Dell 2407 wfpb no backlight

[PlumBum]

Hi i got an Dell 2407 and the backlight stopped working while i was watching a movie.
The power light and input nr is on, but no backlight.
i have opened the monitor and checked the power board an i seams fine, have also checked most of the voltages and they are fine.
The fuse F1 on the inverter board is blown, and i get 19 volt on the one side comming from the connector,
so the question is what caused the F1 fuse to blow ?
I have checked all the diodes, the three big 2.5A fuses and most of the transistors, they all seams ok.
do you have any suggestions about what caused the fuse to blow, maby the 5V voltage regulstor

[budm]

Re: Dell 2407 wfpb no backlight

That is one of those bad black fuse which is used alot in the CMO Inverter board for LCD TV that I replaced lots of them. I believe that is 1.5A fuse, it is used for the PWM IC on the inverter circuit. I use PICO fuse which you will have to bend the legs, no need to remove the old fuse, you can solder one end to the big copper trace and another end to C9 leg.
You can check the resistance on the right side of the fuse and circuit ground to see if it shows very low Ohm, also check the Output pin of the 5V reg to see if it shows low Ohms, that will the idea if you have shorted circuit or not.
I am not sure if that fuse feed the input pin (left pin, center pin is GND, right pin is Output) of the 7805 or not.
You can also put your meter in DCA mode to check the current draw.

[PlumBum]

Re: Dell 2407 wfpb no backlight

You can check the resistance on the right side of the fuse and circuit ground to see if it shows very low Ohm

No short there, it shows 2mega ohm

also check the Output pin of the 5V reg to see if it shows low Ohms, that will the idea if you have shorted circuit or not.

There is 2.3Kohm between out of 78m05 and gnd

I am not sure if that fuse feed the input pin (left pin, center pin is GND, right pin is Output) of the 7805 or not.
You can also put your meter in DCA mode to check the current draw.

the 7805 is fed through Q2 an pnp transistor that also seams to be ok, so i belive the 5v circuit is ok, must bee someware else
the Fuse f1 also supplys U1 an 324, and probably a bunch of other components. there are 3 220uF electrolyts on the board, i have ordered some and will replace them

[bicycle6228]

Re: Dell 2407 wfpb no backlight

PlumBum- Did you get your 2407 Inverter board working?

[DerD]

Re: Dell 2407 wfpb no backlight

I have just to say a BIG THANK YOU, for all the information collected here that allowed me to identify a F1 fuse being responsible for the issue with my monitor. Unfortunately there is no info available how to replace this mysterious fuse with "20" 1:1, a 2A fuse for 500mA regulator seems a bit to much. And 200mA a bit weak, although tests showed a current of 35mA over resistor used for tests in normal mode.
Anyway, I will give a try to a 200mA Polyswitch in 1206, but I have to wait for the shipment from China.