Samsung LN40A650A1F, no backlight

[budm]

Re: Samsung LN40A650A1F, no backlight

Which sch are you using? the 198 or 199 so I can match your designator descriptions with sch.

[CraigParton]

Re: Samsung LN40A650A1F, no backlight

Sorry, should have specified. Its the 198

[budm]

Re: Samsung LN40A650A1F, no backlight

The top drive section of QI1201 more likly to have shorted ZI101, QI101 when the MOSFET sorted out.
QI1201 and QI1202 are driven on each half of the switching cycle, the running frequency of this circuit is may be 50~80Khz. These two MOSFET's should never be on at the same time, it is like having two switches in series and hook up to the 400v power supply, you know what will happen if both switches are turn at the same time!. The center point of the output circuits is connected to the primary winding of the transformer, then current output is coupling to ground through coupling cap C1221, this also acts as a DC blocking cap, if this cap shorted out or has leakage, this one of commonly fail component it this circuit setup, it will take out the MOSFET's, take this cap out and check the value and for any resistance leakage.
The purpose of QI101/102 is for auto limiting Gate drive circuits, if too much current (due to too much drive voltage from the Gate drive transformer) flows through DI101/111 and RI130/133 it will develop enough bias current to turn on QI101/102 which will limit the drive voltage to the Gate of the MOSFET'S.
I hope this help.

[CraigParton]

Re: Samsung LN40A650A1F, no backlight

Thanks for the thorough explanation budm, it will take a few reads to decipher that due to my limited knowledge but I'll read it until I get it completely. I finally was able to get ZI101 off the board (no I don't have a soldering station) and it was indeed the cause of the short. Tested 9 ohms in both directions. Can I use a normal replacement 15v zener or do I need to try to find a surface mounted one?

edit: OK, wow that was a great explanation. The circuit is a LOT less cryptic now. I'll take the cap out and test it. I do have a meter that checks capacitance. Its not an ESR but I guess its better than nothing. At least resistance leakage is easy to test.

[budm]

Re: Samsung LN40A650A1F, no backlight

Any 15V 1/2W Zener diode will do. Verify for sure that QI101/102 are OK otherwise the limiter circuits will not be working and can blow up your new MOSFET's

[CraigParton]

Re: Samsung LN40A650A1F, no backlight

I wish there was an easier way to check QI101 & QI102. I'm having a very difficult time removing these SMD components. They seem to be glued to the board.

I was able to test C1221 in circuit as long as I kept my meter leads reverse biased again D1224. It tested infinite on the 20 Megaohm scale.

[budm]

Re: Samsung LN40A650A1F, no backlight

Also verify that it still indicate 0.47uF on the cap meter.

[CraigParton]

Re: Samsung LN40A650A1F, no backlight

I'll check that in the morning. I forgot to bring my capacitance meter home with me. I'll also need to hunt down a 15V zener somewhere. The only electronics supply store I have nearby is Radioshack and they definitely aren't what they used to be.

[CraigParton]

Re: Samsung LN40A650A1F, no backlight

IT LIVES!! I removed and tested CI221 and it was perfect. Infinite resistance and .447 uF. I was able to get a 15V zener from a local TV shop. Replaced it and the TV lit up and worked like new!

Thanks you guys so much!! Without the schematic and operational theory I would have never been able to fix this. You guys rock!

[budm]

Re: Samsung LN40A650A1F, no backlight

Thanks for the feedback!

[xredhawk]

Re: Samsung LN40A650A1F, no backlight

hi, i have the same set with same problem which zener was bad, thanks

[budm]

Re: Samsung LN40A650A1F, no backlight

Great, glad to hear you solved the problem.

[CRT]

Re: Samsung LN40A650A1F, no backlight

I apologize in advance if this is bad manners by bumping a old thread, but since it is so similar to my problem I thought it would be better to keep the entire discussion contained.

I have a Samsung LN40A550P3F 40" LCD, that was purchased new by me, back in the day. Never had a single problem with it, beautiful picture. It was our first non CRT TV.

This past week, I was watching TV, went into another room and came back to a black screen, but sound still playing. Unplugged the TV for the night and came back the next day with same issue, sound but no picture and no back light. The red light on the front of the TV now flashes when the TV is powered on.

After a lot of searching, reading and more searching; I stumbled upon this thread, which appears to have possibly the same problem. The power board looks identical. Not sure what all is different between the model numbers.

Anyways. I checked the fuse FI1, circled in the picture and it was indeed blown. I am not quite sure where to go next. I de-soldered and checked a few diodes that appear to be good. Hesitant to start pulling the mosfets and checking them as it has been a few years since I've needed to probe out a circuit so I am a little rusty.

If someone could point me in the direction of where to go next, in simple terms I would greatly appreciate that.

[dresinger]

Re: Samsung LN40A650A1F, no backlight

I just wanted to add my experience to this thread in hopes that it may help someone else out. First off I want to thank everyone who has already posted on this thread, as all of the suggestions, advice, pictures and schematics were extremely helpful to me. I had a samsung ln40b530p7n which would power on but and play all sounds but appeared to have no picture. The first thing I did was shine a light on the screen while on to check if it was a video issue or backlight issue. Sure enough with the light on the screen I could see video so I knew my issue was with the backlight. I disassembled the TV down to the power supply board and also found that my FI1 fuse was blown. I tested the mosfets and surrounding rectifying diodes in circuit and found a dead short in mosfet QI202. I replaced my fuse (FI1) and the mosfet (QI202) checked everything once again and powered the board up. Upon powering up I could hear the transformer on the power supply board buzzing and the backlight inverter board buzzing as well. I knew there must be another short somewhere drawing on this power source. Sure enough I found that diode DI224 which is located just below the large white capacitor down beside the transformer was shorted out as well. Upon replacing my set is back up and running. I hope that this helps someone in the future.

[budm]

Re: Samsung LN40A650A1F, no backlight

Thanks for that info about DI224, never run into that bad diode before but did run into lots of bad white cap (lost its capacitance) next to it.