Samsung ln40b630 not turning on

[crosstalk]

The TV isn't turning on. When the red standby light is on, if i try to power the tv up, the red light turns off and then nothing. I checked the power supply and the outputs seem fine without the mainboard connected. When I connect the mainboard, pins 13-16 (5.1v) go to 0v. The backlight comes on with the mainboard disconnected. The 13v and 5v power seem to be working without the mainboard connected.

I am not sure if the power supply is failing under load or something is wrong with the mainboard. I found a spot on the mainboard that has 3.3v for a few seconds and then cycles off for a few seconds, then on to 3.3v again. Maybe the mainboard isn't asserting the power on signal--how is that supposed to work? Not sure what to check next. Thanks for any help.

[budm]

Re: Samsung ln40b630 not turning on

It is either the 5.1VSTBY is bad or the main board is drawing too much current from the 5.1VSTBY power supply.
Do you have the P/N of the power supply? BN44-??????.
You can use 5 Ohms 5W to simulate 1A load on the 5.1VSTBY to see if it can handle and maintain 5.1V output or not.
Any pictures of the boards and the backside of the TV?

[myfastgti]

Re: Samsung ln40b630 not turning on

I've seen this behavior with Samsung LN46C530F1F I bought from Craigslist. If you've heat gun or hair dryer try heating the motherboard, especially the memory chips which has heat sink on top around the processor. I solved my problem by replacing the chip K4T1G164QE-HCF8

[crosstalk]

Re: Samsung ln40b630 not turning on

Thanks for the reply. I am attaching some photos--I hope they are detailed enough. The power supply has board number BN44-00264A. I measured the 5v and 13v outputs again with the mainboard disconnected. The 13v seems a bit low at 12.44v.

With the mainboard connected, I get 0v at all the odd pins of the connector (easiest to probe on the mainboard side). Does connecting the mainboard switch off the 13v rail? In any case, the standby 5.19v (pin 3) goes to 0v with the mainboard connected. I need to see if I have any 5w resistors, but a 1A load might be too much for the standby according to the current limits printed on the board.

[budm]

Re: Samsung ln40b630 not turning on

Hmm, am I reading the label correctly, stby 5.3V at only .040A (40mA)?
That can't be right, according to the SCH, it uses STR-W6252 which is for 40W/60W power supply, it also has two 1000uf filter caps and 10A rating rectifier.

[crosstalk]

Re: Samsung ln40b630 not turning on

Yes--that is what it says. Maybe 40mA is the normal current draw? Is the STR-W6252 also used to drive the 5.3v 4.0A pins? Sorry I don't understand much from the schematic.

[crosstalk]

Re: Samsung ln40b630 not turning on

So I thought I had a bad LDO regulator, but further testing revealed that the output voltage switches on and off every 10 seconds or so. This happens at all the regulators so it seems the board is cycling power somewhere. I'm not sure if the board is switching on the main power supply because I only see something like 0.8v on the control line.

Any thoughts about what could cause the main board to switch on/off in cycles like this? It seems like it's only using the standby supply because the main power supply isn't turning on (unless I disconnect the main board from it).