Samsung LN40B500P3F - BN44-00264A Backlights Flashing

[Mad Capper]

Found this at the road side. As per the many threads regarding this power supply I replaced the bad fuse FM802S and bad MOSFETS QI820 and QI821. Hooked up the dim bulb tester with up to 425W of bulbs. The set has standby light. When turned on the startup chime plays then the LCD lights flash on and off continually as do the tester lights. The screen shows static and a weak signal message as it is on whatever it was when I picked it up.

The capacitors across the replaced MOSFETS C1890 & C1891 show OL measuring their resistance when removed from board.
Capacitor C1892 shows 456nF when removed from board.

Main caps CP815 & CP816 show 162V with set off and 390V when turned on but fluctuates slightly down to 300V and back.

CNM802 voltages when on and lights flashing
PWR - ON/OFF 1.169
STBY5.3V 5.15
GND
GND
5.3V 5.13
5.3V 5.13
GND
13V 12.77
13V 12.77
INV_ON/OFF 4.7
E-PWM 4.23
DET_5V 0-3.7-4.78 It stays at 4.78 for a moment the cycles through the voltages always staying on 4.78 for a moment

None of the voltages change as if they are reacting in time with the flashing of the lights.


Disconnecting the cable between the PSU/Inverter and the mainboard and plugging the TV in the light flashing begins without pressing the power button.
Disconnecting the cable from the main board to the TCON has no effect on the situation.
Disconnecting the FPCs from the TCON to the panel has no effect.

The last attached file, LN40B500P3F.pdf is actually a video of the problem. As video is not a valid upload type you will have to download it and change its file extension to mp4 before you can play it.

I don't know where to go from here. I have a feeling that using the dim bulb tester even with up to 425W of lightbulbs it is causing the issue but I am hesitant to try the set with out the dim bulb tester so as not to ruin what I just put in. So before I did anything foolish I thought I should check with the experts.

What should I check next? Have you ever experienced this behavior?

Your help is most appreciated. Thank You

[budm]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

"Main caps CP815 & CP816 show 162V with set off and 390V when turned on but fluctuates slightly down to 300V and back."
Please verify that 390V again, that is not a slight drop at all, 300V will cause problem (anything below 360V or so will cause the Backlights circuit to go into protection mode).

[Mad Capper]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

The voltage on the main caps is a bit odd. When I tried it again after it being unplugged for a while it started at 380V or so and then rose to 395V.

I then turned it on again while writing this and it was 390V and then once the backlights start flashing it lowers continually. I shut it off when it got to 325V.

Turned it on again and it started at 328V and started to climb to 340V then back down. Turned it on again and it held around 390V

The only consistent thing I see is this:

When I press the the power button the chime plays and and the voltage rises from 162V to 390V. The backlight gives one initial flash on start up. Then the backlights start flashing and the voltages go haywire.

I then press the power button to turn it off, the lights stop flashing. The voltage then holds at 390V for a moment and falls to the 164V for standby.

They still have consistently solid 162V-164V while on standby.

[fredaelsadany]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

Hi
Imeet that fault the problem with the 4 resistors :
1- change value (smaller)
2- need to clean under them
3- resolder them
and I found that voltage to pin "INV' change from 0v up to 5v
and pfc voltage change from 320v up to 390v .
all finished after the 3 steps above .
there is a pic here
thanks

[petehall347]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

check CI892 0.47uf 400v .. its at the side of one of the heat-sinks .

[Alastair E]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

Just bung it direct on full mains. the dim-bulb tester has done its job, time for full beans now!

[Mad Capper]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

I took off CI892 again to check and on my meter it shows 456nF, i.e. 0.456 uF.


Attached wire to pin 1 INV of UP801S and connected to meter and GND on Negative leg of main cap.
On standby it is 1.049V When turned on it is 2.5V then the flashing begins and it fluctuates 1.5V-2.3V

For whatever reason this test only had 200W of light bulbs in the tester. Interestingly the dim bulb tester lights are solid and the backlights still flash but this only lasts for about 15 seconds then both sets of lights shut off and it short cycles but only for about 3 seconds. There are about 4-5 short cycles. After the short cycles nothing flashes and nothing more happens. Then I turned it off.

I repeated this with 425W of bulbs in and then with only 150W of bulbs in. When the dim bulb tester has more wattage in it the flashing of both the bulbs and backlights is quicker and there is not cycling like above. When it had just the 150W bulb in it stayed solid as the backlights flashed and there was cycling but the duration of time of the cycles was longer and the backlights flashed at a slower frequency.

I wanted to further this so I got a 60W and turned on the tester switch. I should have checked, but it turns out that the bulb was bad, so I guess full power went to the set. I had the meter on the one of the main caps and it did not get up to 162V and the front standby light was not on. But it did not blow either fuses, so I hope that means the MOSFETS I put in are still good. Who knows what else it might have done.

I reflowed resistors RP809, RP810, RP811, RP812 which resulted in no change to the situation.

[petehall347]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

i am thinking you have same problem i just had . was the fan7530 pfc controller chip .
up801s .
was powering up on full mains with strange power draw and supply was buzzing .
this one main fuse was blown and one of the pfc fets shorted .
did you test that resistor string mentioned earlier ? these are part of the repair kit for your problem

[Mad Capper]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

I tested the resistors before I did the items in my last post and after I reflowed them.

Before:
RP809 0.793 M Ω
RP810 0.795 M Ω
RP811 0.702 M Ω
RP812 0.704 M Ω

After:
RP809 0.579 M Ω 0.900 0.620
RP810 0.585 M Ω 0.618 0.640
RP811 0.935 M Ω 0.898 0.620
RP812 0.921 M Ω 0.903 0.620

I seem to be measuring them more times in the after condition and keep getting different readings, perhaps this would have occurred before also. They also slowly drift up.

There is no buzzing on mine.

Is there any test to see if UP801S is bad?

Is there a way to test the PFC MOSFETS QP801S & QP802S while on the board? I have a real tough time getting the ones with the collars off.

[petehall347]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

just test for short in the first place in circuit .
only real test i found is replace fan7530 . up801s
if reeceburger finds this thread he will advise you better than i can . he was bang on fixing mine .
i only tried my board on 150w dbt on the bench out of the set and bulb was flashing slightly . i thought it normal so installed board in tv set and give it full mains . thinking back i should have known there was a problem ..switching supplies are still a bit alien to me but i am learning fast .

[Mad Capper]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

I tested for continuity going from pin to pin on UP801S. Only found pin 4 to pin 6 to ring. They are connected through the fusible resistor RP820. If I take it our the UP801S does not have continuity between those pins.

RP820 measures 0.4Ω but its is supposed to be 0.08Ω

First, is there supposed to be continuity between the legs of fusible cement resistors? Second, should the resistance be that high compared to its spec, and if not would this cause the problem?

Or am I just looking in the wrong places?

[budm]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

What is your lead probes resistance? You need to add that to the resistance of the device you are testing.
I doubt that the Source resistor is bad.

[vinceroger69]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

re post 4 did you remove each resistor and clean the pcb i saw a video the other day on you tube with this issue that maybe why your tests keep altering.

[Mad Capper]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

0.4Ω is resistance of probes together. I did not know that I needed to add that. Will put that in the old memory bank. In general why then do some things I measure come to the value they should be and other times I need to use this additive technique?

If I add this to the resistance readings the meter gives then the fusible resistor, RP820, is a dead short, correct. As it reads the same as bare probes together. So would that mean it is bad?

I did not remove or clean under those resistors just reflowed them. I don't understand what needs cleaned but will give it a shot, measure and test. My SMD work is less than stellar so it may take me some time not to rip a pad up. The only thing under them is the glue to hold them down during assembly, right.

Would you happen to have a link to that video.

Thank you all again

[vinceroger69]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

i think its something too do with glue under the resistors? not too sure but there was a video on it i think budm may of saw the videos too where budm mentiond he must of been a samsung tech? i think capwizard linked the videos but cant remember too be honest.

[petehall347]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

that 0.08 ohm resistor will be fine ..
the 1 meg resistors will test better from one end of the string to the other ... if i remember right there are 4 of them so should read 4 meg-ohm as they are in series .

[Mad Capper]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

I removed the four resistors cleaned the pads and adhesive that held them down and that was left on the bottom of them. Cleaned it all with isopropyl alcohol and put them back on.

RP809-RP812 measured 0.994-0.997 MΩ off the board. I measured each one as I put them back on and they measured the same as off except when I put on the last one, RP812. Then they all measured 0.719-0.778 MΩ. Measuring them all in series my meter starts at a higher MΩ reading and slowly drops. I thought it would settle on 4MΩ but it kept going so I don't know what either of these things mean.

I put it back in and tested it with my original 350W in the DBT to make the testing as consistent as possible and nothing has changed, the same thing occurs.

[Mad Capper]

Re: Samsung LN40B500P3F - BN44-00264A Backlights Flashing

I would like to thank everyone who helped me, I appreciate it. I would also like to apologize. My father always said having a little bit of knowledge is a dangerous thing. So it goes, my ignorance of the dim bulb tester has got the better of me.

As I speculated in the my original post the use of the dim bulb tester was the reason for the problems I have been describing. Thinking this after testing all that I did I took the dim bulb tester around to my other working LCD TVs. I worked up in 25W increments until they turned on. Collectively the exhibited the same pattern. This current set did not. It really had no pattern of what would happen as I increased wattage of the bulbs. My other sets turned on and did not show any glow in the bulbs when using between 1.5 and 2.6 times the sets rated wattage in collective bulb wattage. This set took 3.3 times its rated wattage before the bulbs did not glow and the 390V was solid.

I guess the conclusion I can draw is that the dim bulb tester can only show an clear dead short via a constant bright light that does not change. Then I know there is a problem. The blinking, flashing, pulsing or cycling could be the result of the current limitation of the tester. I hope that my experience and observation may help someone in the future who happens upon this.

To reiterate I replaced the bad fuse FM802S and bad MOSFETS QI820 and QI821. These fixed my set, but I checked many other things to be safe.

Again many thanks.

Till the next one