Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

[kevinm34232]

Wonder if someone wants to help me troubleshoot this, I'm thinking it's the T-Con Board (LJ94-16587C). It turns on normally with sound and backlight, but there is no picture/OSD.

I tried reseating the LVDS and ribbon cables. I have 12V at both sides of the SMD fuse FI1 on the T-Con as well as DI1 and some nearby caps, but that's about it (meaning nothing on most other diodes in the area).

Not sure what to check from here. Any help would be appreciated!

[kevinm34232]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

Can someone help identify the part QD1, with the markings 58A on it? Is this a regulator?

[tom66]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

Likely a P-ch MOSFET.
Pin out guess: 1,2,5,6: Drain (Output) / 4: Source (Input) / 3: Gate

[kevinm34232]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

Ok if your guess is correct, I have 13V at Source and Gate, and 0V on all other pins. Does that say whether it's good or bad? Yesterday I ordered the 2 large caps to the left of that IC just in case.

[tom66]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

That is working OK; the G-S voltage is zero, so the FET is OFF (drain is 0V.) However, it should be more than -3V when on, so for some reason part of the T-con is not turning it on. Check the gate-source capacitor (you'll have to trace the circuit a little) is not shorted, small little ceramic thing (0402 or 0603 typically.)

[kevinm34232]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

OK Forgive me I don't understand how MOSFETs work.

When you say G-S voltage is zero, do you mean G minus S voltage? So since they are both equal, the IC is doing what it's supposed to do and remaining OFF?

But... it should actually be ON at this point? And if it was ON, then Drain should be more than -3V if working correctly?

So something before this IC is probably bad (i.e. a capacitor)? Thanks!

[tom66]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

Not quite... The only thing a MOSFET "sees" is the gate-source voltage. It can't measure things wrt to ground. So when the gate is at 0V and the source is at 13V, the gate-source voltage is -13V. For a P-channel FET, the voltage needs to be less than about -3V note the negative sign, ideally at least -5V. (Less than -3V, the FET may be partially conducting.)

Note this is the opposite of an N-channel FET which requires a POSITIVE gate-source voltage, so +3V would turn it on, etc. How do I know it is a P-channel? I don't for sure, but it is rare to use a N-channel FET in this position (it is a load switch to switch current to the rest of the T-con.) This is because using an N-channel FET would require a voltage greater than the input voltage to control it. This just adds far more complexity to the design, so even though a P-channel FET costs about 2x as much in volume (we're talking 5 cents vs 10 cents though) it's much cheaper to control.

Currently, the FET appears to be working as the Vgs (gate-source voltage) is 0V (gate 13V, source 13V) therefore the FET Is OFF as it should be (drain is near or at zero volts, drain-source current is nearly zero.)

You should check for shorts from gate to source, such as a capacitor, or potentially the FET itself (I forgot to mention this initially), because if there is a short, the control circuit will not be able to control the FET and so it will remain always off.

[kevinm34232]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

OK thanks, still digesting this, but does that basically mean that the Gate should be around 8V to turn MOSFET on, if Source is 13V? And so if it was an N Channel it would require around 18V then?

[tom66]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

OK thanks, still digesting this, but does that basically mean that the Gate should be around 8V to turn MOSFET on, if Source is 13V? And so if it was an N Channel it would require around 18V then?

Yes, that's correct.

[kevinm34232]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

OK no short from G-S on the IC or the nearby caps. I just traced it out a little bit in the picture, what next? Is that Gate voltage supposed to be coming from ICD1?

[tom66]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

I don't know, probably ICT1 is more likely.
Is regulator ICD1 working?

[kevinm34232]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

ICD1 is the SM4109 QFN IC, where is ICT1?

[tom66]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

Sorry, meant ICD2.
ICT1 is the AMLCD processor (Custom Samsung silicon)

[kevinm34232]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

No there is no voltages at ICD2 at all

[tom66]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

Could you get a picture of the back of the T-con board?

[kevinm34232]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

Yup, but I just found this, check out QD2, it looks like it may have a burn mark and the pins marked show a short, you think that's it??

[tom66]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

It looks bad.
There is a chance the panel failed and caused this, so beware.

[kevinm34232]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

Is there anyway to identify that part or use something similar, the only thing I can make out on it is a 7 followed by what looks to be an N.

[tom66]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

It is probably an N-channel MOSFET of some kind but exactly what I would not know.

[kevinm34232]

Re: Samsung LN40D630M3FXZA Has Backlight and Sound, No Picture

I got a replacement T-con board and installed, picture came on but looks like the panel is bad.

Likely to be the cause of the burned transistor? Note it did not burn out on the replacement. And that MOSFET we were looking at, yes now there is about 5V at the Gate pin on the new board.

(I think is this burned part which I ordered, I will test on the old board)
http://www.digikey.com/product-detai...7FCT-ND/386059