Samsung LN40B650T1F won't power on

[MEB]

Re: Samsung LN40B650T1F won't power on

bump

[tom66]

Re: Samsung LN40B650T1F won't power on

Does the problem occur on all inputs? Does it affect the menus?

[MEB]

Re: Samsung LN40B650T1F won't power on

OK, before I could get back into trying to answer Tom's questions about the inputs, my friend informed me that the TV died while his wife was watching it. I have been reading posts and wanted to try to find out some information before posting back. Here is what I tried.
I have attached again power board photos, schematic from Tom, and datasheets for easy viewing.
TV is still at my friend's house. After replacing the bad CCFL, the TV worked fine, except for the aforementioned line. Then while viewing a program, the TV quit with no further signs of life. When plugged in there is nothing. Remote will not even light the power LED.
Had my buddy remove the back and told him to check fuses FS801S and FM802S on the power board. FS801S was good, FM802S was open. It is a 3.15A 250V ceramic fuse. I remembered reading a post by budm, and although I didn't really understand the theory, I made a 75W “test lamp” by soldering a bulb to a spent fuse and had my friend plug it in the fuse holder. I told him to watch the light. The light lit up very brightly as soon as the unit was plugged in to AC. See attachment if you wish. I told him if the lamp lit, to power off and then pull the connector to the ribbon cable from the power board at CNM802 and try again. Right or wrong, my thought was that would tell me if the problem was on the power board or farther down the line. The lamp lit up again. So upon getting that report, I figured the power board itself to be bad and told him to remove it and bring it to me. Budm would be proud as this time he left the sockets in the TV! (some earlier humor)
Armed with a meter, the schematic and not much knowledge except countless hours of reading Badcaps, this is what I found up to this point.
Q1821 reads shorted from G-S-D on ohms or diode mode.
Q1820 reads shorted from S-D, G-S and G-D read 120 or 138 depending on lead orientation.
QP801S and QP802S I would say are OK.
UM801S is nearby but didn't know what to do with him.

I would guess from previous reading to remove Q1820 and Q1821 from board and retest, but I would feel better with some advice before I get into trouble. Should I try to check anything else? Thanks in advance for any help.

[MEB]

Re: Samsung LN40B650T1F won't power on

bump

[MEB]

Re: Samsung LN40B650T1F won't power on

fishing

[MEB]

Re: Samsung LN40B650T1F won't power on

I did remove QI801 from the board and it reads shorted everywhere. Guess I will try to find one. There was some sort of reinforcement in the through hole for that device. Is that typical?

[tom66]

Re: Samsung LN40B650T1F won't power on

It is normal.
If this is like the other Samsung boards I remember this is a common fault.
You only need to replace the FETs - you can leave the other components alone because it is driven by transformer, so damage to the control ICs is rare.

[MEB]

Re: Samsung LN40B650T1F won't power on

Thanks Tom. I just checked Digikey and Mouser and it seems this device is obsolete. I will see if there is an equivalent in these posts. Thanks again.

I think I recall July to be your start work month?

[tom66]

Re: Samsung LN40B650T1F won't power on

I start on the 7th, so my availability for help over the next year will decrease significantly.

You only need to match a new FET to the following parameters: Qg same or less, Rds(ON) same or less, Vds same or higher, Id same or higher.

[MEB]

Re: Samsung LN40B650T1F won't power on

Thanks very much for the device info. I don't mind helping myself and with that information I think I can find a suitable replacement. Thanks again.

Good luck with your new job. I have no doubt they will never want to let you go. I (and everyone) will be hoping you have time to still hang out with us.

[MEB]

Re: Samsung LN40B650T1F won't power on

Per Tom's post above I chose a Fairchild FDPF8N50NZU Mosfet as the replacement and replaced both QI820 and QI821. Sent the board back with my friend. He installed it last night and reported that the TV is working wonderfully. Thanks again to Tom and others who helped me learn so much during this adventure.

[triplefour]

Re: Samsung LN40B650T1F won't power on

i just came into one of these tv's and it also has the 3.15A fuse blown... thank you for taking the time to fix this tv and giving me a place to start