HP LA1905wg LCD monitor no backlight

[shelby5041]

hi

i have repaired hundreds of no backlight lcd monitors.......typically its bad caps

however i have come upon 5 of these lcd monitors that when powered on the backlight comes on for 3 sec and then goes off.....the display is there but no backlight....all are same model# and have same symptom no backlight

i tested all of the caps out of circuit using esr meter and they all test and look good

im stumped

any ideas?....i have the boards out and can test any component if advised to do so.....

see photo of power supply/inverter board

[budm]

Re: HP LA1905wg LCD monitor no backlight

Try using test lamps yet to really help narrowing down to find out if it is the board or the lamps?

[shelby5041]

Re: HP LA1905wg LCD monitor no backlight

hi thanks

i do not have a test lamp....and this is just odd because they are all doing the same thing...and i have never come upon bad lamps on over 100 repairs on lcd monitors they have all been bad caps on inverter side of power supply....but i figured since its so common on this model maybe someone has some experience they could direct me on the faulty component

[shelby5041]

Re: HP LA1905wg LCD monitor no backlight

but i do have one of these models with a good backlight so i will swap boards and see how it reacts but that doesnt fix the problem it will just tell me if lamps or power issue

[budm]

Re: HP LA1905wg LCD monitor no backlight

If you fix a lot of unit having test lamps makes much faster in narrowing down the problem.
Test lamps:
https://www.badcaps.net/forum/showthread.php?t=19987

Bad lamps:
http://s807.photobucket.com/user/bud...?sort=3&page=1

[budm]

Re: HP LA1905wg LCD monitor no backlight

but i do have one of these models with a good backlight so i will swap boards and see how it reacts but that doesnt fix the problem it will just tell me if lamps or power issue

You can also then check the resistance of the high voltage windings of the inverter transformer so you can compare the readings between the working unit and the no-working unit.

[shelby5041]

Re: HP LA1905wg LCD monitor no backlight

ok i just tested backlight by taking a good monitor and plugging in the backlight cables from the bad monitor to the good monitor and the backlight comes on fine so it now tells me its not a tube issue its a power board issue....

[shelby5041]

Re: HP LA1905wg LCD monitor no backlight

not sure how to do that can you see my photo and direct me which pins to test...and how to test....regarding the transformer

[shelby5041]

Re: HP LA1905wg LCD monitor no backlight

other question ..when is it safe to test the transformer?

with unplugged of course but do i have to wait some time after powering off?

[budm]

Re: HP LA1905wg LCD monitor no backlight

other question ..when is it safe to test the transformer?

with unplugged of course but do i have to wait some time after powering off?

No waiting needed, the drive circuit will shutdown when power is removed but you should discharge the main filter cap (using resistor) that can have up to 325VDC on it.

[Agent24]

Re: HP LA1905wg LCD monitor no backlight

not sure how to do that can you see my photo and direct me which pins to test...and how to test....regarding the transformer

Post a photo of the underside of the board so we can see which transformer pins connect to where.

Also, have you checked the inverter section for bad solder joints?

i have never come upon bad lamps on over 100 repairs on lcd monitors they have all been bad caps

You're very lucky! (Or I'm very unlucky!) About half my LCD repairs have been bad lamps.

[shelby5041]

Re: HP LA1905wg LCD monitor no backlight

i have not come across any with bad lamps as of yet all have been bad caps on boards....so this one is stumping me....the lamps are good though i used another good monitor and plugged the power cables into the bad monitor and the backlights powered on fine

so its a power supply /inverter issue

checked for obvious bad solder joints and all are good....

maybe transformers are bad?? going to research how to test

[shelby5041]

Re: HP LA1905wg LCD monitor no backlight

see my photo how would i test the transformer..?

[Agent24]

Re: HP LA1905wg LCD monitor no backlight

Find out the correct resistance of the windings and measure them, see if they are correct.

If you have another known working one to compare with, or can find the information online. Search the number on the side of the transformer in Google.