Syncmaster 720 Backlighting - again....

[parabolictech]

Hey guys - First post here so I figured Id make it a good one. I have TWO Syncmaster 720N monitors that have the same problem. The backlight comes on then shuts right off. I recapped BOTH the power and video boards and the only way I can make it stay on is if Im holding my multimeter positive lead (other end grounded) on volts setting to the resistor above one set of CFL plugs (in pic). it will stay lit until I remove the lead then go black. If I put it back on there it wont go back on unless I power cycle it. When its "working" its showing like 65v - when its not its like .11v

I read the threads I could find for this - but seems like it all checks out ok. I have the 2 grounds and 2 5v references on the video board along with about 3.4v on both the on/off and brightness lines.
Any ideas?

[lti]

Re: Syncmaster 720 Backlighting - again....

I attached the copy of the service manual I found a few years ago when I was trying to repair one of these monitors. It's the worst service manual I've ever seen, but it might help a little.

You're connecting your meter directly across one of the CCFL outputs (in parallel with R527 and the CCFL connected to CN503). That makes me think there is either a bad CCFL or a failed component in the current or voltage sense line back to the inverter controller IC. Since you have two monitors, you could try connecting some of the CCFLs from each monitor if you can get the cables to reach without disassembling the LCD panel. You just need four CCFLs from some monitor connected.

Also, using caps from Radio Shack isn't a good idea. Their temperature rating is too low, and you need low ESR caps on the power supply outputs. For some reason, the four 47uF caps on the power supply are rated for 125°C.

[parabolictech]

Re: Syncmaster 720 Backlighting - again....

Thanks! can you test with a meter a bad ccfl?

[budm]

Re: Syncmaster 720 Backlighting - again....

Thanks! can you test with a meter a bad ccfl?

No.