Phihips 47pfl5704d / f7 Back light not working

[cwolford]

Re: Phihips 47pfl5704d / f7 Back light not working

using retiredcaps tuts i'm doing some measurements. Here is T701 backlight
here is the print on it:
2130kp029c-f
gl 47 fhd
ny091106b

here is the measurements
1 to 2= 1.04 ohms
1 to 3, 4, 5, 6= 0L seems to not measure at all
2 to 3, 4, 5, 6= 0L seems to not measure at all
3 to 4=23.94 ohms
3 to 5=67.95 ohms
3 to 6=91.89 ohms
4 to 5=44.1 ohms
4 to 6=68.05 ohms
5 to 6=24.05 ohms

[cwolford]

Re: Phihips 47pfl5704d / f7 Back light not working

Like i said I dont know a lot, but Seems I have a bad MOSFET.

KHB 5D0N50f

all of them Q706-q709 have same readings but q706
q706
1 to 2= 14.92 ohms
1 to 3= 2.109k ohms
2 to 3= 2.116k ohms

q707, q708, q709
1 to 2 = .3190k ohms
1 to 3 = cant get a lock DMM is seeking down from 20M ohms
2 to 3 =Cant get a lock DMM is seeking down from 15M ohms

Since this are the same somebody is bad. the question is who, then why.

[CapLeaker]

Re: Phihips 47pfl5704d / f7 Back light not working

if the drain / source is shorted to the gate, then you have to look at the whole gate drive circuit too. Take the Mosfet out and check it again out of circuit.

[cwolford]

Re: Phihips 47pfl5704d / f7 Back light not working

I will remove it soon and retest. All the diodes seem to test good and all the other transistors on the broad seem ok.

Anyone know of a place in USA to buy a replacement so i don have to get the whole repair kit.

I have used mouser and digikey and ebay but Seems the KEC KHB 5D0N50f is old. I dont know how to find a new one that will have the same specs.

[cwolford]

Re: Phihips 47pfl5704d / f7 Back light not working

if the drain / source is shorted to the gate, then you have to look at the whole gate drive circuit too. Take the Mosfet out and check it again out of circuit.

I could use some help finding the parts that need checked in that circuit. Yow do you guys know this is their like a circuit diagram or something?

[cwolford]

Re: Phihips 47pfl5704d / f7 Back light not working

I removed q706 and q707 with the heat sink as I could not remove screws at that angle. Q706 same measurements. Q707 all I got was 0L on all measurements.

I know how to solder I was in and out quick NP all went easy, I don't think I killed it. What gives

also was wearing esd strap

[cwolford]

Re: Phihips 47pfl5704d / f7 Back light not working

So I got to thinking to find the rest of the circuit as i dont have a diagram, I can just use the PCB traces and dmm to track it from the bad Mosfet. Is there anything hidden between the layers of PCB or should i beable to track by eye and maybe the help of the dmm?

[Davi.p]

Re: Phihips 47pfl5704d / f7 Back light not working

PSU boards don't has inner layers.. for the transistor you have to watch on distributor's site and select the one that meets or exceeds the original's specs as for Vds, Rds-on, Id, channel type. Vds=the more the better, Rds-on=the less the better, Id=more/better, channel=same. I think that if the board tried to switch on the B.Light then probably the drive circuit is ok but it stops for the shorted mosfet and goes in protection, if you don't know methods of testing the rest, try replacing only the mosfet.. without an oscilloscope you can't test much..

[genuineguy]

Re: Phihips 47pfl5704d / f7 Back light not working

To replace the MosFet some parameters to look for the replacement are
1. The Drain current must be equal or higher
2. The on resistance same or less
3. Vds same or higher
4. Vgs same or higher
I used these rules and have replaced MosFets with commonly available IRF series and have repaired the board

Also, the first step is to search the internet and download the service manual for your TV. These days its available. I could get one for mine and it helped me a lot. Neat circuit diagram etc. If you don't one for yours exactly, see if there is one for the nearby models as the boards are more or less similar.. atleast somewhat ...

[cwolford]

Re: Phihips 47pfl5704d / f7 Back light not working

See i know a circuit will only pull the amps it needs. I also knew that the resistance had to be near the same, anyone know how much wiggle room i have?

What i dont get is the voltage for the gate / drain. I know there is wiggle room but Too much cause heat and part blow, too little is bad too the meaning of brown out, not sure what this does tho.

thanks for the replys

[cwolford]

Re: Phihips 47pfl5704d / f7 Back light not working

so i'm not even %50 sure, but based on what i have come to understand from chatting here and the spec sheets.

The circuit applys voltage to the gate of the mosfet to control it
That voltage should be 10v, but no more than 30v right?

Next

The source should be ground and then the drain seems to be the voltage from the 450v caps.

So I should be able to test to make sure there is ground at the source, 10-30v at the gate, and cap voltage of at the drain?

It seems it seems the mosfets control the current as compared to hooking the transformer up straight to the caps right?

[cwolford]

Re: Phihips 47pfl5704d / f7 Back light not working

seems i did not kill that MosFet the holes in the the pcb have the measurements with our with out the Mosfet.

Seems that mosfet is bad to seeing its all 0L yea? If anyone know of anything else i should check because of the bad mosfet plz let me know.
thanks

[Davi.p]

Re: Phihips 47pfl5704d / f7 Back light not working

First.. please use commas.. after, i'm not comfortable using ohms to test mosfets, but i believe it's normal 0L all the way, there's many websites explaining how to test them, search "mosfet test", normally mosfets becomes shorted so you read very low ohms or volts in some ways.. most of the times D-S in your case..

[cwolford]

Re: Phihips 47pfl5704d / f7 Back light not working

First.. please use commas.. after, i'm not comfortable using ohms to test mosfets, but i believe it's normal 0L all the way, there's many websites explaining how to test them, search "mosfet test", normally mosfets becomes shorted so you read very low ohms or volts in some ways.. most of the times D-S in your case..

Will look into to it as I get some free time. Thanks. Why are you not comfortable>? Is it like a fast and dirty way? Like not 100% etc..

[Davi.p]

Re: Phihips 47pfl5704d / f7 Back light not working

It's only that i have learn only to use diode test of the dmm, i dont know the values in ohms..

[cwolford]

Re: Phihips 47pfl5704d / f7 Back light not working

thanks all, got the time to do it.

$6 in MosFet. fixed it. IT boots up, back light flickers, but then after warm up is fine.

been working 3 months.

[Davi.p]

Re: Phihips 47pfl5704d / f7 Back light not working

CCFL lights can be at end of life, your PS board can break again if you don't care of them..