Samsung Syncmaster 940BX backlight problem - Need help please.

[PlainBill]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

can't seem to edit my last post (time restriction perhaps as per FAQs!?) so here are some pictures of the burnt area of the above mentioned board...

It looks as though the track is completely knackered, so could someone with more experience tell me where I can bridge a new diode* to? Correct section of schematic attached I believe.

* I think it is a diode as opposed to a fuse as I said above, based on the symbol printed on the board, but does anyone know what type? I can't really see any characters on it other than '...M...' from another board and although I found a service manual for the 932MW (part of it attached) it doesn't seem to say which it is. D802 isn't listed in the part list! Am I missing the obvious?

I'm about to take the transformer off the burnt board to stick it on to one of the others, here's hoping!

P.s. sorry for the multiple files, had to chop up the pdf to attach it

The schematic and parts list are for the logic card, not the I/P (inverter / power supply). I wouldn't bother trying to identify the diode (although D802 on one of the other boards may provide a clue); it's likely that other components were also damaged.

PlainBill

[hotdig]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

I have the service manual if you need a copy.

[sceva]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

I've marked up the picture of the bottom. Check the resistance from terminal A to B, C, and D The from C to D. Also, some of those solder joints don't look very good; touch them all up.

I'd say you either have a bad solder joint, or a bad transformer.

PlainBill

I have the same symptoms - and readings with the transformer in the board are:
A-B - 1.18
A-C - 0
A-D - 0
D-C - 0

also

Whoops!!! I may have gotten ahead of myself. If the transformer is still in the board, don't do the above test yet. Note that I have marked up the picture of the bottom of the board. This is much quicker. Check for resistance from E to E', F-G to F-G', H-I to H-I', and J to J'. If I understand the circuit, all four of those readings will be very low - much less than 1 ohm.PlainBill

I get less than 1 ohm on all of them except J to J', which is 29.8 with meter at 200k. What is unusual is those leads are on the upper side of the transformer, and the set of ccfls on that end work. I think this means the transformer is bad, could someone confirm/deny? Thanks.

[sceva]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

I pulled the transformer out and got the same readings, open circuit between D&C...

[PlainBill]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

I pulled the transformer out and got the same readings, open circuit between D&C...

Yeah, that's indicates an open secondary.

PlainBill

[sceva]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

Thanks Bill. I finally got a ring tester and confirmed.

[PlainBill]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

Thanks Bill. I finally got a ring tester and confirmed.

Good move. It can quickly pick up a variety of faults on transformers.

PlainBill

[muzzo]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

Hi all, great forum and thanks for the chance to join. I was not sure if I should start a new thread or not - so apologies if I got it wrong. I have read with interest the progress of other members in fixing the Samsung 940BX monitor. Before diving into mine and following guidelines already posted I thought I would run my fault past the list members first, in case there is something in particular for me to check that is not already covered.

My monitor works for two seconds after switch on then goes to a blank screen, with the blue light still on. It seems to be very similar to the problem other members have had. In the two seconds or so that it is on the screen it looks fine and the displayed data is as it is on another screen. Should I just start replacing capacitors as described? Best ones to start with?

Thanks again!

73,
Graham.

[Regenpak]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

I'm just starting here myself... I already checked the caps and they're fine, I believe it's a matter of a blown tube or something. This weekend I'll do more testing. The 2 sec symptom suggests an OLP (Open Lamp Protection) trip. See the datasheet for an example schematic, very helpful. My display is a 940N though.

[Regenpak]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

Sonofabitch... it's the transformer. This is unexpected. The PSU board is Suntronix model # SIP-U5F(M) FWIW. It's nice of Suntronix to have followed the Fairchild FAN7314 datasheet almost to the letter, even using the same component numbering as the application schematic. The AP4525GEH half-bridge MOSFET is fine, although the PCB is a bit discolored. The DC values of the XFMR are OK, primary zero, secondaries 1.2k. Even so, using 390k/1W resistors as dummy load for the CCFLs and an external 15V PSU the OLP gets triggered. The secondary connected to EY610 (eyelet in the PCB) puts out >1kV of AC voltage, the other one almost nothing. The two windings share the same core, so I was baffled.

Removing the transformer and feeding it 50 kHz from a tone generator confirmed the in-circuit findings. Since I'm blessed with a Wayne Kerr LCR meter I hooked it up. The EY610 secondary tests about 0.76 H//120k with a 150k load, the EY609 one 0.71 H//61k. Removing the load gives respectively 0.80 H and an <OVERLOAD> from the LCR tester. I suspect a short circuit in the sec. winding. Will put it in my oven to soften the glue to take the unit apart. OK, it's a completely uneconomical repair, but what the heck. It's a hobby anyway!

Edit: Found the TMS91429CT part here. Not even that expensive at 15 UK quid...

[Regenpak]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

Oh. This happens very often. Thought I was onto something new. Still going to try if I can fix it. I have 0.10 mm wire ready.

[Regenpak]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

So I made one of the most geek-esque fixes in my thirty-year career of repairing electronicks. I swapped the offending transformer out with two (!) transformers liberated from a donor board. No idea where that came from, prolly from a monitor with a cracked panel or something:


According to the datasheet, the output transformers are symmetrical. The Suntronics board uses one transformer with two secondaries, one of which tested bad. The kludge I made uses transformers with a grounded side:


Note that the primaries are in anti-series to make the circuit sort of symmetric. I also had to make sure the secondaries didn't short out anything (the cold end is at the other side):




Anyway, the fix was successful on my workbench, but when I mounted it in the monitor it still gave the "two seconds picture" symptom. Hey, in any case it works! But now I have to figure out why it still goes off. I think it's the OTP that still gets triggered.

Stay tuned!

[severach]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

With those handy tools you should be able to measure the multiplier of the transformers to see how compatible they are. Stuff a volt or two into each and see what comes out. You might discover that the transformers must be placed in parallel instead of series.

[Regenpak]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

No, I already tried that and that didn't work. The primary inductance seemed too low. But maybe it has to be that way anyway. The display as it is now seems too dim.

In any case, to make things a bit easier to understand, this is the original schematic:


So nice of Suntronics to keep the component numbering...

[Regenpak]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

Actually, it's quite simple. The circuit will adjust itself so that EA_IN is at 2.5 volts. Which means 5.2 volts at FB. If it doesn't get there it goes OTP (Open Tube Protection). I'll try tying the primaries in parallel after all. And use an external power supply starting at a lower voltage.

[Urbanhiker]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

Hi all,

Nice posts for the 2 second display problem for the Samsung 940BX LCD monitor. I have one that failed exactly as described here. A flashlight confirmed I can see messages from the monitor when the display goes black. Using your posted information helped greatly in my troubleshooting.

After performing a good inspection under lighted magnification and checking some resistor values, I unsoldered, removed the transformer, TMS91429CT (thanks for the schems!!).
The primary measured about .5 ohms with a multimeter. Since the isolated secondaries seemed to mirror the same CCFL bias circuit, I expected identical DC resistance values using my multimeter. One secondary measured 1200 ohms, the other 3200 ohms. The wires for the secondaries are very thin, and likely so if they need to produce 700 Vac (step up transformer). I don't have a commercial ringer to check for shorted turns (the resistance values given kind of point in that direction) in the secondaries. I made up a crude one with a Pulse generator, 220K resistor and an oscilloscope. Essentially I set the Pulse generator to 10ms rep rate 50us width at 8 Vp, terminated with a 50 ohm load resistor. The positive end of the pulse signal went in series into a 220K resistor, the other resistor end went to one of the transformer secondary winding terminals. The remaining transformer secondary terminal went to ground (return to the Pulse generator and Oscilloscope ground). I placed the Oscilloscope across the transformer secondary being energized by the pulse generator and measured waveforms for each secondary tested. The 3200 ohm winding exhibited a nice decay from the sudden pulse rise. The 1200 ohm winding was sharp, narrow and jagged coming away from the pulse rise. I surmised the 1200 ohm winding was shorted.

This was probably over kill for condemning the transformer but I had the equipment available. It also has been 30 years since I performed a ring test on a transformer as described so forgive me if I did something in error.

I ordered a replacement from China off the famous auction site. I have so far done pretty good getting electronic parts from there when I couldn't find them state side (I would prefer to buy US if I could but then again US suppliers probably gets them from China).

Thanks for the good stuff posted here!!
Urbanhiker

[Regenpak]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

Finally got to try the board with the CCFL tubes attached. It was indeed clear that the two transformers should have been in parallel as the OLP kicked in after the dreaded two seconds. When I resoldered the wires I was awarded with a "check cable" message floating across the screen.



Apologies for the out-of-focus connections but I already closed the monitor when I discovered that...

[hotdig]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

Urban hiker....

You said in post #36.... "I ordered a replacement from China off the famous auction site"..........you mean Ebay?

[hotdig]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

Never mind

I see there on the aution now.

I couldn't find them on E Bay when I wanted one last year.

[Gianni]

Re: Samsung Syncmaster 940BX backlight problem - Need help please.

Thanks to this thread I saved time: today I have received a TV Samsung 932MW that stays on for few seconds and then becomes black.
I thought it was due to usual bulging/leaking caps but I was wrong: one secondary of the TMS91429CT is open.

I have already ordered a replacement for 21.4Euro.

On the inverter/PSU board all caps are Samwha rated at 105°C; AFAIK they are not the best caps but they looks good, at least they are not bulging or leaking: do you think I should replace them?

TIA
Gianni