Sceptre X322BV-HD burnt power supply

[budm]

Re: Sceptre X322BV-HD burnt power supply

It looks like the top is bubled.

[Mr Bill]

Re: Sceptre X322BV-HD burnt power supply

I took a look at U4 and i'm trying to get all the clear off the top of it to see what the numbers are stamped on it but i'm not having much luck. I did set my meter to OHMs and put the black probe on the negative lead from the large cap and checked the pins and all the readings i'm getting are in Mohm range so i'm hoping the chip is ok.

[budm]

Re: Sceptre X322BV-HD burnt power supply

OK, from what I see in the pictures, the MOSFETs that drive the inverter transformer are not shorted since you still have 380VDC feed them directly without any fuse. It seems to me that the Gates drive circuits for the MOSFETs are not working.
Please check those two caps I circle, they should have the same value printed on the bodies, please verify the values and also check for leakage resistance.
Also check to see if the Gate drive transformer is being driven by measuring both DC and AC voltages on those two pins.

[selldoor]

Re: Sceptre X322BV-HD burnt power supply

Looking at the joints in the originally damaged section they dont look great.
perhaps consider reflowing them all especially the Big one which I think is on the
transformer? Check for continuity on the traces under the black.

[Mr Bill]

Re: Sceptre X322BV-HD burnt power supply

I measured voltage using the chasis as ground and i cam up with:

DC - 0.000
AC - starts at .303v then slowly drops to .264 and stays.

Should i use the negative lead of the large cap? The 2 caps you circled are 224K400 NIS. They are both the same value. What do i need to do to determine leakage resistance?

[budm]

Re: Sceptre X322BV-HD burnt power supply

The two points on the transformer, you will just use both probes one on each pad.
The two caps I circle, if they are the same value, one looks fatter than the other, that is why I am trying to see if one is bulging out on the side.

[Mr Bill]

Re: Sceptre X322BV-HD burnt power supply

From the picture one cap does look fatter than the other but i put my dial calipers on them and they are both about the same thickness (.165). I measured the voltage again and i'm still getting 0.000 dc but i'm getting 2.663v AC.

I'm going to reflow the joints around where it originally arced next.

[budm]

Re: Sceptre X322BV-HD burnt power supply

How about the capacitance reading on both of them, so the Gate drive transformer is active since you can read some AC on the primary winding . See simplified diagram. Any luck on that U4 and U3 P/N? The two brown caps are part of the bridge drive setup.

[selldoor]

Re: Sceptre X322BV-HD burnt power supply

It was U4 but it may well be the same part no as U3

[Mr Bill]

Re: Sceptre X322BV-HD burnt power supply

How about the capacitance reading on both of them, so the Gate drive transformer is active since you can read some AC on the primary winding . See simplified diagram. Any luck on that U4 and U3 P/N? The two brown caps are part of the bridge drive setup.

Googling 224k400 i've come up with .22uF 400v. My capacitance meter doesn't read that low. It's not showing it's shorted testing it with my VMM. I did check the voltage on those 2 caps and i have 385V dc on one end and 190V dc on the other end.

I can't get a part number from U4. The more i tried to clean off the clear varnish the more un readable it became. I can give you U3 if you want. I've been trying to find my jewelers loupe all afternoon.

And finally i re-soldered the joints all around the area where it arced originally and it made no difference.

[budm]

Re: Sceptre X322BV-HD burnt power supply

"i have 385V dc on one end and 190V dc on the other end." That is corect to have Half the B+ (385/2 =~190V at the center point.
It looks like U4 drive a bunch of transistors that drive the primary side of the Gate drive transformer.
Lets try this, put the meter in DC, Black probe on the chassis, red probe on any one of the two pads on the primary side of the Gate drive transformer (we measured across the winding before).