Samsung LE40D550K1W no backlight

[kleini]

TV starts up, shows picture (checked with flashlight) and plays sound but the backlight doesn't come on. Very seldom the backlight flickers for a few seconds (usually a few minutes after being powered on).

Part # (Haven't found any matching datasheets so far.)
PSU: BN44-00440A
MainBoard: BN41-01751
Inverter: SST400_12A01 INV40T12A

What i did so far:

1) Visually checked all electrolytic capacitors. They looked fine but i replaced them anyway, except the big main caps.

2) Checked for cold solder joints.

3) Measured voltages. The 13V lane seems a bit low but should be OK I think? High Voltage towards the inverter is missing.

Main Caps: 390V

CN801 (towards inverter - see attachment - not listed Pins are NC)
Pin# Label: measured voltage (target voltage)
1 High(FET): 0V (280VAC)
3 Low(FET): 0V (190V ACDC?)
8 GND: 0V (0V)
9 OVP: 0V (?V) (Seems to be a backlight-confirm Signal.)
10 CNT_PRT: 11.79V (?V)
11 VCC: 12.38V (13V)
12 LD: 11.04V (?V)

CN802 (towards main/interface -board)
Pin# Label: measured voltage (target voltage)
1, 3, 5, 7 B13V: 12.38V (13V)
2 P_DIM: 1.43V
6 BLU_ON: 5.07V
8, 9, 10 GND: 0V (0V)
11, 13 B5V: 5.2V (5.3V)
12 A5V: 5.2V (5.3V)

4) Checked the two MOSFETS (MI801S, MI802S) that drive the Inverter. Both are OK!
Realised there is no voltage at the gate pin of the MOSFETS.

5) Traced it back to UI801 - marking is SEM2106 1132 - found a datasheet (attached).

6) Measured voltages on SEM2106.
Pin1,24 VREF,P_DIM: 0.7V
Pin14 VCC: 12.36V
Pin16 Enable: 4.91V
Pin 12,13 N1,N2: 0V (These are connected to a tiny transformer that drives the two aforementioned MOSFETS)


Everything seems to point towards the SEM2106 being damaged but would that result in a sporadically flickering backlight?

[luisparada]

Re: Samsung LE40D550K1W no backlight

Did you solve your problem? I have the same issue, but in my power board (same as yours) I found capacitor CP811 completly fried.

do you happen to know the specs for that capacitor?

[budm]

Re: Samsung LE40D550K1W no backlight

The snubber cap (in parallel with the Diode DP804) CP811 for the PFC Diode DP804 is 100pf 1KV ceramic cap.
See similar cap CP811 used in the similar circuit.

[luisparada]

Re: Samsung LE40D550K1W no backlight

Thank you a lot for your help! I made a quick call to my local store, would a 100pf/500 V work?

[budm]

Re: Samsung LE40D550K1W no backlight

No, do not go lower than 1000V rating

[kleini]

Re: Samsung LE40D550K1W no backlight

Hi luisparada, sorry for the late reply...
I just took a look at CP811 and on my board the value is 220pF ±10%/1kV, if I decoded the markings correctly. The complete marking is "LB 221K 1KV".
At the moment I'm still waiting for the ICs to arrive (had some trouble with a seller on Alibaba).
I'll post results when they get here (in one or two weeks).

[kleini]

Re: Samsung LE40D550K1W no backlight

Well I'm back at the beginning.
Replacing the SEM2106 changed nothing.
If anybody has any ideas I'm all ear.

[kleini]

Re: Samsung LE40D550K1W no backlight

Problem solved.
Since replacing UI801 didn't fix my problem I started checking the diodes and z-diodes around it.
Sure enough one of the zeners was broken. ZDI805 a 6.8V zener had a breakdown voltage of ~4.5V.
Replaced it and now everything works.

[HANSPAL4U]

Re: Samsung LE40D550K1W no backlight

Hi Kleini

I found MOSFETS (MI801S)completly fried , but Mosfets (mI801s) is seem to be fine advise tha part name/ details of MI801s

Thanks & Brgds
Singh

[digital]

Re: Samsung LE40D550K1W no backlight

zdi804 6,8 v ,radi nje neradi inverter

[rauffmia]

Re: Samsung LE40D550K1W no backlight

invetoreka ti kassu kama toa

[S7YH]

Re: Samsung LE40D550K1W no backlight

Problem solved.
Since replacing UI801 didn't fix my problem I started checking the diodes and z-diodes around it.
Sure enough one of the zeners was broken. ZDI805 a 6.8V zener had a breakdown voltage of ~4.5V.
Replaced it and now everything works.

solved and working.......
You were correct....replaced the Zener ZDI805 ans ZDI804 and presto...all is working fine

[raj mehra]

Re: Samsung La32d450 no backlight

sir pls help me about repair inverter circut useing with sem2106 or mosfet 5n52u

[Donazan]

Re: Samsung LE40D550K1W no backlight

Samsung LA32E420 LCD 32 INCH TV, power not coming on at all. there is about 390v dc on the main power capacitor I have replaced STR W6052S and 10N60 transistor on the same heat sink of the STR but still not working. any idea pls help me out.

[Herendi László]

Re: Samsung LE40D550K1W no backlight

solved and working.......
You were correct....replaced the Zener ZDI805 ans ZDI804 and presto...all is working fine

My problem was the ZD1801 9.1 zener

Best regards László

[shivamtechnology]

Re: Samsung LE40D550K1W no backlight

Samsung LA32E420 LCD 32 INCH TV, power not coming on at all. there is about 390v dc on the main power capacitor I have replaced STR W6052S and 10N60 transistor on the same heat sink of the STR but still not working. any idea pls help me out.

[budm]

Re: Samsung LE40D550K1W no backlight

Samsung LA32E420 LCD 32 INCH TV, power not coming on at all. there is about 390v dc on the main power capacitor I have replaced STR W6052S and 10N60 transistor on the same heat sink of the STR but still not working. any idea pls help me out.

Start a new thread and upload GOOD CLEAR pictures of the whole backside of the TV showing all the boards.
What other Voltages did you check beside the Voltage on the main cap?