DELL 2405fpw

[budm]

Re: DELL 2405fpw

I am still trying to locate D19? Can you tell me which board it is on?
OK, I see it now.

[Mirage00]

Re: DELL 2405fpw

Logic board. Upper left by the Capacitors. In the area that was marked by a square in a prior post. It's to the left of the screw holding the rubber wire on the board

[Mirage00]

Re: DELL 2405fpw

I am still trying to locate D19? Can you tell me which board it is on?
OK, I see it now.

Did you find it

[budm]

Re: DELL 2405fpw

On the bottom side of the board, there is one component that I circled, what is that part?

[budm]

Re: DELL 2405fpw

So I went back to that URL again, I can see now they were talking about circuits in the main board around the area I circled, if that the case, do you get short circuit on that Q1 (F5305S)? You already found out that D19 Zener is shorted out. If that Zener is for the Gate protection, I would expect it to be in 12~15V range.
So it looks like you are working at the right area.

[Mirage00]

Re: DELL 2405fpw

Its the component covered in black rubber. It's had sticker on it that read

19-40
229-011
LSE

[Mirage00]

Re: DELL 2405fpw

So I went back to that URL again, I can see now they were talking about circuits in the main board around the area I circled, if that the case, do you get short circuit on that Q1 (F5305S)? You already found out that D19 Zener is shorted out. If that Zener is for the Gate protection, I would expect it to be in 12~15V range.
So it looks like you are working at the right area.

How do I test if I get a short circuit on that Q1 as you asked.

[retiredcaps]

Re: DELL 2405fpw

How do I test if I get a short circuit on that Q1 as you asked.

You really should read my entire guide (section 2 on mosfets)

https://www.badcaps.net/forum/showpo...4&postcount=19

[Mirage00]

Re: DELL 2405fpw

Ok all readings on the Q1 are 1 (yes left side of DMM)

[retiredcaps]

Re: DELL 2405fpw

Ok all readings on the Q1 are 1 (yes left side of DMM)

Not being shorted is no guarantee that the mosfet is working properly. It still may be turning on properly.

If it were me, I would get the replacement diode in addition to the mosfet. It is $2.47 at digikey.

http://www.digikey.com/product-detai...FCT-ND/1769871

[Mirage00]

Re: DELL 2405fpw

Not being shorted is no guarantee that the mosfet is working properly. It still may be turning on properly.

If it were me, I would get the replacement diode in addition to the mosfet. It is $2.47 at digikey.

http://www.digikey.com/product-detai...FCT-ND/1769871

Ok will do and what diode? thx

[budm]

Re: DELL 2405fpw

The Zener shouLd be 12~15V, I would use 15V Zener 1/2Watt (the F5035S GATE is made to handle up to +/-20V), 15V should be OK.

[Mirage00]

Re: DELL 2405fpw

Ok. BTW, I found this website and wound up ordering a 6.8v zenner. Apparently this guy had a similar problem with the video after he fixed the power supply issue. He measured the output of the drain on a working monitor and determined it should be 5v. Interesting how he fixed the issue. Let me know what you think.

http://www.cse.iitb.ac.in/soumen/Sys...2405fpwRepair/

and I also found this one.

http://forum.eserviceinfo.com/viewtopic.php?t=63329

[budm]

Re: DELL 2405fpw

Looks like he replaced the badly damaged 5V power supply section with the new 5V supply board. If 6.8V Zener is not too low for the Gate drive, I do not see why not, if it works.
So you are going to replace the Zener and the MOSFET and see what happen?

[Mirage00]

Re: DELL 2405fpw

Yeah I ordered the Zener and Mosfet from digikey last week. Should be here this week. I will let you guys know what happens. I just realized didn't get a surface mount zener. Hopefully it wont be an issue soldering it?

[Mirage00]

Re: DELL 2405fpw

One more question.. in the other thread he said "I am not sure I want to replace the FET, for fear that the gate controller logic may be fried"

I would imagine the zener is their to protect the circuit in case the FET blows. What did he mean by gate controler logic. Isn't that within the FET?

[budm]

Re: DELL 2405fpw

There is an IC that drive the Gate of the MOSFET (at 40~80KHz a second) , when you have Drain to Gate shorted, it will usually take out the protection Diode and sometime the output circuits inside the IC also, I hope it is not in your case. At this point I am not sure which IC that is for this buck regulator.

[Mirage00]

Re: DELL 2405fpw

Got it.. thats why he said "without further help from DELL (schematics)" he had to replace the regulator. Thanks again budm

[Mirage00]

Re: DELL 2405fpw

Ok everyone I have one last question before I proceed since the parts (Zener diode for D19 and 5305S MOSFET) arrived today. In speaking with the person who repaired his 2405fpw by adding its own SMPS to supply 5volts to the T-CON board, he said if I replace the MOSFET and diode, chances are it will blow again and could cause more extensive damage to the unit. He reccomended just replacing the 5v supply as he did. What do you guys think? Doesnt the diode (d19) prevent downstream circuits and gate logic from damage?

Thanks

[budm]

Re: DELL 2405fpw

If I read his post correctly, that Zener Diode is for protecting the Gate of the MOSFET, not for protecing the whole 5V circuit., If that 5V is for feeding the T-CON only and NOTHING else, then you can find out if the it is putting out 5V or not by not connecting the cable to the T-CON, but install 100 Ohms 1/2Watts resistor as a load instead.
IF the IC that drives the MOSFET is bad and causes the MOSFET to be stuck on instead of swithing, then the output voltage will be the same as the input voltage, which is around 20V (it basically not stepping down the voltage as it should).
I forget, does the MOSFET has shorts between Source and Drain? You may want check when you remove the MOSFET.