Samsung SyncMaster 940BW no backlights

[Jim on the river]

PWI1704SV LCD backlights don't come on LED does

Same symptoms appeared about 1 year ago, detected 2 bulging capacitors and replaced for successful operation.

Although no visible problem with caps, at Badcaps' suggestion I replaced all but the big one with quality caps.

Symptoms remain with video detectable with front light, LED on, but no backlights.

I assume either the inverter is not working or the bulbs are bad. How can I determine which one is the case?

[ratdude747]

Re: Samsung SyncMaster 940BW no backlights

Moved from FAQ.

---

Could be the lamps... The only way to test would be to use another screen with similar lamp connectors...

...But I'd first suspect an issue with the inverter transistors and/or the inverter transformer's soldering to the PCB.

Did you make sure that you got the caps in straight, soldered them in well, and trimmed the leads enough (so they're not shorting anything)? Even I've bungled all 3 before, just a good thing to check when things don't change after a repair..

[jetadm123]

Re: Samsung SyncMaster 940BW no backlights

Try checking fuse F201, located directly below the small green board next to cap C30.

[Jim on the river]

Re: Samsung SyncMaster 940BW no backlights

I checked the caps and continuity from all their leads. They all seem to be OK, no cold solders. They are snipped short enough not to be shorting.

The fuse at F201 checks out OK also.

The leads from the transformers have continuity. If the inverter transistors are the ones in the heat sinks, as I expect they are, they too have continuity. So, it appears those components are soldered in good contact with the PCB.

There are also no visible signs of anything gone awry.

[jetadm123]

Re: Samsung SyncMaster 940BW no backlights

Inverter transistors are not heatsinked (at least not to metal heatsinks). I'm thinking your inverter circuit is located on that small green hybrid board between the two inverter transformers. It probably has the ccfl controller and 1 to 2 8-pin mosfet transistors mounted on it. Before doing anything else, measure the point marked "vcc" on that board. It should read approx 13-15Vdc. Also, keep in mind that no backlights can be caused by:

a bad inverter tranformer

bad ccfl

bad ccfl wiring

[Jim on the river]

Re: Samsung SyncMaster 940BW no backlights

The vcc voltage is 14.72 V.

[Jim on the river]

Re: Samsung SyncMaster 940BW no backlights

I appreciate all the help, but I must say at this point I am a bit stumped. I read elsewhere on this forum that a mosfet requires some sophisticated equipment, that I don't have, to test in circuit.

I could order a new board from an OEM source for 20 bucks, which would solve the possible mosfet and/or transformer issues, but I can't be sure it is not the ccfl's.

I started to take the screen apart to access the ccfl's and realized it would be pretty easy to ruin it in the process. The bulbs have been in use regularly for about 5 years so they could be worn out, but there was nothing indicative of them going. The screen was fine with no perceivable problem one minute then blank the next without a sound.

Anyone willing to risk a guess as to how likely it is that the problem is with the board?

[Jim on the river]

Re: Samsung SyncMaster 940BW no backlights

Gambling that it was the board I ordered one from China for a little over 20 bucks. It must have come on a slow boat, but it finally arrived. Now I have my second monitor once again! It was like wallpapering with one arm in a sling there for a while.

[orfeous]

Re: Samsung SyncMaster 940BW no backlights

Trying to problems on a friend monitor too.. :P just got the cover up and watching visually over the powerboard.. no bulging caps.

[orfeous]

Re: Samsung SyncMaster 940BW no backlights

Symptom is like this...
Got a video to make it easier to explain.
Hooked monitor up with VGA to computer.. same symptom with DVI so it doesnt matter.

Video: https://dl.dropboxusercontent.com/u/...IDEO0028-0.mp4

Service manual:

[orfeous]

Re: Samsung SyncMaster 940BW no backlights

Update on my friends Samsung..
Changed all 5 small capacitors.. put everything back together and fired it up.
Got picture for 2sec then it went black.

What to try now?

[selldoor]

Re: Samsung SyncMaster 940BW no backlights

You need to read through and do the tests in here

https://www.badcaps.net/forum/showthread.php?t=10419

start at post19.

Post clear pictures of front and back of boards- use manage attachments
do not file off site