Sasung M87, problematic back light inverter.

[Dang]

Hi, I hope some one can clarify the part I need here. I have a Samsung LE40M87BDX which has been suffering from a non starting back light, it powers on fine, has sound but just no back light(a torch shows up menus), upon looking and with a little testing using a DMM and a hair dryer, it seems part number KIA78M12T is at fault, it's a three terminal positive voltage regulator. There's a 12 volt feed that should run to the inverter, but this is at 8.13v only raising to 8.43v once warmed up, if I heat it up with the hair dryer for about 10 seconds it shoots up to over 10v allowing the Tv to power up properly, at about 9.5v the back light struggles to get going, once on it holds at a constant 9.48v. The problem I'm having is finding an equivalent, I can't find the exact part and I'm not familiar with these so not sure what is suitable.

I have found a data sheet for the exact part but it's a little alien to me, page No5: http://pdf1.alldatasheet.com/datashe...KIA78M12T.html If someone could help me source an equivalent replacement from the likes of eBay, I'd be very grateful, thanks, Andy.

[budm]

Re: Sasung M87, problematic back light inverter.

What input voltage do you get feeding this 12V regulator? May be Vin is too low for it to regulate.

[tom66]

Re: Sasung M87, problematic back light inverter.

Depending on the package it would be equivalent to a 7812 in TO-220 or DPAK, but check Budm's suggestion first. A 78xx requires at least 1.8V more on the input than the output to regulate correctly.

[Dang]

Re: Sasung M87, problematic back light inverter.

Thank you to both of you, I shall have a look tomorrow morning and let you know, Andy.

[Dang]

Re: Sasung M87, problematic back light inverter.

OK, measured V-in and it's only at 13.43v, I believe it should be at something close to 19v so quite a discrepancy, any ideas on what else could be causing this? thanks again, Andy.

[tom66]

Re: Sasung M87, problematic back light inverter.

Bad capacitors on the input... what is BN44 number of power board?

[Dang]

Re: Sasung M87, problematic back light inverter.

Hi, it's a BN44-00165A. From the voltage regulator I did follow the track back from V-in and yes, it leads to the usual capacitors that fail, as to what one or few I don't know, none are raised, bulged or leaking???

[tom66]

Re: Sasung M87, problematic back light inverter.

Schematic

[tom66]

Re: Sasung M87, problematic back light inverter.

The 78M12T looks to be a low dropout variant of 7812 so 13.3V is fine (and is indicated on schematic as MULTI13V.) You could probably use an LM2940-12
https://cdn.badcaps-static.com/pdfs/...daece7257c.pdf

[budm]

Re: Sasung M87, problematic back light inverter.

Disconnect CNI806 to see if the output will go back up to 12V. The output also feed 22 Ohm resistor for the IC VCC.

[Dang]

Re: Sasung M87, problematic back light inverter.

Thanks again to both of you. OK, so upon looking at the schematic I'd agree, V-in looks to be correct although the datasheet does show 19v as V-in, the resistor R1814 measures at 22.3 Ohms so perfectly fine, there is a cap just beside the voltage regulator which I've changed as a precaution, no change to the output though. I've found this: http://www.ebay.co.uk/itm/1108307299...84.m1438.l2649, is this possibly what I'm after?

CN1806 was disconnected and tested, still exactly the same.

[budm]

Re: Sasung M87, problematic back light inverter.

If you disconnect all the loads from the output pin and install 100 Ohm 2W (or 47 Ohms 5W) as the load, if the out put is still not 12V then we know for sure that the IC is bad, we are trying to find out if the output is low due to heavy load or due to bad IC.

[Dang]

Re: Sasung M87, problematic back light inverter.

If you disconnect all the loads from the output pin and install 100 Ohm 2W (or 47 Ohms 5W) as the load, if the out put is still not 12V then we know for sure that the IC is bad, we are trying to find out if the output is low due to heavy load or due to bad IC.

OK, just to clarify, I disconnected the stated connector, the two main inverter power cables and then tested pin 1, still exactly the same result, I then reconnected them back up and used the test pad just below the connector on the inverter for V-out, still exactly the same result. I'm not too sure on your method but I would guess the above would show the inverter to be OK, so seen as V-in seems correct it would point to the voltage regulator being bad, but that's just my guess.

[tom66]

Re: Sasung M87, problematic back light inverter.

This is what I would use
http://www.ebay.co.uk/itm/LM2940CT-1...item2a0fb48f45

The 7812 is not low drop out so may not work properly.

Also, turn the TV on, run it for about 15 mins with fault. Then turn it off and feel regulator, is it burning hot? Or just cool/warm to touch?

[Dang]

Re: Sasung M87, problematic back light inverter.

This is what I would use
http://www.ebay.co.uk/itm/LM2940CT-1...item2a0fb48f45

The 7812 is not low drop out so may not work properly.

Also, turn the TV on, run it for about 15 mins with fault. Then turn it off and feel regulator, is it burning hot? Or just cool/warm to touch?

Thanks for the pointer, I'll get back to you a little later on the 15 minute power up, thanks again, Andy.

[tom66]

Re: Sasung M87, problematic back light inverter.

Just if you use that part, I would recommend you solder a 22uF or greater capacitor (16V rated or more) between output and ground because it might be unstable (oscillate) otherwise. Any capacitor will do - does not have to be low ESR type.

[Dang]

Re: Sasung M87, problematic back light inverter.

Thanks again, had the Tv on for the 15 minutes, just stayed cool if not cold. I shall try my local Tv repair centre for the part tomorrow, they're usually quite good, if a no go I shall try your recommendation, I'll post up my findings in due course. Once again thank you to the both of you for your time, it has been a great help, Andy.

[Dang]

Re: Sasung M87, problematic back light inverter.

Hi again, I have one last question before ordering the part, the part you recommended requires a decoupling capacitor to smooth out it's output, why would the original not require one?

I have found this :http://www.maplin.co.uk/1a-low-dropo...egulator-46321 at Maplin(it's just down the road from me), it says it's a TS78M05 if you hover over the picture, I'm sure the 05 should be a 12 so hopefully in stock, do you think this would be suitable and still need the decoupling capacitor?

[tom66]

Re: Sasung M87, problematic back light inverter.

Just because the original part is hard to get hold of, this one is an alternative part but requires a small capacitor to make it work properly. The Maplins one is not a low dropout regulator and is 5V rated, you need one rated for 12V. Low dropout is important because it might not produce the right supply voltage once the unit has been running for long.

[Dang]

Re: Sasung M87, problematic back light inverter.

Hi, yes, I realize the above and bought this one: TS78M12CZ, , upon checking it's specification sheet it looks very close to what I had and I've now installed it, the Tv now works with an 11.8v feed to the inverter, a small drop of 1.5v andit's been on for over an hour without issue, just what the doctor ordered. Thanks again, it was certainly informative and of course, as always, very helpful, Andy.


The next I have is a Sharp 37HV4E, usual stuck in stand by start up issue, thanks again.