HP LA1905WG lcd monitor no backlight

[shelby5041]

hi i have 4 of these monitors manuf 2011

all 4 do not have backlight.....backlight does come on for 2 seconds but then goes off......

you can see the image but no backlight

i have repaired hundreds of lcd monitors 99% of the time its just bad caps but on these the caps are all good
and its just weird they are all same model# and all have same problem

im hoping someone can help me....i would love to find out what component is causing this issue....must be common

thanks

[shelby5041]

Re: HP LA1905WG lcd monitor no backlight

here is a photo of power board

[stj]

Re: HP LA1905WG lcd monitor no backlight

could be the ransformer or a bad tube.

[budm]

Re: HP LA1905WG lcd monitor no backlight

You can make two test lamps to test the inverter circuits.
https://www.badcaps.net/forum/showth...er+testing+cfl

[shelby5041]

Re: HP LA1905WG lcd monitor no backlight

hi...thanks for replying the tubes are good i tested them a while back....im just trying to get back to this repair....have time now

so it has to be a power supply inverter issue........

the 1000uf cap in front of transformer to lights is good....how can i test the transformer?

here is a pic of underside of board

[budm]

Re: HP LA1905WG lcd monitor no backlight

What is the P/N as printed on the inverter transformer? Did you also check those two SMD 8-pin MOSFETs that drive the inverter transformer?
Check the two legs on the right side of the transformer as show in you bottom view of the board.

[shelby5041]

Re: HP LA1905WG lcd monitor no backlight

hi no i havent checked any mosfets...how do i test the 2... 8 pin mosfets? underside board

[shelby5041]

Re: HP LA1905WG lcd monitor no backlight

too hard to read

darfon 80TL22t-4-dn

not sure......

[budm]

Re: HP LA1905WG lcd monitor no backlight

hi no i havent checked any mosfets...how do i test the 2... 8 pin mosfets? underside board

Get the P/N as printed on top of MOSFET and then do the search to get the spec sheet that will show the pinout, then you check the resistance between S and D pins.

[shelby5041]

Re: HP LA1905WG lcd monitor no backlight

ok thank you...ill try that when i can.......

[shelby5041]

Re: HP LA1905WG lcd monitor no backlight

part# too small cant read it.....oh well

[budm]

Re: HP LA1905WG lcd monitor no backlight

OK, on the transformer, just check the resistance between the two legs on the right of the transformer.
http://www.ebay.com/itm/80TL22T-4-DN...-/161158475930

[shelby5041]

Re: HP LA1905WG lcd monitor no backlight

seems on 8pin mosfets one side has 4...d and other side 1 s and 3 g

ok trans has 2legs one end and 6legs other end.....check res between 2legs one end?

what value would be good?...and bad?

[shelby5041]

Re: HP LA1905WG lcd monitor no backlight

do i need to worry about main cap voltage?..board has been out of monitor for 2days

[stj]

Re: HP LA1905WG lcd monitor no backlight

meter it and see.

[shelby5041]

Re: HP LA1905WG lcd monitor no backlight

transformer 2 pin...resistance is 442 ohms on all 3boards

[shelby5041]

Re: HP LA1905WG lcd monitor no backlight

mosfet may be APM4548A

[shelby5041]

Re: HP LA1905WG lcd monitor no backlight

on mosfets getting 17 m ohm s-d on all of 3boards

[budm]

Re: HP LA1905WG lcd monitor no backlight

17 m (mili) Ohms = 0.017 Ohms.
Or you mean 17 M (Million) Ohms.

[shelby5041]

Re: HP LA1905WG lcd monitor no backlight

meter read 17M ohms