Samsung plasma PS50A550S1F - VS voltage droop

[Dans2530]

Hi everyone, I have a weird problem with a samsung plasma PS50A550S1F, the tv will operate normally, but only with the cell brightness on the lowest level. Even then, it will cut out occasionally at random.
I have checked all the voltages, all are within spec except for VS, according to the panel it is supposed to be 205. When the panel lights up it drops down to 197v and i cannot adjust it higher with the pot. It fluctuates by about +/- 2v. As i turn the brightness up it drops down further until the relay clicks out at about 191v. It is worse on a bright screen, i can restart with the antenna pulled out so it has a dark screen and drop the brightness level again.
When i got this tv there were a couple of obviously bad caps on the y-main which were replaced but didn't help the problem. I checked over the psu components and couldnt find any obviously busted parts.
The model of the psu is bn44-00222A, i couldnt find a schematic, so if anyone has one i would really appreciate if i could get it off you.

Has anyone come across a problem like this?
Thanks all.

[mmartell]

Re: Samsung plasma PS50A550S1F - VS voltage droop

I'm having the opposite problem with voltage INCREASING on the XMain (Ve, and I suspect other voltages on the YMain as well) so I will be watching this.

[tom66]

Re: Samsung plasma PS50A550S1F - VS voltage droop

I would check the poly coupling cap on the primary for the VS transformer... can you post a picture of your PSU?

[Dans2530]

Re: Samsung plasma PS50A550S1F - VS voltage droop

Thanks for the quick reply!
Here is my psu, would the caps you are talking about be the blue ones below the larger transformer?

[tom66]

Re: Samsung plasma PS50A550S1F - VS voltage droop

Yep - the two right next to each other. I've replaced a few before. None for this model, but have seen them cause similar issues in other models before.

[Dans2530]

Re: Samsung plasma PS50A550S1F - VS voltage droop

Thanks for the tip Tom, I'll pull them out and test/replace and see if that helps.

[Dans2530]

Re: Samsung plasma PS50A550S1F - VS voltage droop

Update: i have pulled out both caps and they both test within 1nf of their 47nf capacity. I put them back in and i cranked the brightness up to full and the vs voltage started dropping again, but not as fast as it did earlier. So it may be the problem.
I am going to replace them once i find some suitable replacements.

Tom, is there anything else you can think of for me to test in the meantime, just in case that isnt the problem?
Cheers.

[tom66]

Re: Samsung plasma PS50A550S1F - VS voltage droop

One other thing is try checking the PFC voltage. Across big 450V caps. Should be about 390V or so, with only a slight variation due to image brightness. You can probably check it from one end of FS803 to a hot ground.

[Dans2530]

Re: Samsung plasma PS50A550S1F - VS voltage droop

I did check the main caps voltage and it's around the 340v mark which seems kind of low, as I have read on here it should be around 380-90v or so.
Do you think the Pfc circuit is not boosting correctly under load and the vs controller is hitting max duty so the voltage starts to sag?
The controller for the PFC is the L4981AD, do they have a history of going bad?

[tom66]

Re: Samsung plasma PS50A550S1F - VS voltage droop

Yes, I think you have found the fault. Since you are in Oz with 240V mains, the non-boosted PFC voltage should be about 340V. (This is given by 1.4 x 240V.) Your PFC boost stage is not working and during the bottom of the mains peak, it is causing the Vs supply to brown out due to hitting peak duty cycle or current limit.

You can try checking the Vcc voltage for the PFC controller, and make sure the boost diode is not shorted.

[Dans2530]

Re: Samsung plasma PS50A550S1F - VS voltage droop

Now we are getting somewhere!

I will update when I check out the Pfc circuit. Let's hope its something simple.

Thanks again for your time Tom!

[Dans2530]

Re: Samsung plasma PS50A550S1F - VS voltage droop

Update: i have checked the voltage going to the power factor controller board, all good, but not getting any output to the totem pole transistors that drive the mosfets.
I pulled the controller IC off and you can see in the picture that there looks like a burn mark on the underside near pin 20 (bottom left), which is the gate drive output according to the datasheet.
I have ordered a new IC so it will be a couple of weeks before i can see if it works. In the meantime, i have checked all the other components on that board and they are all good.

[Dans2530]

Re: Samsung plasma PS50A550S1F - VS voltage droop

Update:
I have received the new power factor IC, soldered it in, and.....nothing. Same as before. Voltage sits around 340v on the main filter cap.
What an anti-climax!

The power factor controller is getting power, just no output to the totem pole transistors. If anyone has a schematic for a bn-44 00222a board or can suggest a working theory on how to fix this i will be eternally grateful. I will try anything at this point!
Cheers everyone.

[Dans2530]

Re: Samsung plasma PS50A550S1F - VS voltage droop

Anyone? Would love to hear from those experienced in smps repair.