Samsung LN19B361C5D Backlight, No Picture/Sound

[budm]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

So now the backlights still go on right away like you had in post 1? Or you can now turn the backlights on and off with power switch.

[caphair]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

So now the backlights still go on right away like you had in post 1? Or you can now turn the backlights on and off with power switch.

Now the backlights are off and the standby light blinks on and off. No response to power button.

[budm]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

So at least the backlights on/off has changed state from being high all the time (which makes the backlights to stay on). Looks like it will need in depth Main board troubleshooting, I did not find the service manual for it either.

[caphair]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

Something is making the input voltages fluctuate now. Would it definitely be a main board problem? Any ideas where to start?

[budm]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

Without diagram will be almost impossible, we know that the inverter and the power supply is OK since you are getting the backlights to be on before. Not sure will be worth to put so much money on 19" TV.

[caphair]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

Yeah, I guess part of me would just want to figure it out as a learning experience. If there were steady voltages with the bad regulator, but not with new, could the circuit that uses that 3.3v be the possible culprit? Like you said it'd be hard to pinpoint it without schematic.

[caphair]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

Not sure if this will help determine anything but I measured voltage across the caps on psu that are linked to the 15v which is only output that measures a fluctuation of voltage coming from the psu, and I noticed there are three 470uf 25v caps that are part of the inverter section.

[caphair]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

What would the 15v be used for? Is 24v generated from that for backlights? Just trying to understand LCD circuits bit more b

[caphair]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

Ok, I know I should probably just give up, but I had a little time on my hands today and I was curious if I could somewhat isolate where on the mainboard is causing the problem.

I decided to desolder a coil, L101 to the left of the t-con board connector, powered tv up and had a steady standby light! looks like it connects to the little 8 pin ic to the left of it.

I then measured voltages of the three regulators to see if they had stopped fluctating at the input and they did but the middle one which is adjustable, had no voltage at the input at this point (with coil out of circuit)

I measured voltage at top of coil with it soldered and it measured a varying 1.11-1.16v the bottom had a steady 1.26v with it desoldered I measured voltage at top connector point where coil would be and it was a steady 4.67v I have no idea what that means, hoping maybe someone does? Included additional pics of the area and also underside of mainboard.

[budm]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

That inductor is part of the switching power supply. Try removing that ribbon cable that goes to the panel and put the parts back on the board and see if the TV will power up OK.

[caphair]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

That inductor is part of the switching power supply. Try removing that ribbon cable that goes to the panel and put the parts back on the board and see if the TV will power up OK.

What role does that inductor play in the psu? With the panel disconnected and parts back in, same symptom of blinking standby light returns.

[caphair]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

So I discovered while taking voltage measurements at a network resistor above the mcu that if I short out to pins the standby light goes steady until unplugged. Power button doesn't respond at this point though.

[tom66]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

The inductor is used as one component in a switching converter, likely a buck converter.

[caphair]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

The inductor is used as one component in a switching converter, likely a buck converter.

Wouldn't it be part of the psu circuit rather than placed on mainboard? Also what does it mean then that by removing it I get steady standby light?

[tom66]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

It's on the main board due to being specific for that part or because the voltage is too low to send over cables with low losses.

[caphair]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

Anyone have any other suggestions or should I write this one off?

[caphair]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

Would a bad optocouplar or start up cap cause the fluctuating voltages with a load (connected to mainboard)?

[caphair]

Re: Samsung LN19B361C5D Backlight, No Picture/Sound

Ok I've noticed the voltage regulator I replaced gets a lot hotter than the other three which makes me think something on it's output is shorted? I jut can't trace it to any other components except for a smd capacitor and smd ferrite bead.

Wondering if the mcu uses this output and maybe that's the problem?