Samsung Syncmaster 400pn (no backlight)

[danieloh69]

I have a Syncmaster 400pn. I replaced 5 bad caps on the power board, checked the 24v power pins going to both inverter boards, checked both fuses on both inverter boards, checked the power on pin (3.3v) on both inverter boards. I still have no back lights. Any ideas??? I am a very novis
electronics person, all self taught. So please no hi tech jargen if possible try to explain in laymens terms.

[jetadm123]

Re: Samsung Syncmaster 400pn (no backlight)

Wow! A 40" monitor! You'll have to post some good photos of the power supply (top and bottom), inverter and logic card per the link below:

https://www.badcaps.net/forum/showthread.php?t=1868

1) Does the power blink? solid?

2) Do you get a flash on screen when you press the power button? Then it goes black after 1-2 seconds?

3) Can you see an image on the screen if you hold a flashlight up to the screen?

4) What type of caps did you use as replacements? Do you double-check to make sure they're installed correctly?

5) Is there a 5 volt ouput on the power supply?

6) Check fuses on power supply and logic card?

[danieloh69]

Re: Samsung Syncmaster 400pn (no backlight)

Yes I checked all 5 fuses. the power supply board has 3.3v pins one for each inverter board called "on" (2 inverter boards) and has 24v pins for each board (4-24v pins for each). I did the flash light thing and the monitor is making a picture just fine,I am sure it is just a "back light" problem. See attached photos.
photo 1 I circled some parts that I tested. the top part tested 24v top and bottom the bottom part tested 24v on top and nothing on the bottom.I don"t know what these parts are (as I said I am a novis) so I don"t know if it is normal to test the way they do (or I did for that matter).
photo 2 the copper coil looking parts tested at 24v on all 5 bottom pins and nothing on the top 2 pairs, I assume these are the high voltage coils for the back lights. Any ideas would be appreciated.

[retiredcaps]

Re: Samsung Syncmaster 400pn (no backlight)

the top part tested 24v top and bottom the bottom part tested 24v on top and nothing on the bottom.I don"t know what these parts are (as I said I am a novis)

That looks like a diode. Diodes are designated with a "D" on the PCB.

photo 2 the copper coil looking parts tested at 24v on all 5 bottom pins and nothing on the top 2 pairs, I assume these are the high voltage coils for the back lights. Any ideas would be appreciated

The copper coil is the inverter transformer. The top 2 pins can easily carry 600V to 1000V. This voltage will do some serious harm to you.

If you are getting no backlight, you have possibly 3 problems.

1) One or more of your fuses are blown. A good fuse should measure under 1.0 ohms.

2) The transistors or mosfets are shorted on the inverter board. We will need much better pictures to identify the mosfets or transistors. Get a clear focused 2000x2000 resolution. 2 out of the 3 pictures you have posted are too small and out of focus.

3) The inverter transformers have a shorted turn or open winding.

If you have a manual range multimeter, set it to 2k ohms (two thousand). You can test for open windings "in circuit" on the transformer by (with power off and unplugged)

Power off and unplug monitor. WAIT 5 MINUTES BEFORE STARTING the ohms measurements.

a) Number the pins on the transformers anyway you like
b) black probe on pin 1, red on pin 2 - record ohm
c) black probe on pin 1, red on pin 3 - record ohm
d) black probe on pin 1, red on pin 4 - record ohm
e) etc
f) black probe on pin 1, red on pin last - record ohm
g) black probe on pin 2, red on pin 3 - record ohm
h) black probe on pin 2, red on pin 4 - record ohm
i) etc
j) black probe on pin 2, red on pin last - record ohm
k) black probe on pin 3, red on pin 4 - record ohm
l) etc
m) finally, black probe on pin last-1, red on pin last - record ohm

I'm guessing the secondaries should read 950 ohm (this will vary for different transformers). The primaries should read 1.3 ohm. Some will be 0L. If one secondary is 950ohm and the other is 750ohm, that would indicate a problem.


PS. Please keep everything in one thread (stop opening new threads). It makes it difficult to follow if everything is spread out.

[danieloh69]

Re: Samsung Syncmaster 400pn (no backlight)

Well hopefully these pictures are better, I took the monitor outside and used a tri pod. in picture 4 you can see I drew an arrow to the fuse, I soldered a small jumper across to be shure that it wasn't the problem. Also sorry about the extra thread but it was giving me a hard time posting (and I am new to this) a reply. I really do want to thank you for your help. I will check the tranformer as soon as I can. My wife has me doin a "honey do" project (installing bedroom carpet). So it will be this evening befor I can get to it. Again thanks for your help.

[retiredcaps]

Re: Samsung Syncmaster 400pn (no backlight)

in picture 4 you can see I drew an arrow to the fuse, I soldered a small jumper across to be shure that it wasn't the problem.

I would personally remove those jumper wires. A good fuse (with power off and lcd unplugged) will measure 1.0 ohms or less.