Backlight issues with LN46D550K1FXZA

[Pioneerx01]

When the TV cold starts, it works mostly fine. I have noted that all backlights are uniformly dimmer towards the middle of the screen. As the TV remains on, the dimmer spot progressively goes away. Right about the same time the backlights are uniformly on throughout, they shut off, about 3 min after cold start. Sound works, picture is there with flashlight test. If I turn the TV off and back on again, backlights stays on for only few seconds and then I am back to sound only, and picture with flashlight only.

At first glance all internal parts and components look OK. At all times I am getting psu on and backlight on signal from main board, even after backlights shut off. Even though it was a long shot, I have visually inspected the CCFLs themselves, and they all look OK, no burned spots or overly darkened area near connectors. I have noticed that right before the backlights go out, they start pulsing about 4 times per second as if someone is adjusting the backlight intensity in menu.

I believe that the problem lies with PSU. If I bypass the main board, I still get same issues with backlights. When the backlights go off and I unplug the cable between PSU and inverter, wait few seconds, and plug it back in they do not come on. If however I unplug the PSU from the wall, wait few seconds, and plug it back in they come on only for few seconds. Backlights are on for few minutes only if TV sits for some time before I turn it back on again.

Capacitors on the PSU look OK. In my research I came across someone mentioning that the Polyfilm caps like to go bad. I have removed mine form the circuit, and they all read withing less than 10% of their value. The two main filter caps also read OK at all times.

I am not sure what to look at next at this point.
Thanks for your input



PSU: BN44-00441A
Inv: 27-D056702
Main: Part number sticker is missing, but they seem to be cross compatible with each other

[budm]

Re: Backlight issues with LN46D550K1FXZA

Are you getting about 380~400Vdc measured between the two legs of the main filter cap (there are two of them connected in parallel)?

"Capacitors on the PSU look OK. In my research I came across someone mentioning that the Polyfilm caps like to go bad. I have removed mine form the circuit, and they all read withing less than 10% of their value. The two main filter caps also read OK at all times." Are you talking about CI818?

[Pioneerx01]

Re: Backlight issues with LN46D550K1FXZA

Yes, original article I read was in reference to CI818. Mine was not bloated or dis-formed, but I gave it a try anyways.

After I tester the Polyfilm caps, I have soldered them back to the circuit and went to work on something else. After about hour cool down, I plugged everything back in and went to play around with the TV again. This time, the TV remained on way past the 3 min mark well into 10-15min range, but the backlights went out anyways; still the sound and picture were there. From that point on, the backlights never came back on again, even with cool down period.

When I was originally writing the first post the main filter caps (that are in parallel) were reading 396V, now they are reading 162V only.

[budm]

Re: Backlight issues with LN46D550K1FXZA

The power supply for the inverter circuit requires that boost Voltage to work so that 390 something Volt should be present when TV is turned on.

[Pioneerx01]

Re: Backlight issues with LN46D550K1FXZA

Right, the TV turns on fully (but backlights) there is bl_on and psu_on voltage from main. Main caps are reading only 162V.

I have been reading this thread which has a very similar problem to mine with very similar PSU as well: https://www.badcaps.net/forum/showthread.php?t=53599

I have swapped out the DP810 diode as it was only around .410V, did not help. Pin 8 on ICP801 reads 16V when psu_on is on, which is consistent with the other thread.

I have kind of got lost towards the end of the second page, with swapping out resistors and capacitors. There does not seem to be a solution posted at the end.

ShopJimmy does seem to have a repair kit for this PSU which does include "missing PFC" as a symptom among others. I have looked at the ICs that require swapping with this kit, however on top being soldered on the board, they appear to be glued from the bottom as well. I have issues removing such IC's due to lack of skills and equipment. I'll probably just get another PSU, as I don't think I will have much luck with IC replacing on this PSU.

[alfredomfr1]

Re: Backlight issues with LN46D550K1FXZA

I have same problem with my TV, sound and video but no backlight
I have this CI818 reading 455nf but I replaced for a new one.
TV fixed !!!!

[neilc6]

Re: Backlight issues with LN46D550K1FXZA

Have same model TV with PFC circuit not boosting. Checked usual suspects CP820, CI818 and DP810 which all tested fine. Both fuses ok so don't think there is a transistor short. Four electrolytic caps are crappy Sam Wa but not bulging and two tested ok.

Will probably order the Shopjimmy repair kit to get the SMPS IC after checking the rest of the components in the SMPS circuit. Anyone have a schematic? Repair kit for BN44-00441A includes F801S, QI802 808 QM801 QP801 802 RI805 806 RP882 CM809 811 818 825 IC8101 ICM801 ICP801

[neilc6]

Re: Backlight issues with LN46D550K1FXZA

A bit more investigation. Red front led is red and when the soft on/off switch is pushed, it flashes 5 times and goes out. Tv appears to be in the ON state with no backlight. Checked BLU_ON and PS_ON and these are getting sent from the main board. Put alligator clips on the big cap and when the the TV is first plugged in, the voltage spikes up to about 430V and then drops down to around 165VDC. Voltage doesn't change again when the on button is pressed. Thinking maybe overvoltage is being tripped on the PFC IC.

[budm]

Re: Backlight issues with LN46D550K1FXZA

Are you sure that the Voltage on the main cap went up to 430V? If that is true, the PFC over Voltage protection will kick in and shut down the PFC circuit.
Normal Voltage is around 390V, so it may have feedback problem in the PFC circuit.

[neilc6]

Re: Backlight issues with LN46D550K1FXZA

Did the test a couple of times since it is difficult to see on a digital meter but it jumps up when first plugged in. I am going to replace the 1.2M ohm resistors in the top resistive network divider (RP802, 805, 807, 810 and one more) but need to order the parts. I had low PFC voltage on another TV that was corrected when I did this before. The resistors had measured ok with a meter but I replaced them anyway and it brought the voltage back up.

[neilc6]

Re: Backlight issues with LN46D550K1FXZA

I may order ICP801 as well just in case. Can anyone confirm it is FAN7530? The top marking isn't visible.

[neilc6]

Re: Backlight issues with LN46D550K1FXZA

I had put the TV aside since I ordered the parts on ebay from China and shipping takes 1-2 months but decided to look at it again to confirm ICP801. Couldn't find any schematics online. Closest is for board BN44-00341B.

All parts and designators match except for the circuitry on pin 2. This pin is the main difference between FAN7530 (MOT), FAN7930 (RDY) and FAN7930B (OVP). On BN44-00441A, pin 2 connects to a jumper and then a series of 6.8M 1206 resistors sampling the PFC output so this suggests ICP801 is a FAN7930B. I can also clearly see a B on the marking and 7530B doesn't exist.

Good news is that Digikey carries the 7930B.

[neilc6]

Re: Backlight issues with LN46D550K1FXZA

Was curious so replaced the 1.2M ohm resistors and it didn't help. Replaced ICP801 (FAN7930B) and PFC voltage is stable at around 397VDC and TV turns on. I overheated the area trying to get the IC off and it was likely held down by glue.

[budm]

Re: Backlight issues with LN46D550K1FXZA

So the problem solved?

[neilc6]

Re: Backlight issues with LN46D550K1FXZA

Yes tv is working. Will run it for awhile.