Samsung LN40B500P3F - Won't turn on

[veaux]

Re: Samsung LN40B500P3F - Won't turn on

OK, remove both caps and check the resistances of the caps again off the board. It is either shorted caps or shorted power MOSFETs.

CI891 and CI890 have "infinite" resitance off-circuit. I got no reading at 200ohms and at all the intermediates up to 2Mohms.

Probing both capacitor places on the board without the capacitors on still shows less than a ohm resistance at both places. Is it the mosfets then?

[budm]

Re: Samsung LN40B500P3F - Won't turn on

The POWER MOSFETs are shorted. As you can see in the diagram, each cap is connected in parallel with Source and Drain of each MOSFET.

[veaux]

Re: Samsung LN40B500P3F - Won't turn on

The POWER MOSFETs are shorted. As you can see in the diagram, each cap is connected in parallel with Source and Drain of each MOSFET.

Both Mosfets are dead then. Those components drive the CCFL transformers right? I'll get them out and see if the short between the Fuse line and ground disappears.

What could be the reason of those mosfet to die while the TV was on Stand-by and so with CCFL off? I'm curious. Is is common?

Thank for your help

[budm]

Re: Samsung LN40B500P3F - Won't turn on

"What could be the reason of those mosfet to die while the TV was on Stand-by and so with CCFL off?"
Post 1: "I have a Samsung LN40B500P3F that just powered off when I was in the middle of watching House of Cards Season 2" But it it was not in standby when it stopped working. Any parts can fail. You also need to check other components in that section of the circuit to make sure there are no other damaged parts otherwise you may damage the new MOSFETs.

[veaux]

Re: Samsung LN40B500P3F - Won't turn on

Alright. I'll look for a replacement for FDPF7N50U. Apparently they are obsolete. FDPF8N50NZU is a replacement per Mouser Electronics webpage. Any recomendation?

[budm]

Re: Samsung LN40B500P3F - Won't turn on

That should be fine.

[veaux]

Re: Samsung LN40B500P3F - Won't turn on

Mosfets "resistance":

G-D-S Looking in front. Red terminal on the left pair in each test:

1)
G-D: 1.6
D-G: 1.6
G-S: 1.4
S-G: 1.4
D-S: 0.7
S-D: 0.7

2)
G-D: 1.5
D-G: 1.5
G-S: 1.4
S-G: 1.4
D-S: 0.7
S-D: 0.8

Now there is no more short between the 2nd pole of the fuse and teh common ground.

I'll put back the two condensators. Those have no polarity right.

Thx a lot I'll post back when I'll find the MOSFET and reassemble the whole thing. Any test I should do meanwhile?

[budm]

Re: Samsung LN40B500P3F - Won't turn on

The 1.4, 1.5, 1.6, 1.7 readings, are they in Ohms, K Ohms, M Ohms? They are tested off the board, right? If Gate to Drain is <2 Ohms then you need to check the components connected to the GATE pin for damages.

[veaux]

Re: Samsung LN40B500P3F - Won't turn on

All in ohms. I had my DMM in the 200ohms position.

[budm]

Re: Samsung LN40B500P3F - Won't turn on

Then you better check all the resistors, transistors, and diodes in the Gate drive circuits as shown in the Diagram.
Also check the capacitor CI892 (0.47uF 400V) to make sure it still shows 0.47uF and no leakage resistance.

[veaux]

Re: Samsung LN40B500P3F - Won't turn on

And yes. All measurements off the board. I got some trouble getting them out but I finally managed to do it after rocking them a lot.

[budm]

Re: Samsung LN40B500P3F - Won't turn on

See post 30.

[veaux]

Re: Samsung LN40B500P3F - Won't turn on

Then you better check all the resistors, transistors, and diodes in the Gate drive circuits as shown in the Diagram.
Also check the capacitor CI892 (0.47uF 400V) to make sure it still shows 0.47uF and no leakage resistance.

Those? Can I do it in circuit?

[budm]

Re: Samsung LN40B500P3F - Won't turn on

Yes, for now, just make sure to report the readings if they are in Ohms, K Ohms, or M Ohms.

[veaux]

Re: Samsung LN40B500P3F - Won't turn on

There you go. All is in OHMS unless noted otherwise.

RI870= 69
RI871=57.1
RI872=30k
RI873 / RI874= 237

RI875 = 68.5
Ri876 = 56.5
RI877 = 30k
RI878/RI879 = 237

At the transistors QI 874 and QI811 I get the resistance values of surrounding resistors.

On the diodes DI 830/831 and DI832/833 I also get the readings of the resistor, not sure how to test those.

At CI892, I get no reading, infinite resistance or at least more that my DMM can read.

[budm]

Re: Samsung LN40B500P3F - Won't turn on

OK, they are good, I would expect the transistor to short out due to high voltage dump through the shorted G to Drain pin and would have also blow the RI871, 876 open circuit, but they are OK.
Leave that fuse that feed this circuit out for now and test the DC Voltage right at the toe pins of the main filter cap (450VDC rating) in the primary side, it should show about 170VDC when the TV is plugged in, and >360VDC when power switch is activated, if it does show >360VDC, then the PFC Voltage booster is still OK.
You should build one of this automatic current limiter (lamp is series with power feeding the TV) so will not keep blowing the fuse and new parts. 100W will be used for this TV.
https://www.badcaps.net/forum/showpo...&postcount=112

[veaux]

Re: Samsung LN40B500P3F - Won't turn on

Can I so that without the new mosfets in ? I remember the 2 big caps at 170ish volts when the fuse was blown. What is the cap # you are talking about in the primary ?

[budm]

Re: Samsung LN40B500P3F - Won't turn on

"Can I so that without the new mosfets in ?" Yes,It should go to >360VDC when TV is on, once you see that you can turn the TV off right away. BTW, you do have to put the new fuse in otherwise the standby power supply will not be running.

[veaux]

Re: Samsung LN40B500P3F - Won't turn on

Ok, let me ensure I got that correctly:
1- put back the card in the tv as is (without Mosfets so the backlight will not work... I soldered the caps back in btw)
2 - put back a fuse
3 - connect the wire harness that goes to the signal card
4 - connect the tv to the outlet. At that point TV shroud be in std by with stdby led on
5 - measure voltage at capacitors CP815/CP816. Shroud be 170VDC
6 - turn on tv with remote.
7 - measure voltage at capacitors CP815/CP816. Shroud be >360VDC
8 - turn off

So, if all that go well, when receiving the new Mosfets, I just solder those on and the TV should be working.

I'm I correct?

[budm]

Re: Samsung LN40B500P3F - Won't turn on

Yes, but
"I just solder those on and the TV should be working." That , we will find out, that is why you need to make that electrical box to put the lamp in line with the power feeding the TV