Samsung LN46C600F3FXZA No backlight

[1997eclipsegst]

Hi everyone,

I am working on this set I picked up off Craigslist.

When I turn it on I get the normal Samsung chime but the backlights never come on. I can take a flashlight to the panel and see snow being displayed on a channel with no signal. If I hook up my cable line I get sound as well.

A couple things I have tried already:

Unplugged the mainboard from the power supply to see if the backlights would come on. No difference.

I read on the forums that the JP852 jumper may cause the backlight to flicker and that it should be cut, which I did with no change.

Voltage readings from plug JP824 from mainboard to power supply:
1- B13 12.31vdc
2- Lampdet 0.00vdc
3- B13 12.31vdc
4- Sw Inv 5.14vdc
5- B13 12.31vdc
6- Pwm_Dim 1.66vdc
7- Gnd
8- H_Sync 1.67vdc
9- Gnd
10- Gnd
11- B5v 5.15vdc
12- Gnd
13- B5v 5.15vdc
14- B5v 5.15vdc
15- 18vs 12.31vdc
16- A5v 5.16vdc
17- 13/18vs 12.31vdc
18- Pow_on/off .88vdc

Voltage readings from plug CN1802 from power supply to inverter
1- 12v 12.23vdc
2- FB 0.00vdc
3- FB 0.00vdc
4- Gnd
5- Gnd
6- LD 8.94vdc
7- LD 8.94vdc

I have checked the fuses on the power supply and both are good.

Voltage across the large 450v caps is 390vdc on each

Anyone have any ideas what to check next?

Thanks in advance!

[budm]

Re: Samsung LN46C600F3FXZA No backlight

"18- Pow_on/off .88vdc" This voltage should be 2.5V or more when power switch is activated. So far the power supply/inverter board is getting the correct signals from the main board. SCH for the BN44-00341B.

[1997eclipsegst]

Re: Samsung LN46C600F3FXZA No backlight

Hi Budm

Thanks for the reply and schematic

I just went and double checked pin 18- Pow_on/off and it reads 0.00vdc when the set is off, and when the power button is pressed it will change to .88vdc and nothing higher.

[budm]

Re: Samsung LN46C600F3FXZA No backlight

I need to look at the SCH, but it is enough to turn on the PFC and the power supply, otherwise you will not be getting the 390VDC.
OK, 0.8V is correct, that the Vf of B-E of QM805 + small voltage drop on RM838 (220 Ohms).
This power supply-inverter board combo has built-in pull-up resistors for the PS_ON and the BL_ON, so if you disconnect the main cable between this board and the main board, the backlights should come on the minute you plug the TV into the Outlet.\
Fuse FS802S which is for the inverter circuit is OK?

[1997eclipsegst]

Re: Samsung LN46C600F3FXZA No backlight

Thanks for the reply

With the cable disconnected between the power supply and mainboard I plugged the power supply in and get nothing from the backlights, not even a flicker.

I checked Fuse FS802S and reads 307vdc on each side of the fuse

[budm]

Re: Samsung LN46C600F3FXZA No backlight

That should read 390Vdc since it goes back to the 450VDC filter caps, please see the SCH.
By the way, we are checking the voltage in the primary side of the circuit, so the negative leg of the 450Vdc cap is the primary side Ground ref.

[1997eclipsegst]

Re: Samsung LN46C600F3FXZA No backlight

Budm,

Testing off the negative leg of the 450Vdc cap fuse FS802s reads 390vdc on each side. I originally was testing with the ground probe on one of the grounding screws of the power supply.

[Zalinar]

Re: Samsung LN46C600F3FXZA No backlight

Hi, I just thought I would throw in my two cents. I have the exact same set with the exact same problem. After replacing the inverter controller IC, which was NOT the problem, I found that C1818 on the power supply had gone bad. It read full open and registered no ESR. I replaced it and am good to go! It is just a .47uF 1Kv block style film capacitor.

Hope this helps someone who might have the same problem!

[budm]

Re: Samsung LN46C600F3FXZA No backlight

OK, now we know that the PFC voltage booster is OK and working as it should, so next thing will be testing that DC blocking cap C1818 as mention above, verify it is still show 0.47uF on your cap meter, that is one of the commonly failure cap.
If it is bad, it should be replaced with high impulse current type poly film cap.

[Zalinar]

Re: Samsung LN46C600F3FXZA No backlight

Thanks Bud,
I intended to post here some time ago since I have the same TV with the same problem, but didn't want to Hijack the post. I hope I am not...
It seemed that the original poster abandoned this thread some time ago.

_______
So, after a long day yesterday, I accidentally replaced the capacitor in my good power supply that already worked. I got excited prematurely.

I replaced the same C1818 cap in my bad supply, and now the back lights only flash on for a split second and then shut off again. I guess that is a step forward since they didn't come on at all before. Any ideas are welcome and I can tinker with it after work this evening.

Thank you very much!

[jugador87]

Re: Samsung LN46C600F3FXZA No backlight

Zalinar, I've got the same issue. Still need to go get a multimeter to test out the pins. Did you ever resolve your issue?

[budm]

Re: Samsung LN46C600F3FXZA No backlight

Zalinar fixed his board, it has bad C1818 (0.47uF 1000V).

[Zalinar]

Re: Samsung LN46C600F3FXZA No backlight

Actually, no. As I said in my November post (which must have looked like a signature file since I got no replies), replacing the capacitor only made the back lights flash on for a second which was a step forward from them doing nothing. There was still something else wrong.
I ended up purchasing another board since I didn't know what to replace next.

[budm]

Re: Samsung LN46C600F3FXZA No backlight

Oops, I missed that follow-up.

[jugador87]

Re: Samsung LN46C600F3FXZA No backlight

Yup, that's what I thought I read. So replacing the board worked? I have replaced my power supply board (albeit from eBay, the only source I can find anymore), with no change. I also replaced the mainboard, with the same result.

Thankfully I have a multimeter and some time this weekend. Any objection to hijacking the thread? Or should I just start a new one?

[stj]

Re: Samsung LN46C600F3FXZA No backlight

what's on that long board under the black cover on the right?

[jugador87]

Re: Samsung LN46C600F3FXZA No backlight

That would be the inverter board. I replaced that one as well.

I should add that all parts were sourced from eBay, so quality is suspect.

[efren hernandez]

Re: Samsung LN46C600F3FXZA No backlight

I had the same sound and image problem but no backlight what solved it by removing the bridge jp852

[joellamoreaux2]

Re: Samsung LN46C600F3FXZA No backlight

Hey guys I am new to the site as a member but have been using the site for a while. Did anyone get the backlight to work after Replacing the C1818 Box style Cap and cutting JP852?

[mradamrada]

Re: Samsung LN46C600F3FXZA No backlight

Does anyone know what the FB pin is for?