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Samsung SyncMaster 906BW 19" LCD - No Backlight

Tesla

May 10, 2010
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It just stopped working. No fluorescent backlight. No "No Signal" display (like it usually does when not connected to PC) or able to see main menu Setup Menu display. However, if I look close, I think I can see something (so I think the logic board is working).

So, looking close at Power Board first. No obvious cold solder joints. No obvious bulging or leaking caps.

It's circa 2007 widescreen model. Any ideas on continuing appreciated (including where exactly to test for voltages). See attached pics


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Tesla

May 10, 2010
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Ok, off of CN2 (to logic board), I'm getting 5.1v DC and 16.0v DC ... so that looks good. Also means a large part of the Power Board is working.

Off of CN 301 and CN 304 ... how exactly do I attach the meter leads to check the 650v (I assume AC) that I'm suppose to get there? Notice (in pic) there are 4 smaller plugs that fit into those 2 connectors (above the long black coil). They drive the back-light.
 

Tesla

May 10, 2010
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So, it looks like there are 4 tubes in this panel. Laptops I have worked on before only had one.

So, I set meter to 1000 VAC scale. I connect both leads to each set of 2 pins (just like the 4 tubes connect). Do that 4 times (once for each pair). I get no AC voltage on any of those.

Also, about 2 seconds after I power it up, the PS makes a squeaking noise, and I think shuts down ... but just the 650vac high-voltage part I think. Seems something is holding it down.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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A failure in the transformer is a possibility.

Given the cost of these devices, at this point I buy a new one or swap the power supply out for one that works. (not saying you have to, but I've not bothered fixing them)
 

Tesla

May 10, 2010
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A failure in the transformer is a possibility.

Which transformer (the main yellow one or the black one for the blacklight tubes)?

Like many boards, it's divided into two parts. The high voltage section, and the low voltage section. In this case, the low voltage section happens to have the AC ballasts section also.

Again, like many board, the main yellow transformer connects the 2 sections. Some of those windings must be good because the DC voltages are being generated properly.

Since there are 4 tubes, I think the black transformer is actually 4 transformers in one package. I have seen HDTVs, and they often have one mini-transformer per. fluorescent tube... fed by one larger main transformer.

Do you know of a source for a replacement power-board, schematic, or service-manual? It's got a good panel, so I hate to put it in land-fill.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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The yellow one is for the low voltage supply. The black and blue one is for the high voltage section.

It's one transformer, but it may have multiple secondary windings.

Hmmm 906 -- I'll have to check. I inherited one with a broken panel. Maybe I still have it. I believe the power supply was still good. I'll have to check...

It would probably cost a bomb to send it to the US though. Maybe you can search for a monitor with a broken screen?
 

Tesla

May 10, 2010
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Strange ... I found a genuine Service Manual for a Samsung 2443 which uses a very similar Power Supply board. It contains full schematics for the Logic Board. But for the Power Board ... only block diagrams.

Yes, there 4 outputs (4 sets of windings) inside that black transformer. Also, 4 Protection Blocks, feeding a central Feed-Back Block. Also a central Inverter Controller controlling/monitoring the whole thing.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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The power supplies may be just a purchased module.

I know that I've seen very different manufacturers with almost identical power supplies inside.
 

Tesla

May 10, 2010
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All the electrolytic caps look fine with no bulging or leaking, but I see a lot of people selling "cap kits" for this exact Samsung branded and made board IP-35155A.

Ok, so I un-solder and removed all the electrolytic caps, so I could test them out-of-circuit with my ESR-Cap Tester.

Cap reads very close to rated value on all.
ESR reads fairly close to chart I have. If anything, some ESR values are quite a bit lower than they should be. Can an ESR value be too low? Is low ESR bad? Some are readings in 0.06 where it should be 0.4 ... but again, this is very little ( all under 0.5).

It would not be too hard to upload a chart of values if you think it would help.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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No, low values are perfectly fine (and expected if they are Low-ESR capacitors).
 

Tesla

May 10, 2010
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So, if a cap tests good with ESR & Capacitance meter, the caps are good, right?

If so, would you still replace all the caps, and try it installed? Or would you move to the determination that since the removed caps are good ... it must be something else bad. Something harder to get, like probably that long/black 4 output transformer?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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I wouldn't suspect the caps.

ESR can be tested with the caps in circuit, and that's probably the only test I would have done on them.

By a process of elimination, I guess that means it's something else.

If the low voltage rails appear OK, then if the fault is on this board, it can really only be in the high voltage supply for the CCFLs.
 

Tesla

May 10, 2010
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I wouldn't suspect the caps.

ESR can be tested with the caps in circuit, and that's probably the only test I would have done on them.

By a process of elimination, I guess that means it's something else.

If the low voltage rails appear OK, then if the fault is on this board, it can really only be in the high voltage supply for the CCFLs.

I'll keep that in mind about ESR. but I also wanted to test capacitance. That is the part that wasn't working/reading properly in-circuit. Plus, if I was the replace them, they needed to be removed anyway... that was my thinking.

Thanks. It really helps to bounce these ideas off of someone.

On the larger HDTVs, the mini-transformers-per-tube go out. Via the Feedback circuit, they will hold-down all tubes in the TV from coming on (until the individual one is replaced or removed/jumped out). I think that is what is going on here. But with one larger combo-4-output transformer, I have nothing to compare against or remove partially.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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If you can identify the 4 sets of output windings you could compare the resistance of each. In theory they should be very similar. If any one shows a significant deviation, this might point the finger at the transformer, or the circuitry connected to that output.

This assumes there is are multiple output windings though.
 

Tesla

May 10, 2010
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If you can identify the 4 sets of output windings you could compare the resistance of each. In theory they should be very similar. If any one shows a significant deviation, this might point the finger at the transformer, or the circuitry connected to that output.

This assumes there is are multiple output windings though.

This is a different model monitor, but the Power Board (IP Board) decribed here is basically the same. The component numbers are a little different and instead of T602 and T603, mine is just one part (too bad they don't show the windings). Block diagram is the same.

http://linfotech.co.uk/schematics/Samsung/Samsung 932BF.pdf

Mine is marked SP-35155A, but there are several versions. Some with different layout but same connectors (just a different revision ... no big deal). But some have different connectors (obviously won't work ... ie, connect to logic board) :confused: but they still call them SP-35155A (and that number it etched onto boards). :(
 
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Wabajig

Apr 14, 2012
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Just a quick note. Did you notice the different color around the output diodes on the secondary? Check the connections on the diodes, again. Also what voltage rating on the 2 identical caps near yellow transformer, 25V or 35V? The reason I ask is in my experience at least with lcd tv the supply for the inverter section is 24V which would require a 35V cap. So, if those caps are 25V, then a 16V supply would be normal. And there is pin coming from the signal board that should vary when you press the power button. If not, suspect signal board. Otherwise I've ran into alot of bad transformers, too.
 

Tesla

May 10, 2010
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Just a quick note. Did you notice the different color around the output diodes on the secondary? Check the connections on the diodes, again. Also what voltage rating on the 2 identical caps near yellow transformer, 25V or 35V? The reason I ask is in my experience at least with lcd tv the supply for the inverter section is 24V which would require a 35V cap. So, if those caps are 25V, then a 16V supply would be normal. And there is pin coming from the signal board that should vary when you press the power button. If not, suspect signal board. Otherwise I've ran into alot of bad transformers, too.

That coloring is just my camera (not heat). But yes, I did check those diodes for cold solder joints.

Those caps are 820uf-25v ... but they check good. When replacing caps (especially if the bulged or leaked), I often try to go next voltage rating higher (if they will fit).

Yes, pretty sure it's the black transformer. I think one of the 4 outputs is blown.

Thanks for the post.
 
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Tesla

May 10, 2010
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I ordered a whole new Samsung Power Board (IP Board / SP-35155A) off of eBay and that fixed it. It looks new ... for $20. Time will tell if it was money well spent.

So, my sister's computer is back in action. Thanks for the help guys.
 
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