Samsung LCD Power Board turning on and off.

[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...

[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.

[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..

[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.

..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...

[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..

[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.

[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?

[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.

[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.

[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.

[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.

[greencroft]

Re: Samsung LCD Power Board turning on and off.

Which capacitors are replaced for switching in and out of standby

[retiredcaps]

Re: Samsung LCD Power Board turning on and off.

I am going to burn test it to make sure that we have the issue buttoned up.

The last suggestion is to replace the remaining non Panasonic caps so your TV set will last longer.

Good job!

[waitery]

Re: Samsung LCD Power Board turning on and off.

OK guys, we're watching TV! Cold solder joint found on Fairchild Mosfet located at QB802, just a step passed PC802S. I just kept getting these huge voltage swings at PC802, so I started backtracking and when I got to the FET and I touched it with my Fluke probe to get a reading and it was just enough pressure to start the TV without me having to press the on/off switch.....so thrilled! The joint was super weak looking before reworking it and applying three new beads of solder.

Looks like she is holding steady, voltages all look good and there seem to be no problems. It fires up perfectly every time I hit the remote and touch the set's power button. I am going to burn test it to make sure that we have the issue buttoned up.

Retired and PlainBill, THANK YOU SO MUCH for your help, patience and most of all, YOUR TIME! I would love to sit down and troubleshoot with you guys one on one! I appreciate you both!!

[PlainBill]

Re: Samsung LCD Power Board turning on and off.

The diode in question is DB801. It's on the drive to the primary SMPS controller. It's probably a bad idea to put a DMM on that point - there are some very high voltage pulses on it.

It's fairly easy to localize the area of the problem. I would prefer you download the schematic; it will make following along a lot easier. I'm going to use grid locators.

On page 1, location B8 is the ON/OFF input. When it is high, QB851 (1-B7) begins to conduct, and current flows through the LED of PC802S (1-B7). This causes the phototransistor of PC802S (1-B7) to conduct. This pulls the base of QB802 (1-B6) high. Since QB802 (1-B6) is in an emitter follower configuration, the emitter also goes high, which provides power to the main SMPS controller.

Troubleshoot by measuring the voltages from under the following conditions: TV in Standby; TV switched ON, but not powering up; TV operating. You are interested in every point from the ON/Off input through PC802S. Suspect a bad solder joint.

PlainBill

[waitery]

Re: Samsung LCD Power Board turning on and off.

I replaced all of the startup caps with new 105 degree Panasonic caps. Still having the same issue. One thing I did notice when I got it fired up once is that the standby voltage on the #1 pin shot up to 3.95vdc. Usually it will just go up from .35 to 3.7 and do nothing. Not sure what would drive it up higher, but the higher voltage is making it start up. Still very inconsistent and when it wants to start up.

As I am taking some test mesurements on some of the components, I come across a diode (D8801) in the primary side. The anode side is violently changing from 95vdc to 108vdc to 121 vdc, fluctuating rapidly and then beleive it or not, it flips to VAC (yes that is VAC) for a second then back again to do the same thing 95/108/121vdc. I could not belive what my Fluke was telling me. Tried it again and same thing, it was switching over to VAC and the voltage range was all over the place. The cathode side is much more stable, but flipping every few seconds from 192vdc to 199 vdc and back again.....no VAC though.

As I was sitting here typing this out, my set just came on by itself....I didn't even push the power button....very frustrating!!

Any thoughts on how I can chase this down or a good starting place? Thanks guys!

[PlainBill]

Re: Samsung LCD Power Board turning on and off.

Hi PlainBill-

Yes, BN44-00214A is the board designation.

The 16 pin IC is listed as MC33067P, so it is really close to what you listed.

Are you thinking that we can use the schematic to get the part numbers listed in the repair kit advertisment? Let me know what you think....

I think we haven't yet confirmed this is the same problem. The kit includes the input fuse, clearly that is not the problem in your case.

I also think I will be spending the next 6 hours of so watching football.

PlainBill

[waitery]

Re: Samsung LCD Power Board turning on and off.

Hi PlainBill-

Yes, BN44-00214A is the board designation.

The 16 pin IC is listed as MC33067P, so it is really close to what you listed.

Are you thinking that we can use the schematic to get the part numbers listed in the repair kit advertisment? Let me know what you think....

[PlainBill]

Re: Samsung LCD Power Board turning on and off.

Ahh, Fiddlesticks!!! This appears to be a variation of the BN44-00195A. Currently I am chasing problems (same general problem) on a BN44-00195A, and it's slow going. Please confirm a few points:

This is model number BN44-00214A

The 16 pin IC is MC34067P

A few points - a number of the BN44-00214A are available on eBay.

Someone in Great Britain has a repair kit for this power supply.

I have been unable to find a schematic for the other version; I wounder if this one will be easier.

EDIT: Yes!!! I found the schematic on Elektrotanya. And Ohm Supplies Ltd lists the parts in their repair kit.

PlainBill