Samsung Syncmaster 940BX backlight problem - Need help please.

[hotdig]

Hello all and first off, thank everyone for the wealth of information within this forum. Your help to people like me is very much appreciated. After reading this forum daily for the past few weeks I decided to make the plunge into repairing my Samsung Syncmaster 940BX.

monitor status...With monitor on, no vga cable hooked to computer, have blue power light. This does not change at all.

The problem...."Digital" box appears on screen for 1 or 2 seconds, disappears and then the "no cable attached" box appears and starts to move up screen. Then the backlights switch off.

From power on until backlights switch off is 18 seconds.

What I have done so far
- I replaced 5 caps on power/inverter board with multiple Panasonic FM and one FC caps from Digikey. None of the original caps had any bad signs but replaced anyhow.
- resoldered any joint I thought looked even slightly needing it. Used mag glass to check joints.

There was no change in problem after install new caps.

My measurements....120V input
Big Cap= 172.3V DC across legs

Power connector out to logic board pins 1-9
1- BDIM
2- 14.45V
3- GRD
4- GRD
5- GRD
6- 5.18V
7- 5.18V
8- ADIM
9- ON/OFF

Cap specs 1 thru 5 ( numbered in pic)
1- 330uf 16v 105
2- 1000uf 16v 105
3- org was 68uf 25v 105, replaced with 68uf 35v 105
4- 100uf 450v did not replace
5+6- 1000uf 25v 105

I have double checked all my caps for correct polarity on board.

Attached are front and rear of power/inverter board and monitor sticker info.

I have read many, many of the threads but I am now stumped and need some help from all of you knowledgable guys. Up until now I was hopeful it only needed caps but it must be something more.

I need someone to point my in the right direction or step me through it.

[hotdig]

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

Pic of the back of inverter board attached.

[magistertc]

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

hotdig,
See if you can get the ID off the chip on the bottom - 16 lead surface mount. And if you feel skillful enough, try to read the voltages, particularly on pins 6, 7 and 8. I had a Samsung with a bad resistor on the overvoltage ref pin and it took me most of a day to figure it out. Worth pursuing before you do anything drastic.

[Bobdee]

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

Hello Hotdig
Lots of info there, but you havent told us what happens when VGA is connected to comp
I ask because a lot of Samsungs have video board problems
It will help any members that help you

[hotdig]

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

Hey all....I isolated part of the problem..see attached pics
Lamps are good...used substitutes One lamp output is bad..does not light lamp at all

Magistertc
That is a Fan7314.....data sheet attached
I checked the following on that chip
pin 17 = 15.69V Voltage In
pin 6 = 2.49V... ref voltage
pin 5 = grd

Any other tests?

Bobdee
I was afraid you would ask about that...LOL
I have a vga cord on the way...don't have one in hand now.

Thanks for the help guys.

[hotdig]

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

Another observation about this situation.

The digital/analog comes on for about 2 secs then goes blank but I can still see it with flashlight. It then cycles to the no input signal box for 2 seconds then blank. The box is still visable with flashlight.

That's all the info I have at this point.

Thanks again for you comments on this.

[PlainBill]

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

Another observation about this situation.

The digital/analog comes on for about 2 secs then goes blank but I can still see it with flashlight. It then cycles to the no input signal box for 2 seconds then blank. The box is still visable with flashlight.

That's all the info I have at this point.

Thanks again for you comments on this.

This is a classic example of the 'two seconds to black' problem. Yes, it takes 18 seconds, it still the same problem. The flashlight test is definitive.

There are some discrepancies in your description. It appears this uses four CCFLs, ganged up in two connectors. In the picture of the top you seem to be indicating that one set is not working. Is this correct, or is only one CCFL of four not working?

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

[hotdig]

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

Hey Plainbill....thanks for responding. I was hoping you would help me out. I've read a heck of a lot of your posts and responses. I'm very impressed with your knowledge along with many others in here.

Onward

To your answer above....The one set is not working. I plugged the same known good lamp into each individual socket. Each socket on the good set would fire the lamp breifly. Neither of the sockets on the one I have marked bad would fire the lamp.

Measurements asked for.......
A2B= 1.207K ohms
A2C= open...infinity
A2D= open...infinity
C2D= 1.141K ohms

I am going into town tomorrow to get some soldering supplies so I can clean up some of those joints.

Thank you

[PlainBill]

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

Hey Plainbill....thanks for responding. I was hoping you would help me out. I've read a heck of a lot of your posts and responses. I'm very impressed with your knowledge along with many others in here.

Onward

To your answer above....The one set is not working. I plugged the same known good lamp into each individual socket. Each socket on the good set would fire the lamp breifly. Neither of the sockets on the one I have marked bad would fire the lamp.

Measurements asked for.......
A2B= 1.207K ohms
A2C= open...infinity
A2D= open...infinity
C2D= 1.141K ohms

I am going into town tomorrow to get some soldering supplies so I can clean up some of those joints.

Thank you

That confirms it. There are two independent secondaries A-B and C-D with a 6% difference in resistance. That doesn't sound good. Someone with more experience may chime in on the variation between windings, but to me that sounds like an indication something is wrong. I did a search on the transformer part number - TMS91429CT - there seems to be some indications that that transformer fails, causing this problem. Many of the sites aren't in English. One site is Fixya; someone seems to have them at 5 Euros each.

PlainBill

[magistertc]

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

Hotdig,

Good of you to grab the datasheet - it is not the IC that I expected, however.

Please check the voltage on OLP pin 1. Note that it should start somewhere around 2.2v (per datasheet) and it if drops to ~1.5v the chip shuts down the inverter. That might give you a clue as to where to look next. Keep us posted
M

[hotdig]

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

Hey Magistertc

I measured it and it's very strange. As I have said before, the lamps come on very briefly upon power on.......then.....off....then after about 15 or 16 secs...they cycle on again very briefly. Then nothing.

What's strange is...when the lamps first cycle on, I measure 1.55V then it shifts to 3.65V upon turning off. When they cycle on again, it drops back to 1.55V then goes to 3.65V again upon shutdown of lamps.

I'm stumped. Anything else you can point me to? Anyone else have any suggestions?

Thanks for all the help

[kc8adu]

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

one driver for the single primary.one secondary works.the other is bad.the resistance readings point here too.you need a transformer.

[hotdig]

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

PlainBill or Kc8adu

I have pulled the transformer and remeasured from A2B and C2D again

My measures are:
A2B= 1.275
C2D= 1.310

Can you confirm if you still think the trans is bad?
Any other measurements I can do on the trans now that it is out or maybe check some points in the circuit while the trans is out?

Thank you

[PlainBill]

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

PlainBill or Kc8adu

I have pulled the transformer and remeasured from A2B and C2D again

My measures are:
A2B= 1.275
C2D= 1.310

Can you confirm if you still think the trans is bad?
Any other measurements I can do on the trans now that it is out or maybe check some points in the circuit while the trans is out?

Thank you

Something is strange. I would have expected the resistance to be the same in and out of circuit. Check the resistance between the solder pads (on the circuit board).

PlainBill

[afo]

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

grrr...just when there seems to be some light at the end of the thread, the responses stop!! hotdig if you're subscribed to this then I hope you can get back to me/us with your findings - did you end up changing the transformer, if so did it do the trick?

I have pulled two boards each from a Samsung 932MW, with the same problem, same transformer.

I've just taken readings with the following results:
A-B = 1.175
C-D = 1.178

Seeing as they are near identical does this mean it's not the trans at fault?

[PlainBill]

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

grrr...just when there seems to be some light at the end of the thread, the responses stop!! hotdig if you're subscribed to this then I hope you can get back to me/us with your findings - did you end up changing the transformer, if so did it do the trick?

I have pulled two boards each from a Samsung 932MW, with the same problem, same transformer.

I've just taken readings with the following results:
A-B = 1.175
C-D = 1.178

Seeing as they are near identical does this mean it's not the trans at fault?

That is PROBABLY correct. You would also have to measure the primaries, and do a ring test to be absolutely sure.

PlainBill

[afo]

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

thanks for the reply PlainBill - would you mind elaborating on your advice, should I measure across the same pins on power-up? also if it's not too long-winded to explain, how would I carry out a ring test?

I have inspected the board closely and can not see any bad points. Caps are fine. I had re-flowed the points of the transformer (on both boards) before coming across this thread, to no avail.

I was hoping I was probably NOT correct, because I've been in touch with the chap mentioned earlier about supplying the TMS91429CT and he does them at 5euros each if bought in a tray of 6, or 10euros each if single...might as well get the tray! If it turns out not to be the trans then I'm at a dead end with both TVs.

Thanks again

[PlainBill]

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

thanks for the reply PlainBill - would you mind elaborating on your advice, should I measure across the same pins on power-up? also if it's not too long-winded to explain, how would I carry out a ring test?

I have inspected the board closely and can not see any bad points. Caps are fine. I had re-flowed the points of the transformer (on both boards) before coming across this thread, to no avail.

I was hoping I was probably NOT correct, because I've been in touch with the chap mentioned earlier about supplying the TMS91429CT and he does them at 5euros each if bought in a tray of 6, or 10euros each if single...might as well get the tray! If it turns out not to be the trans then I'm at a dead end with both TVs.

Thanks again

Take a look at the picture (originally from hotdig) I have labeled. I'm not sure of the actual winding layout, so you will have to make a number of readings with the transformer out of the board. It's a simple progression:

Measure from E to F, E to G, E to H, E to I, E to J

Measure from F to G, F to H, F to I, F to J

Measure from G to H, G to I, G to J

Measure from H to I, H to J

Measure from I to J

Many of these readings may be greater than 1 Megohm - open. Others may be only a few ohms.

A ring test requires either a ring tester, or an oscilloscope. If a sensible pattern appears for the primary windings I would suggest we work on investigating other possible causes.

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

[afo]

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

PlainBill,

Thank you for the detailed help - my apologies for seemingly disappearing for the last month, I'm not one of those 'post then do something else and never reply' kinda guys, honest just had a busy month and not been able to look at the boards. I now have 4 of these (and 4x 932mw tv's!) but have also sourced the transformers and am expecting 6 of them any day now, got them at a vastly reduced rate compared to ebay. I will carry out the test you have advised before doing anything else.

On one of the four boards I have removed, there is visible burning to what I think is either a thermal fuse or diode (?!) These boards are slightly different to the OP's but it is to the right of the large capacitor labelled "4" on the image in post#1. I intend to pull one from another board so I can read off the numbers and get a replacement. Figure the transformer on this one might be good so will try a swap.

I will post pics later and also let you know the results from your advice.

Thanks again!

[afo]

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