Samsung 931BF

[pamr4]

I have one monitor Samsung 931BF with the next problem: power on 2 sec then the image is black, but monitor is still power on. I think this is a common problem with the inverter. I measure the inverter transformer TMS91429CT and the results are : secondaries 1.258 kohm and 1.254 kohm; primary 0.2 ohm. These values are ok, everythink else is ok. I replace that transformer with a new one and it's working. I am a little confused, maybe someone can help me to undestand why it doesn't work with the first one.

Thank you.

[PlainBill]

Re: Samsung 931BF

I have one monitor Samsung 931BF with the next problem: power on 2 sec then the image is black, but monitor is still power on. I think this is a common problem with the inverter. I measure the inverter transformer TMS91429CT and the results are : secondaries 1.258 kohm and 1.254 kohm; primary 0.2 ohm. These values are ok, everythink else is ok. I replace that transformer with a new one and it's working. I am a little confused, maybe someone can help me to undestand why it doesn't work with the first one.

Thank you.

A resistance test on a transformer isn't a very good test. It will detect a short across a large number of turns. If only a few turns are shorted it won't detect that. A ring tester is the best way, but the usual member doesn't have one available.

PlainBill

[pamr4]

Re: Samsung 931BF

A resistance test on a transformer isn't a very good test. It will detect a short across a large number of turns. If only a few turns are shorted it won't detect that. A ring tester is the best way, but the usual member doesn't have one available.

PlainBill

Thanks PlainBill, it seems that measure the resistence on transformer can be tricky.

[retiredcaps]

Re: Samsung 931BF

http://lcdparts.net/TransformerDetai...ProductID=3712

says resistance should be around 1.21K IF it is accurate.

[pamr4]

Re: Samsung 931BF

http://lcdparts.net/TransformerDetai...ProductID=3712

says resistance should be around 1.21K IF it is accurate.

I agree, but the new transformer measure: 1.264 and 1.269 kohm.

[retiredcaps]

Re: Samsung 931BF

I agree, but the new transformer measure: 1.264 and 1.269 kohm.

I hear what you are saying.

Based on your initial measurements back in post #1, I would have guessed that your inverter transformer was good, but as PlainBill pointed out, the resistance test does have its limitations.

If you are hobbyist or this is an "one time" fix, then it doesn't make sense economically to buy a ring tester. I personally don't have one and don't plan on getting one unless I get a really sweet deal.

[alexanna]

Re: Samsung 931BF

I agree, but the new transformer measure: 1.264 and 1.269 kohm.

This is an extreme example but it may give you an idea.
This is how I understand it, when you use a DMM to measure the secondary's resistance it applies a very small current through the circuit to make a measurement which produces very little load and very little heat.
When the inverter energizes the transformers there is a lot more current flow thru the circuit, the increased current flow generates heat which causes the windings to expand, this will cause the resistance to change or if there is a fault in the windings they could become open circuited or the voltage could find another path to ground.

Here is my analogy: you go out to your car and try to start it all you here is a loud click. If you take your DMM and measure the voltage of the battery you read 12V, but when you try to start it and it clicks you read really close to 0 volts. You also look at the negative battery cable and it looks like a science project all kinds of white stuff growing on the terminal end and the cable. You disconnect the battery cable and with your ohmmeter you read 0 ohms from the cable end to the engine block ground. Well according to the ohmmeter reading the cable is good. The correct way to test it would have been to do a voltage drop test with a load on the circuit. I have never used a transformer ring tester but I suspect it puts some kind of a load on the coil and would be a better test.

[retiredcaps]

Re: Samsung 931BF



talks about the limitations of the Blue Ring tester in the 2nd last paragraph on page 8.

The PDF also talks about how a ring tester works.

[PlainBill]

Re: Samsung 931BF

Good link, but it still doesn't explain the basic physics. Let's go to Electronics 102, transformer theory.

At it's simplest, a transformer consists of two magnetically linked coils of wire. An AC voltage flowing through one coil (the primary) will induce an equivalent voltage in the other coil (the secondary). The ratio of the number of turns in the primary to the number of turns in the secondary dictates the ratio of the primary voltage to the secondary voltage.

Now, there are a lot of factors that affect the efficiency of a transformer - the core, the frequency and wave shape of the driving voltage, any DC current, the load on the secondary, and the degree of magnetic saturation in the core.

Let's use an example. The 4006A transformer used in the Darfon V144-301 inverter. The primary has 26 turns, the secondary has 2220; a turns ratio of 1:85. The inverter operates off 24V, as a SWAG (silly wild assed guess) this results in a drive waveform of about 13V peak-peak, and gives an output voltage of about 1100 volts. That means each turn of the secondary has about .5 volts. I just checked the one I have on hand, the resistance of the secondary is about 990 ohms, each turn has a resistance of about .5 ohms.

Now let's get into some more speculation. The secondary is wound in 6 sections, so each section has 370 turns. If each layer is 37 turns, there are 10 layers in each section. Layers are wound in alternating directions. That means there are anywhere from 1 turn to 74 turns between a wire in one layer and the wire directly above (or below) it.

Let's take a look at some examples. With 1 turn shorted, the resistance of the secondary drops by .5 ohms. With 74 turns shorted, the resistance drops by 37 ohms. On a resistance test, 1 turn is undetectable, 74 turns is slightly over 3%, and is detectable.

Now look at the impact of the shorts: The short between adjacent turns is a .5 ohm resistance with .5 volts developed across it. And the current is left as an exercise to the student (This IS Electronics 102). With 74 turns shorted, there is 37 volts generated across 37 ohms - the current is the same. In each case, the output of the transformer is severely reduced. I would guess even a single turn would reduce the output by 10%, perhaps more. The result will be a dim CCFL. ALSO, because of the design, less current will flow through that CCFL, so the controller will increase the drive, eventually triggering over current protection.

PlainBill

[pamr4]

Re: Samsung 931BF

Ok PlainBill, i get the point (anyway that was a very good explanation). So, to get a real measure i have to get the ring test. There are many lcd monitor with this problem and i wonder: the transformer is poor quality or the power supply is bad?

[PlainBill]

Re: Samsung 931BF

Ok PlainBill, i get the point (anyway that was a very good explanation). So, to get a real measure i have to get the ring test. There are many lcd monitor with this problem and i wonder: the transformer is poor quality or the power supply is bad?

Let's define some terms. To me 'Bad' has two meanings. 1. It has failed and should be repaired or replaced. 2. It was poorly designed and even replacing it with a new one is not a permanent fix.

For what it's worth, many computer power supplies suffered from a bad design. A common failure would send excess voltage to the motherboard and destroy it. I am not aware of any monitor supplies that have a bad design. There are many (most?) that are built using inferior components (capacitors). I'm of the opinion that a power supply with failing caps would not damage the transformer.

This is a totally unscientific and probably biased opinion. The major cause of 'two seconds to black' are bad capacitors in the power supply, specifically those in the inverter supply section. We have seen two seconds to black when new standard (not low ESR) caps were used to replace failed caps. I don't own stock in Panasonic or Digikey, but my first recommendation is to ALWAYS replace all the caps.

Number two cause is the CCFLs and their wiring. Frayed insulation, CCFLs at the end of their life, bad solder joints, broken tubes all can cause two seconds to black.

Number three is a transformer. Sometimes you can detect this with an ohmmeter, sometimes not.

Number four is the over voltage and over current sense circuitry. Troubleshooting this is extremely difficult, even with an oscilloscope. I can only recall one case where we were successful.

PlainBill

[pamr4]

Re: Samsung 931BF

Let's define some terms. To me 'Bad' has two meanings. 1. It has failed and should be repaired or replaced. 2. It was poorly designed and even replacing it with a new one is not a permanent fix.

For what it's worth, many computer power supplies suffered from a bad design. A common failure would send excess voltage to the motherboard and destroy it. I am not aware of any monitor supplies that have a bad design. There are many (most?) that are built using inferior components (capacitors). I'm of the opinion that a power supply with failing caps would not damage the transformer.

This is a totally unscientific and probably biased opinion. The major cause of 'two seconds to black' are bad capacitors in the power supply, specifically those in the inverter supply section. We have seen two seconds to black when new standard (not low ESR) caps were used to replace failed caps. I don't own stock in Panasonic or Digikey, but my first recommendation is to ALWAYS replace all the caps.

Number two cause is the CCFLs and their wiring. Frayed insulation, CCFLs at the end of their life, bad solder joints, broken tubes all can cause two seconds to black.

Number three is a transformer. Sometimes you can detect this with an ohmmeter, sometimes not.

Number four is the over voltage and over current sense circuitry. Troubleshooting this is extremely difficult, even with an oscilloscope. I can only recall one case where we were successful.

PlainBill

Yes, the question was for bad design power supply (sorry for miss understanding). In "pro" series of TV's or monitors i hardly see bad caps, but other component like transformers, mosfets, etc fail. In "budget" series capacitors are on top of failing components.

[Duke7213]

Re: Samsung 931BF

My monitor would stay on at first for about 20 minutes then shut off. The problem gradually got worse and the monitor would no longer stay on for more than a few seconds. I thought for sure it needed new caps so I bought a cap kit. I removed the power supply and visually inspected the capacitors and they appeared to be fine. I figured one must still be bad but I decided to take took a closer look (with a magnifying glass) at the board looking for weak solder joints. Sure enough, I counted 12 joints that had the infamous solder "ring" indicating the joints that had gone bad. I used a fine tip soldering iron and touched them all up. The monitor has been working flawlessly for over a month! I should have replaced the capacitors while I had it apart but I'm good to go for now. Bad solder joints may be something to consider with your monitors.

Duke7213