HP 1955 - Backlight not working

[fabianard]

Greetings,

I have a HP H1955 monitor. The monitor is obviously old (from 2005) and a similar replacement monitor wouldn’t be particularly expensive and hard to get hold on. The problem is that I'm using it for a custom built special purpose so it would be tedious and might take a while to find an exact replacement.

Description of problem
Suddenly it went black during use. When I restarted it, it stayed on for 1-2 seconds then went black again. This was repeated a couple of times. Then a burned smell came from the monitor and from that point the backlight was not coming on at all anymore. However the video processing is obviously still working. If I light on it with a flashlight I can clearly see the image. Thus I conclude that it’s a problem with the backlight.


Troubleshooting so far
I found lots of information regarding problems with this type of monitor in this thread:
https://www.badcaps.net/forum/showthread.php?t=8598
However, my board doesn’t seem to be identical so I’m not entierly sure what to check more than I have already done.

I have found the following problems so far:

• What appears to be a faulty solder on one of the gray caps (see attached image).
• Shorted transistors: When I measure the resistance, the two C5707s transistors on the sides of the gray cap with faulty solder show no resistance, however I measure them. The two others seem fine. (See attached images.)
• If I measure resistance over the fuse (see attached image marked "PF701" on the board) I get 139 Ohms when tested in circuit. I guess that may indicate that it’s been burned. Or do I need to desolder it to test it? (according to the link above, no backlight at all may indicate a burned fuse).

Questions

1. Before ordering parts, what additional troubleshooting can I do and how can this be done?
   
2. According to this post https://www.badcaps.net/forum/showpost.php?p=144403&postcount=15 the “fu9024” should be checked for shorts. However. I’m not sure which part this would be and how it should be tested.
   
3. Is it worth a shot to fix this? Or is it likely that any irreparable damage has occured?
   
4. If I find another HP monitor of similar age, say with other dimensions, are the power supply boards swappable?

Sorry I'm not used to working on PCB:s but I’m a quick learner and have basic knowledge and soldering tools. Any help in getting this sorted would be greatly appreciated.

Thanks so much and cheers from Sweden.

[senndogg]

Re: HP 1955 - Backlight not working

Hello fabianard and welcome to the forum!

This is a well known problem which i remember very well, repairing lots of these Benq PSUs.

In general, there a 7 elements failing, 4x C5707 , 2 x FU9024 also the PF701 3A fuse.
You can buy them as a kit, (if they are still available, its been a while i dont see these monitors around) or separate.
It is discussed in this forum before, that it is essential to resolder the joints mainly the transformer legs, as this board suffers a lot.

I think its a simple job, follow the first link you have mentioned, you cant go wrong.
Good luck!

[fabianard]

Re: HP 1955 - Backlight not working

Hello fabianard and welcome to the forum!

This is a well known problem which i remember very well, repairing lots of these Benq PSUs.

In general, there a 7 elements failing, 4x C5707 , 2 x FU9024 also the PF701 3A fuse.
You can buy them as a kit, (if they are still available, its been a while i dont see these monitors around) or separate.
It is discussed in this forum before, that it is essential to resolder the joints mainly the transformer legs, as this board suffers a lot.

I think its a simple job, follow the first link you have mentioned, you cant go wrong.
Good luck!

Thanks for your answer! Unfortunately, parts seems to difficult to get hold on. I found that a Dell 1908FP with its bezel removed has almost the same physical dimensions as the HP monitor. In the end it was a cheaper option to just scrap the HP monitor.

For future reference, I think that the NTE2668 available at TME.eu maybe could serve as a replacement for the C5707.