Samsung LCD Power Board turning on and off.

[Cessna170]

Re: Samsung LCD Power Board turning on and off.

Hey Guys looking for some help I have a 32' Samsung Model #LN32A450 with a power board Dong Yang MK32P5B, this unit runs for about 30 mins and then shuts off, and it will always restart right away....I replaced all the electrolytic caps 13 of them and I am still having the same issue, can anyone suggest any other component that might causing the issue...Please let me know and Thanks for reading my post.

[selldoor]

Re: Samsung LCD Power Board turning on and off.

Hi First question would have to be what caps did you use for the recap
make/series. Have you checked your work for cold/cracked joints and did you
use leaded solder.
This could be a heat issue - with the tv on, can you try and warm each board in turn with a hairdrier to try and speed up the switching off. If you find a board that does this then try heating the board in sections to pinpoint the problem. If this does not work we will have to think of something a bit more scientific.

[waitery]

Re: Samsung LCD Power Board turning on and off.

I would check that same FET that was giving me problems and resolder the leads. That might do it for you. Please see my earlier post for location.

[mbsqw1d]

Re: Samsung LCD Power Board turning on and off.

Hi, I'm new and I have the same issue. The TV is an LE26A457C1D and PSU is BN44-00214A Rev1.1- MK32P5B

Just wanted to document for anyone else having the same issue and hopefully add more evidence to OP's claim of the badly soldered MOSFET. I will check out and resolder the joints and report back.

[mbsqw1d]

Re: Samsung LCD Power Board turning on and off.

found that the T0-220 MosFET QB802 has a loose joint, solder has cracked on one of the pins. Hope a resolder will do the trick. else i wouldnt know what part id need to replace it?

[selldoor]

Re: Samsung LCD Power Board turning on and off.

Is there a part number on it? try to clean with isopropyl alcohol- even some of the number might help.

[thevolatile1]

Re: Samsung LCD Power Board turning on and off.

Hi, currently got one of these power supplies in bits.. Had a completely dead 150uf capacitor - yes the big one !! Replaced & tested - gives blinking red light on sammy 28 lcd its in. Further checks reveal all other caps are pretty much bang on stated values (all electrolytic caps on this pcb are samwha branded). Mosfet QB802 has 2 loose pins & looks like its had some massive heat - cooked board glue gone black & crispy - part tag - F H45 D526Y - Fairchild Bipolar NPN Transistors. Alternative to220 is 2sd526. I shall resolder & retest & come back with results later..

[thevolatile1]

Re: Samsung LCD Power Board turning on and off.

..well, tested & now the tv just sits with a blinking red light - responds to remote but doesnt turn on.. further checks revealed another dead part - took both to220s from qm801 & qm802 right behind the big smoothing capacitor - one is dead !!! I'm assuming it was qm801 since thats the one leading straight to the positive side of the capacitor I replaced. My psu schematic states the to220 is 500v 5a - actual part markings - fairchild m h47. fqpf. 5n50c. Part is FQPF5N50C - 500v N-Channel mosfet... now to go look in my spares box.... I'll be back later with results...

[selldoor]

Re: Samsung LCD Power Board turning on and off.

There is a repair kit listed here
http://www.ohmsupplies.co.uk/REPAIR-KIT-POWER-SUPPLY

It lists the parts in the kit so you may have more that have gone- advising in case there is a knock on effect where if you dont replace all the bad ones they kill the new replacements.

[thevolatile1]

Re: Samsung LCD Power Board turning on and off.

Thanks for the tip... I'm pulling each of those parts & testing now - except the MC33067P IC on ICM801. For anybody else tracking this fault, the other parts to check / replace are - QM801 / 802 FQPF5N50C mosfet, DM802 40v 1a diode, DM804 100v 250mA diode sm, RM811 / 813 10 ohm resistor sm, CM801 332k capacitor sm, CP803 450v 150uf electrolytic capacitor.
This applies to psu MK32P5B / BN44-00214A.
In my case I will be halting this job after testing the remaining parts as I'll have to order the mosfet since the only ones I have left new are K2843.. shame.. but I'll be back to tell you which parts were actually gone from that list..

[thevolatile1]

Re: Samsung LCD Power Board turning on and off.

update... All other parts checked out ok from the list & don't you just hate looking for tiny numbers on pcbs.. or maybe my eyes are just getting old...
So this fix - just the 2 parts - QM801 FQPF5N50C mosfet & CP803 150uf @ 450v capacitor.

[JonathanAnon]

Re: Samsung LCD Power Board turning on and off.

I have exactly the same board with the exact same problem. LED blinks four times and then goes off... On testing there is 330V across the filter cap, but no voltage on the rails or the schottky diodes on the secondary side of the SMPS... Found this thread when I was searching for the schema.....

Looked up the same parts mentioned and look what I spotted QB802...

[budm]

Re: Samsung LCD Power Board turning on and off.

It looks like some one had worked on this board and did not solder the part.

[JonathanAnon]

Re: Samsung LCD Power Board turning on and off.

It looks like some one had worked on this board and did not solder the part.

Hi again Bud, I did think that... cos the joints just look too clean, as if the chip was just screwed in to the heatsink and not actually resoldered... also the mess solder below the middle pin indicates that somebody was at it... but I know the guy that owns it and he says it's just started having problems and did not mention that he had anybody else looking at it.. ??

[budm]

Re: Samsung LCD Power Board turning on and off.

I can tell it was worked on because the Green PCB masking is scratched up, it is not that easy to remove the green masking unless it was heated up and scraped by the soldering tip. By the way, does this transistor check out OK?

[jetadm123]

Re: Samsung LCD Power Board turning on and off.

Does that mosfet have a date code stamped on it? Compare the year of the mosfet to the manufacture date of the TV. If it's the original mosfet, then the year of manufacture should be the same (or very close) as the TV set.

[JonathanAnon]

Re: Samsung LCD Power Board turning on and off.

It checked out fine when I tested it ... I tested it out of circuit first .. (i.e. the way it was in the photo above)..... The telly is coming back on now after soldering it..... I'm not sure what's going on, I'm gonna have to have a serious conversation with the chap....

At the very least I've sorted out the mystery of why his remote control doesnt work anyway



You just dont get the twenty years out of a set of batteries like you used to..

[budm]

Re: Samsung LCD Power Board turning on and off.

Oooo, but they are 'EXTRA HEAVY DUTY'!

[thevolatile1]

Re: Samsung LCD Power Board turning on and off.

update... All other parts checked out ok from the list & don't you just hate looking for tiny numbers on pcbs.. or maybe my eyes are just getting old...
So this fix - just the 2 parts - QM801 FQPF5N50C mosfet & CP803 150uf @ 450v capacitor.

update.... this tv is still working fine with the mosfet & capacitor replacement.

[mbsqw1d]

Re: Samsung LCD Power Board turning on and off.

found that the T0-220 MosFET QB802 has a loose joint, solder has cracked on one of the pins. Hope a resolder will do the trick. else i wouldnt know what part id need to replace it?

UPDATE: Re-soldered the pin. TV has been working 100% since.