Samsung le37b530 no backlight

[hey_monkey]

Hello,

I have a limited knowledge, but I want to learn.

My SAMSUNG LE37B530 is causing me problems.
Most of the time when i start it, it makes a sound of welcome, then the red LED is flashing few times but the backlight doesn't start.
I'm saying backlight, because in the dark with a flash light, i can see taht the image is there, it is just not lighted from the back.
Sometimes, if i try several times to turn it ON and OFF, it will finally start, but it is more and more rare.

My Powerboard is a BN44-00262A, the main is a BN41-01165A.

Here are answers to question I saw in other posts:

I see no badcaps.

If I plug the tv on without the connector between the MainB and the PowerB, the backlight is turning ON.
and I read at the connector CNM802:
DET_5V = 4.6v
E_PWM = 6.8v
INV_ON/OFF = 1.6v
13V M = 12.5v
5.3V = 5.2v
13V S = 12.5v
STBY 5.3V = 5.2v
PWR ON/OFF = 0.8v
I_PWM = 0v
H_SYNC = 0v

CM804 = 15v

If i plug the TV on with the connector berween, PB and MB, I have WELCOME SOUND but NO BACKLITE and the following changes :
DET_5V = 0.3v
E_PWM = 3.2v
INV_ON/OFF = 5.1v

CM804 = 18v

I hope it is enough for you to give me some directions.

Thanks,

Cheers.

[tom66]

Re: Samsung le37b530 no backlight

On the power supply there are one or two large 450V electrolytic capacitors, on the primary (danger) side. Can you check the voltage across these? A common fault with the B-series LCDs is failed PFC circuit which will cause intermittent backlight or flickering backlight.

[budm]

Re: Samsung le37b530 no backlight

"PWR ON/OFF = 0.8v" What do you get on this pin when all the boards are connected?
Same question for this one "INV_ON/OFF = 1.6v"

[hey_monkey]

Re: Samsung le37b530 no backlight

Thanks for the quick replies,

PWR ON/OFF stays at 0.8V
INV_ON/OFF goes from 1.6v(MB not connected) to 5.1v(MB connected).

[hey_monkey]

Re: Samsung le37b530 no backlight

On the power supply there are one or two large 450V electrolytic capacitors, on the primary (danger) side. Can you check the voltage across these? A common fault with the B-series LCDs is failed PFC circuit which will cause intermittent backlight or flickering backlight.

I see only one of those big Capacitor, i'm reading 315v.

[tom66]

Re: Samsung le37b530 no backlight

I see only one of those big Capacitor, i'm reading 315v.

That will be your problem then - it needs to measure around 370-390V. You should replace FAN7529/FAN7530 IC on bottom of the power board.

[budm]

Re: Samsung le37b530 no backlight

Check the DCV between pin 8 (VCC) and pin 6 (GND) of the PFC IC P801S.
Also check diodes DP804, 810.
Diagram provided by TOM66

[hey_monkey]

Re: Samsung le37b530 no backlight

I'm lost. Here is the bottom of the PowerBoard. Are we talking of those 3 pieces in black?

[tom66]

Re: Samsung le37b530 no backlight

BTW, a common "trick" on the 'net regarding this fault is to disable the UVLO circuit for the inverter (PC803S, etc.) This is not a solution as it will be running the PSU under much greater line ripple - sometimes causing issues with the audio or main board, and damaging the 150uF/450V filter cap due to high ripple current. (And when one of those pops, boy do you know it!)

[hey_monkey]

Re: Samsung le37b530 no backlight

PWR ON/OFF stays at 0.8V

Is this normal to keep the same value?

[budm]

Re: Samsung le37b530 no backlight

Look for 8-pin SMD device with designator P801S next to it.
Your pcitures are not in focused for me to read the designators.

[tom66]

Re: Samsung le37b530 no backlight

I'm lost. Here is the bottom of the PowerBoard. Are we talking of those 3 pieces in black?

PFC IC is between holes 1 and 2 from the bottom left of that board. The capacitor CP809 can be used to check Vcc for this IC, it is on the top and the board should be installed and connected (including power) to measure this. You should be able to bend it over slightly and measure on the pins. (DON'T touch it with your finger when power is applied it is live!)

[budm]

Re: Samsung le37b530 no backlight

Is this normal to keep the same value?

It should toggle, but we will know if it is working or not by observing that the VCC for the PFC IC will be toggle on and off or not.

[hey_monkey]

Re: Samsung le37b530 no backlight

Check the DCV between pin 8 (VCC) and pin 6 (GND) of the PFC IC P801S.
Also check diodes DP804, 810.
Diagram provided by TOM66

Can I check the diodes on the board, or I'have to remove it?

[tom66]

Re: Samsung le37b530 no backlight

Can I check the diodes on the board, or I'have to remove it?

Ok to check in circuit for now with power removed, if the readings are suspicious then remove them to confirm.

[budm]

Re: Samsung le37b530 no backlight

Can I check the diodes on the board, or I'have to remove it?

You can test them on board, if they are not tested good, then you will have to test them off board later.

[hey_monkey]

Re: Samsung le37b530 no backlight

ok, i found it, now i have to identify the pins from the low res pdf? or is there a trick with the black dot on one size of the PFC?

[hey_monkey]

Re: Samsung le37b530 no backlight

PIN6-8 =15v when connected to the MainBoard! good?

[tom66]

Re: Samsung le37b530 no backlight

The PFC IC is getting power.

Check diodes as Bud suggests, they could be bad. (Hah, just noticed we posted basically the same thing a few seconds apart...)

If diodes are good it's probably the IC. I had an LE40B530 pass through here a few weeks ago with the same fault and it was that IC. On that model, the backlight would work ok in my house (did not work in previous owner's place) but when tumble dryer/washing machine spun up it shut down. Turns out this slight drop in mains voltage was enough to cause the inverter UVLO to trip out. After replacing IC my PFC voltage returned and TV worked properly.

[hey_monkey]

Re: Samsung le37b530 no backlight

OK,
I told you, i'm novice; so i put my multimeter on diode mode:
for DP804 it shows me 620 in one direction and 600 in the other
for DP805 it shows me 002 in one direction and 002 in the other
for DP810 it shows me 630 in one direction and 600 in the other