Problems with Samsung 740N monitors

[raokd]

Re: Problems with Samsung 740N monitors

@Behemot:

The drain voltage when the Lamps were on was shuttling between 11V-14V. Not sure if my multimeter was jumpy, or the voltage itself.

When I removed both the lamps completely, the voltage on 820uF remained steady at 13V.

Taking your advice I resoldered many points....especially since my monitor was manufactured in Feb 2006. The monitor turned on for atleast half hour without flickering. But soon the symptoms came again.

So am going to resolder, rest of the points and then give it a try again...although visually, the joints look fine. Will repost the pics of my board after all the soldering...

@budm
Thanks for the link...I dont have test lamps...will try to engineer based on your guidelines...also am trying to get back an old monitor that I had given in for sell off...

The voltage on the large DC cap comes between 315-330V...is this normal?

Will be taking a small break for 2 days...so will come back to follow your guidelines...pls be there when i come back

Thanks for your advice

[selldoor]

Re: Problems with Samsung 740N monitors

The voltage on the large DC cap comes between 315-330V...is this normal?

Perhaps should not fluctuate that much - or was that testing at different times.
It should be 1.4 * your mains voltage ie 322 vDC

[raokd]

Re: Problems with Samsung 740N monitors

Yes it was taken at different times....normally fluctuates 3-4V....for example during 1 reading 319V-323V...

[Behemot]

Re: Problems with Samsung 740N monitors

Taking your advice I resoldered many points....especially since my monitor was manufactured in Feb 2006. The monitor turned on for atleast half hour without flickering. But soon the symptoms came again.

It's probably lead-free already. Try adding slightly more leaded solder, may help. Lead-free sodler is shit and has many known problems, including bad thermomechanical characteristics, resulting in often cracking.

You may want to read What is RoHS? in your free time. I have added nice document from NASA couple days ago.

[raokd]

Re: Problems with Samsung 740N monitors

Hello Behemot,

The solder that i am using is TIn-Lead (60-40) composite. Hope thats good enough.

Have resumed now...will finish re-soldering the remaining joints and then post my board's pics.

Thanks...

[Behemot]

Re: Problems with Samsung 740N monitors

Yeah, that's the good one (: Wish you luck

[raokd]

Re: Problems with Samsung 740N monitors

Today I re-soldered all the remaining joints. Now when I did a cold start (after all the soldering)...the backlights remained on for an hour or so.

But again the flickering resumed. Although now i notice the flickering is less frequent.

Am attaching new pics of the solder side of the board.

Next I plan to physically check the lamps.

Please advise if any other checks need to be done to reach the core issue.

Thanks

[bowlofpudding]

Re: Problems with Samsung 740N monitors

this is a pretty old thread but thought i would add to it. i have a syncmaster 930b, with the same board that everyones having capacitor troubles with (ip-35135b). the monitor made a buzzing noise, the backlight dimmed and i just got around to taking it apart to take a look. on the board the 2 8-pin mosfets going to the inverter transformer looked like they had a tiny hole burned through the center. when the monitor died it let out the fried electronics smell but since i waited so long to look at the problem the smell was long gone. i just ordered a new set of mosfets and am waiting for them to arrive and hope that fixes the problem. after reading all these posts im surprised the caps weren't swollen. the 820uf caps are samxon branded and the 1000uf caps are capxon branded. im starting to think i should preemptively replace them. im also curious why the mofsets just burned up in the first place, these things shouldnt burn up if the circuit was properly designed. i tested the resistance on the transformers and the readings on both were nearly identical. anyway thought i would share my experiences here since this forum gave me some insights

oh and i only find 1 fuse on the board labeled f101, cylinder shaped orange thing in line with the ac plug. i looked where everyone says the pico fuse is and i never found it. everything else on the board seems to be working fine though, i still see an image on the screen when i hold a light up to it. would be strange if they didnt include a fuse on this board but might explain why the mosfets blew

and 1 last edit. i just looked more closely at the 820uf samxon caps and the top isnt flat, its kinda of pushed out a little so im thinking it will be a good idea to replace these

[Behemot]

Re: Problems with Samsung 740N monitors

Most likely Samxon GF, their only bad series I am aware of. Replace on sight. Samxon GT is good substitute. Other than that, I source Chemi-Con KY's from Digi-Key with 20 mm height.

[Pentium4]

Re: Problems with Samsung 740N monitors

Most likely Samxon GF, their only bad series I am aware of. Replace on sight. Samxon GT is good substitute. Other than that, I source Chemi-Con KY's from Digi-Key with 20 mm height.

I believe KM and GK aren't very reliable either, if I remember correctly

[Behemot]

Re: Problems with Samsung 740N monitors

Oh yeah, GK, too, booth green and very similar, I usually take them as the same Not familiar with KF though…

[Pentium4]

Re: Problems with Samsung 740N monitors

Or maybe it's KM...I've never actually seen them in person, just bad ones on the net

[Behemot]

Re: Problems with Samsung 740N monitors

KM is good series, mostly used as big high-voltage caps (often on primary side of PSUs or dispalys).

[Pentium4]

Re: Problems with Samsung 740N monitors

Hmmm....I forgot the other bad series....c_hegge would know

[bowlofpudding]

Re: Problems with Samsung 740N monitors

the ones i ordered were 565-1553-ND from digikey. chemi-con KY series 25mm height (hopefully theyre not too big) i figured they would be a suitable replacement, since the ones were a bit bulged on top i figured wouldnt hurt to replace them, i would at least like to get a little more life outta this monitor even though i bought it used, and for cheap.

[bowlofpudding]

Re: Problems with Samsung 740N monitors

just got the 25mm height caps today, test fit them and theyre too big by like 2mm. they hit the top of the cage. probably shouldve done my homework before i ordered. so i guess ill be looking for the same caps but at 20mm height

[bowlofpudding]

Re: Problems with Samsung 740N monitors

before i order, anyone have any opinion on the chemi-con lxy series?

[Behemot]

Re: Problems with Samsung 740N monitors

I'd take KY again, last time I checked, they've been somewhat cheaper than other series (LXZ, LXY). They also seem to have highest capacity in every particular diameter with 20 mm height.

[LuSiD]

Re: Problems with Samsung 740N monitors

Hey guys,

Am i the only one that has this powerboard in my Samsung 740N?

See the attached pictures. The caps seem fine and are labeled 'JH'. What should I do? Does anyone knows what this board is and if it has the same problems with the common ones?

Thank you

[Rtech]

Re: Problems with Samsung 740N monitors

The TMS Inverter transformers do fail,you need to check the secondary resistance of them