Samsung LN46D630M3F, TV powers on, sound works but no backlight

[Five_magics]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

Shorthing the BL_on signal, I'm only getting 160 vdc on the main caps.

EDIT: BL_on is at 5v.

[budm]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

Shorthing the BL_on signal, I'm only getting 160 vdc on the main caps.

EDIT: BL_on is at 5v.

But the PS-ON is present, right? Before your PFC Voltage was over 400VDC, and now you are not getting the Boosted Voltage, so I would check your works and make sure you do have VCC to run the PFC controller IC.

[Five_magics]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

PS_on coming from main board is present, but ICP801 has no Vcc.

[budm]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

It has been awhile, see post 22.

[Five_magics]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

Wow ok my mistake, measure with the cold ground as reference, hope I didn't screw anything up. VCC is at 16vdc (was at 12,9v before).

[Five_magics]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

Judging from the 7930B datasheets, operating supply voltage should be 13-20v, so at 16v I should expect it to work properly. Could ICP801 be fried?

[budm]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

BTW, are you sure that those resistors are the correct value as the old ones, since you were getting PFC Voltage before and now you don't after replacing just the resistors.

[budm]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

Also make sure PFC diode DP810 is not shorted out.

[Five_magics]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

They are. 5x 1,2Mohm and 4x 6,8Mohm. By the picture from your post #22, that's exactly what's on the board (it's also what was on my board originally). Didn't measure the new ones before soldering them on board tho.

Also, when the TV crapped again I took for granted the resistors failed so didn't bother confirming it was the same trouble as before(now that I think of it I probably should have).

D810 is good.

[budm]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

You can check the Gate drive output pin of the IC to see if it shows any action or Voltage there at all to drive the Gate.

[Five_magics]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

Will do tomorrow. I'll check if there's any change going on at all when shorting the BL_on signal.

[Five_magics]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

Removed the new resistors, tested them out of circuit(they're good), resoldered and the TV works.

A solder joint looked shady, probably my mistake. Will test and report back.

[Five_magics]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

Right now, the TV works for a few minutes, before the backlight shuts off.

When it works, PFC voltage varies between 390 and 410v.

Also, the inverter transformers now make a buzzing sound that wasn't there before, if that's any indication.

[Five_magics]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

Got to test the TV some more tonight, here's what happens. At first, the backlight works, but the PFC output fluctuates (390vdc > 410vdc > 400vdc > 410vdc, etc). After 5-10 minutes, the output reaches 450vdc, at which point the backlight/PFC shuts off. Shorting the BL_on signal at that point is useless, and the PFC won't turn back on.

With the PFC off, the Gate ouput of the IC is at 0vdc (6vdc when working), so I figured there must be something wrong with the voltage dividers (or IC). Decided to take out bypass capacitors C809 and C810 (shown on the attached file, altho not the actual schematic).

The TV has now been working for over an hour, with the PFC output stable at 390vdc. Either one of the caps is leaky, or one of the adjacent resistors(or the IC) is failing, but working after being heated.

Now in my clumsyness I managed to lose one of said capacitors. Wondering if you guys could tell me what's a safe value to use. Was thinking 1nF as it's what's shown in the 7930B's datasheet.

[budm]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

1nF is probably fine, it is there just to filter out high frequency noise.
Voltage jumps up over 400V sounds like it is still loosing the feedback Voltage or the IC is flaky.The resistor on the bottom of the string that goes to Ground is good fro the resistor network that feed the INV pin? If the resistance of that resistor goes down, the output Voltage will go up.

[Five_magics]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

1nF is probably fine, it is there just to filter out high frequency noise.
Voltage jumps up over 400V sounds like it is still loosing the feedback Voltage or the IC is flaky.The resistor on the bottom of the string that goes to Ground is good fro the resistor network that feed the INV pin? If the resistance of that resistor goes down, the output Voltage will go up.

Voltage fluctuation was before taking out the caps. After taking them out and leaving them out, the voltage is stable at 390 volts. TV been working for 2 hours now.

Haven't measured the grounded resistors on the INV and OVP pins yet, will do that tomorrow. Will most likely replace them anyways, aswell as the bypass caps and IC (have to order online and pay shipping anyways). I'm still thinking the heat from the soldering iron might be "fixing" the resistors/IC temporarly. Either that or one of the caps I removed was leaky.

[budm]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

Since that cap is in parallel with the bottom of the resistor string, if it has intermittent leakage resistance that will cause the PFC Voltage to go up due to lower resistance of the bottom resistor. If it is one of those MLC Cap type (tan color with rectangular body), those kind of cap will fail in shorted out mode or has leakage resistance.
The OVP pin is for monitoring the output Voltage and will shutdown the PFC if it gets too high, it will not cause the output Voltage to go high.
When the output Voltage try to go high the more Voltage will appear at the INV (Invert input pin) pin will cause the regulation circuit to bring down the output Voltage back to the set point.

[Five_magics]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

Since that cap is in parallel with the bottom of the resistor string, if it has intermittent leakage resistance that will cause the PFC Voltage to go up due to lower resistance of the bottom resistor. If it is one of those MLC Cap type (tan color with rectangular body), those kind of cap will fail in shorted out mode or has leakage resistance.
The OVP pin is for monitoring the output Voltage and will shutdown the PFC if it gets too high, it will not cause the output Voltage to go high.
When the output Voltage try to go high the more Voltage will appear at the INV (Invert input pin) pin will cause the regulation circuit to bring down the output Voltage back to the set point.

Thanks for the info on the INV and OVP pins. INV works like I thought, but wasn't too sure about the OVP pin.

The cap on the OVP pin was a ceramic cap, like these; http://2.imimg.com/data2/JF/EC/MY-89...or-250x250.jpg

The one on the INV pin was grey/white-ish. That's the one I lost, so can't measure it. Can only say it wasn't shorted.

I will be ordering both caps, and both resistors(2x in parallel) for the INV pin, aswell as the IC considering it's only a buck or so.

[budm]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

Thanks for the info on the INV and OVP pins. INV works like I thought, but wasn't too sure about the OVP pin.

The cap on the OVP pin was a ceramic cap, like these; http://2.imimg.com/data2/JF/EC/MY-89...or-250x250.jpg

The one on the INV pin was grey/white-ish. That's the one I lost, so can't measure it. Can only say it wasn't shorted.

I will be ordering both caps, and both resistors(2x in parallel) for the INV pin, aswell as the IC considering it's only a buck or so.

It does not have to be shorted, it can have leakage resistance which will act like having another resistor in parallel with the bottom resistor which will bring down the total resistance of the bottom resistor, as you can see, the bottom resistor is in the low K range and the top string are in the M range so it does not take much lower resistance of the bottom resistor to cause the output Voltage to go high.

[Five_magics]

Re: Samsung LN46D630M3F, TV powers on, sound works but no backlight

Took out the resistors to test them out. One is at 38,8kohm, the other at 2Mohm (they're in parallel). In theory this should give me 38,061 ohm.

But there's no 38,8kohm resistors available over at digikey or mouser. Thought I'd get different values to end up as close to 38,061 as possible. For example, a 44,2k in parallel with a 274k should give 38,060 ohms.

I'm thinking that would be just fine, but I'm wondering how much of a difference is acceptable.