fuse on inverter board.

Second most common cause was an open secondary winding on a transformer.

Last bad IC's and MOSFETs.

A single bad CCFL or transformer will shut the lot down.

Questions:

With transformer you mean the ones on the inverter board? I have seen some videos where they determine which is faulty and then connect one pin to the ground. Does that make sense? Which pin would that be?

When one of the CCFL is bad, shouldn't the TV at least flash light for 1 second, before the circuit shuts all lamps down? That does not happen with me.

What do you mean with fuse? There is a regular fuse close to the power input. This is of course ok. I have not found any other fuse, much less near the transformers close to the connectors that provide power to the inverter boards.

Thanks!

[shakalnokturn]

Questions:

With transformer you mean the ones on the inverter board? I have seen some videos where they determine which is faulty and then connect one pin to the ground. Does that make sense? Which pin would that be?

When one of the CCFL is bad, shouldn't the TV at least flash light for 1 second, before the circuit shuts all lamps down? That does not happen with me.

What do you mean with fuse? There is a regular fuse close to the power input. This is of course ok. I have not found any other fuse, much less near the transformers close to the connectors that provide power to the inverter boards.

Thanks!

Yes transformers on inverter board for CCFL high voltage. Don't know about tying a pin to ground, I've always replaced them when defective.

True when one transformer secondary or CCFL is bad you can expect a flash of backlight on power-up.

By fuse I mean on inverter PCB close to 24V input connector, often at least one large SMD fuse >2A, sometimes a second smaller one to the control IC supply.

That was on the most common types... I've been a bit less lazy and looked for pictures of PCB's used in your model, a pity we have none in the opening post.

Yours doesn't have a typical setup, in yours I'd start with th H.V. supply in the corner of the PSU board (bad solders on the transformers were very common because they're bulky and get hot) also check on ceramic capacitors on output and film capacitor on primary.
You should be able to check if there is anything happening at all on power-up by leaving your oscilloscope probe close (without hooking it up) to the secondary, the voltage is dangerous here.

[Bicurico]

Hello.

Here is an update:

I checked the CCFL transformers and the high voltage section with a CCFL light bulb. In a video they explained that you would just need to bring the bulb near the regions with high voltage and it would glow. That did not happen, so I assumed that the inverter is not getting any power.

This led me to concentrate on the PSU and in fact I noticed 3 diodes that will pass current in either direction. The multimeter will beep no matter what way around. It is D1806, D1802 and D1805. The other seem fine.

Now the difficult task is to find where to source replacements...

Regards,
Vitor

[Bicurico]

I desoldered the 4 diodes. One pair is definitely broken with a short, the other pair is OK.

Each pair is composed by:

1x MU460 --> https://www.vishay.com/docs/88686/mur440-e3.pdf
1x UHF8JT --> https://www.mouser.com/datasheet/2/427/88956-339625.pdf

Question to the experts:

1) Can I assume that replacing the broken pair (or both, to make sure), might fix the problem? Or could it be some catastrophic failure caused by something else? I did not find any signs on the board.
2) What replacements to both references could I use, in case i cannot find them?

Kind regards,
Vitor

[shakalnokturn]

If none of the transistors are bad I'd look towards what the diodes are there for. Clamping, part of snubber?
They could have failed because of electrolytics you replaced in the main PS section, could also be bad capacitors in the snubber.

[Bicurico]

One MOSFET ist bad, too. The three are one group close to the inverter transformer.

The mirrored components are ok, for the other transformer.

But ordering equivalent parts would cost me around 20 euro, so I just bought a whole PSU board for 45 euro on eBay (with shipping). Hopefully that fixes the TV.

If not, I will try to sell everything for spares.

[Bicurico]

The TV is working again.

It was just the PSU board. I have now ordered the three components to repair the original board. I found a shop that can provide me with equivalent parts, but they need some time to order them themselves.