Samsung LN40E550 Dim Screen

[browserjoe]

Few weeks ago my tv screen became dim, the whole screen is dim with part of it being a little darker than the rest. I'm trying to figure out if the dim screen is a bad backlight inverter or power supply.

I am a beginner with electronics so I'm not too sure if I was testing the inverter transformers correctly, but the resistance values I measured were pretty much the same on both inverters (around 2k ohms, and I couldn't find specs on the transformer) so I decided to test supply voltage.

These are the part #'s for the 2 boards
Power supply BN44-00440A
Inverter LJ97-00207A

On the power supply board there is a chart that says:
High (Fet): 280v ~
Low (Fet): 195v ACDC

The voltages I measured with tv on:
High (Fet): 37v
Low (Fet): 16v

Is this low? Should it be closer to the 280v and 195v the board says?

I also noticed the voltage going to the transformers was different to each one, not sure if this is normal. Picture below.

T1
Pin 1 = 36v
Pin 2 = 23v

T2
Pin 1 = 23v
Pin 2 = 16v

I also don't see any swollen or leaking caps.

[budm]

Re: Samsung LN40E550 Dim Screen

Your meter more likely not to have high frequency band with to be able to measure high frequency (>40 KHz) AC Voltage.
It sounds like the inverter transformers are not getting enough drive Voltage.
What DCV do you have between the two legs of the main filter cap?
Check the capacitance of cap CI826 to see if it still has the correct capacitance.
Pictures provided by shopjimy.

Samsung BN44-00440A (I40F1_BDM) Power Supply Unit
http://www.shopjimmy.com/samsung-bn4...upply-unit.htm

[CapLeaker]

Re: Samsung LN40E550 Dim Screen

post a picture of your "dim" screen, so we can see what's going on

[browserjoe]

Re: Samsung LN40E550 Dim Screen

Are the 2 large 450v caps the main filter caps? I see CI826 off to the side.

Before I try testing anything I just want to make sure, the caps have to be removed before doing any testing correct? What about discharging them? From my research I see some things that say discharge with the cap removed, and others show discharging with it still on the board with a 10w 10k ohm resistor.

Is there a way to identify if the problem is on the power supply board or inverter board without removing any components? I'm not too confident in my circuit board soldering skills at the moment, and was thinking of replacing the whole board with one from shopjimmy for now, then I can take my time and learn on the old one and try to fix it.

If the only way to find out where the problem lies is to remove components, then please bear with me, I am new to this so may take me awhile.

As far as a picture of the tv goes, I tried but the screen looks normal in pictures. Just imagine turning the brightness way down, with the right side of the screen being a little dimmer. Adjusting the backlight setting on the tv makes no change, setting 0-8 look the same.

[budm]

Re: Samsung LN40E550 Dim Screen

No need to remove the cap to check the DCV to find out if it has correct Voltage or not, but you will be working in the deadly side of the circuit so make sure not to touch anything on the board with your hands and do not let the meter probes slipped on you.
BTW, I would expect the unit to go into protection shutdown mode if the inverter, lamps are bad.
May be need to go to service mode to do the reset.

[browserjoe]

Re: Samsung LN40E550 Dim Screen

The 2 big caps measured 164v on each one with tv plugged in power off.

[budm]

Re: Samsung LN40E550 Dim Screen

what do you get when the TV is on with dim screen, 380Vdc?

[browserjoe]

Re: Samsung LN40E550 Dim Screen

158v tv on and video showing on screen, 160v tv on with no video just black, and goes back to 164v when tv is turned off

[budm]

Re: Samsung LN40E550 Dim Screen

158v tv on and video showing on screen, 160v tv on with no video just black, and goes back to 164v when tv is turned off

OK, the PFC Voltage booster circuit is not working, the Voltage on the cap should be around 380VDC or more when the TV is ON.
The power supply board will need more in-depth troubleshooting in the PFC section to get the Voltage backup at this point.

[browserjoe]

Re: Samsung LN40E550 Dim Screen

What would need to be done to continue the troubleshooting?

[budm]

Re: Samsung LN40E550 Dim Screen

Need to find out if the PFC Diode is shorted out or not, if not then need to find out what the P/N of the PFC Controller IC is and then get the spec sheet so you can test and find out if it is getting the VCC to run the IC or not.
Lots works involved.
BTW, do you do any repair with SMD parts?

[browserjoe]

Re: Samsung LN40E550 Dim Screen

I have no experience with repairing smd parts.

I removed the power supply to look at the backside of it. PC801S (not sure if it is part of the voltage booster circuit) looks discolored, was wondering if it could be bad, or if the discoloration is just from when it was soldered to the board?

[dick_barton]

Re: Samsung LN40E550 Dim Screen

Can't really tell from the picture. Can you supply a close up. PC801S looks as though it's an opto-isolator.

[budm]

Re: Samsung LN40E550 Dim Screen

Get the P/N of the IC (I cannot read the designator), and also check the PFC diodes.

BTW, PCxxx designator: PC = Photo Coupler.

[Alastair E]

Re: Samsung LN40E550 Dim Screen

How are you measuring the PFC voltage?--ref to ground (wrong) Or across the big cap(correctly)....?

I suspect nothing wrong with PSU or inverters--more likely panel is stuffed, seen this a few times with 40" Samsung...

[browserjoe]

Re: Samsung LN40E550 Dim Screen

get the p/n of the ic (i cannot read the designator), and also check the pfc diodes.

Btw, pcxxx designator: Pc = photo coupler.

smva0u
sem3040
9u1151

[budm]

Re: Samsung LN40E550 Dim Screen

http://www.datasheetcafe.com/sem3040-datasheet/

Did you check the PFC DIODES?

[browserjoe]

Re: Samsung LN40E550 Dim Screen

Sorry, I'm going to need help identifying the diodes. I see DP804, and DP805, but I'm not 100% on DP802, DP803, and DP807. Are 802, 803 and 807 the 3 legged surface mount parts circled in the picture?

Diode test on both DP804 and DP805 measure 0.117 in both directions. Will I have to remove them and retest?

[browserjoe]

Re: Samsung LN40E550 Dim Screen

Voltage at the VCC pin on the IC is 14.56V

[budm]

Re: Samsung LN40E550 Dim Screen

PFC Diode DP805 is the big barrel body diode right next to cap CP801S.

https://www.badcaps.net/forum/attach...7&d=1473114608