LG Flatron 1950s-sn 2 sec to black

[sceva]

I got this used, and the first time I plugged it in it worked fine. The next day it was 2 seconds to black, and every time since. The first thing I tried was plugging in some external ccfl's. They came on dim - only about 1/2 the tube lit up, then off. As I replaced caps and resoldered some areas, I got differing results, sometimes the tube would be a little brighter, sometimes dimmer, but always goes off.

I replaced the smaller cap in the inverter section, C306, 33u, 50v, Nichicon, and c206, 1000u, 16v cap in the bottom left corner. The other caps tested ok, and the ones I replaced weren't too terribly bad on the esr meter. (note: I replaced c206 with a 1000u, 25v - didn't have a 16v handy.)

On the top of the board between the transformers there is some discoloration, and on the bottom of the board, the components at u303 and u304 look strange, kind of shiny on top. I can't read any identification markings on them. I think it is possibly a mosfet?

The transformers tested ok with the ring tester, and I resoldered some connections. I don't know where to look next, and don't want to just replace u303 & 4 (assuming I can figure out what they are) if something else needs to be checked.

Thanks.

[PlainBill]

Re: LG Flatron 1950s-sn 2 sec to black

I got this used, and the first time I plugged it in it worked fine. The next day it was 2 seconds to black, and every time since. The first thing I tried was plugging in some external ccfl's. They came on dim - only about 1/2 the tube lit up, then off. As I replaced caps and resoldered some areas, I got differing results, sometimes the tube would be a little brighter, sometimes dimmer, but always goes off.

I replaced the smaller cap in the inverter section, C306, 33u, 50v, Nichicon, and c206, 1000u, 16v cap in the bottom left corner. The other caps tested ok, and the ones I replaced weren't too terribly bad on the esr meter. (note: I replaced c206 with a 1000u, 25v - didn't have a 16v handy.)

On the top of the board between the transformers there is some discoloration, and on the bottom of the board, the components at u303 and u304 look strange, kind of shiny on top. I can't read any identification markings on them. I think it is possibly a mosfet?

The transformers tested ok with the ring tester, and I resoldered some connections. I don't know where to look next, and don't want to just replace u303 & 4 (assuming I can figure out what they are) if something else needs to be checked.

Thanks.

How many CCFLs did you try to substitute, and were they of comparable length? If the original CCFLs are shorter (lower voltage) than the test CCFLs you could get a partially light tube. Do you get full brightness with the original CCFLS? If so, it's probably an issue with the sense circuit.

PlainBill

[sceva]

Re: LG Flatron 1950s-sn 2 sec to black

I plugged in two sets of working ccfls. They are the same length or smaller than the originals. When turning on the originals I don't think they come on full bright, but they are on so short it is hard to tell for sure.

I have repaired several monitors, but still a lot I don't know - can you elaborate on the "sense circuit"?

(My father in law had the same eye surgery. He didn't need glasses anymore, but had worn them for so long he felt naked without them!)

[PlainBill]

Re: LG Flatron 1950s-sn 2 sec to black

I plugged in two sets of working ccfls. They are the same length or smaller than the originals. When turning on the originals I don't think they come on full bright, but they are on so short it is hard to tell for sure.

I have repaired several monitors, but still a lot I don't know - can you elaborate on the "sense circuit"?

A common description of a CCFL inverter is 'A constant-current high voltage AC source'. A constant current source will tend to destroy itself if it doesn't have some provision for limiting the output voltage. Also, some provision must be made for the rise in current when the CCFL reaches the end of it's life, especially if there are multiple CCFLs driven by one inverter.

I've attached the schematic of a typical inverter. Note the circuit running from pin 2 of CN2, through CR1, R16, R15, R13, and to pin 13 of U2. The voltage on that line is proportional to the current through the CCFL. As the voltage on pin 13 rises above the set point, the drive to the transformer decreases, reducing the output voltage. Thus, constant current.

Next, look at the circuit from pin 1 of CN2, through C8 and C13, CR2, R14 and C16, and to pin 2 of U2. As the voltage out of T1 rises, the voltage at pin 2 of U2 rises. If it rises above a set point, the inverter shuts down and 'latches' - stays off until the control input turns it off, then on again. That protects against a voltage that goes too high.

Other designs sense the current through the CCFLs and shut down if there isn't enough current through a CCFL. Different design, same reason.

Many inverters have another circuit in parallel with the current control circuit. If the current through any CCFL rises too high, the inverter shuts down and latches. Sometimes they trigger

'Two Seconds to black' occurs when the sense circuitry detects a problem - output voltage too high, current through a single CCFL too high. Sometimes this occurs because of a broken CCFL, a broken wire to a CCFL, a CCFL that is dying, etc. And sometimes it occurs because one of the components is defective. For example, if C8 was leaky, the inverter would shut down shortly after the CCFL lit.

If you look at the back of your inverter, you may be able to see the equivalent of these components. I didn't work with that because it is very difficult to trace green circuit paths on a green circuit board.

(My father in law had the same eye surgery. He didn't need glasses anymore, but had worn them for so long he felt naked without them!)

I know how your Dad feels. I've worn glasses since I was (roughly) 8 years old. The first thing I did when I woke up was grab my glasses. Now they aren't there!!!

PlainBill

[sceva]

Re: LG Flatron 1950s-sn 2 sec to black

Thanks for the info. I knew there was a feedback/sense circuit, and that is what causes the 2 sec to black, but that was about it. Using your schematic and directions, I was able to trace the path from pin 1 of one the ccfl connectors to pin 2 of U2 (on my board, using the labels on your diagram). Some places I had to use the ohm meter to figure out the path as it runs under other components. But tracing pin 1 from the ccfl I got lost in the maze of components and paths under all the smd's.

After checking that out, and not seeing anything visibly wrong, I went back to the area where the discoloration was. I can't keep power to the area long enough to test voltage, but I checked continuity on U303 and U304. From pins 2 to 8 (actually 2 to 5-8 as they are on the same path) sometimes it would charge up to 1. (I mean it would read close to 0, then go up to 1 and stay there.) It would not discharge if I reversed the probes.

The smd's I am a little unsure of what they are, so don't know what to test. I assume the ones labeled starting with c are caps, d's are diodes, and r's are resistors. But some are labeled jr, q, and some others I forget. Also, the large caps (I think) between the inverter transformer and the ccfl connection - is there a way to test those?

Thanks.

[lucky13]

Re: LG Flatron 1950s-sn 2 sec to black

Which large cap? Do you mean C101? That one almost never fail...

J is jumper.
JR is ... "Who shot JR"? OK, maybe a jumper resistor
Q is transistor

BTW, those CCFL sets you used to test, were they from another LCD panel or are they for computer mod?

[PlainBill]

Re: LG Flatron 1950s-sn 2 sec to black

Thanks for the info. I knew there was a feedback/sense circuit, and that is what causes the 2 sec to black, but that was about it. Using your schematic and directions, I was able to trace the path from pin 1 of one the ccfl connectors to pin 2 of U2 (on my board, using the labels on your diagram). Some places I had to use the ohm meter to figure out the path as it runs under other components. But tracing pin 1 from the ccfl I got lost in the maze of components and paths under all the smd's.

After checking that out, and not seeing anything visibly wrong, I went back to the area where the discoloration was. I can't keep power to the area long enough to test voltage, but I checked continuity on U303 and U304. From pins 2 to 8 (actually 2 to 5-8 as they are on the same path) sometimes it would charge up to 1. (I mean it would read close to 0, then go up to 1 and stay there.) It would not discharge if I reversed the probes.

The smd's I am a little unsure of what they are, so don't know what to test. I assume the ones labeled starting with c are caps, d's are diodes, and r's are resistors. But some are labeled jr, q, and some others I forget. Also, the large caps (I think) between the inverter transformer and the ccfl connection - is there a way to test those?

Thanks.

Common identifier codes:

C = Cap
D = Diode
R = Resistor
J, JR, JP = Jumper
Q = Transistor
T = Transformer
L = Inductor

There is no easy way to check the blue caps, but it's usually not necessary. If they fail it will be obvious by the black color and smoke.

Here is something to check. If you look at the full-length picture of the bottom, at the right end you will see C403. That is the equivalent of C8 in the schematic above. It goes to C415 and possibly C404. One (or both) of these are the equivalent of C13 in the schematic above.

Lecture time again. C8 and C13 form a voltage divider. The voltage at the junction of those two caps is .00067 (22/33022) of the voltage at pin 1 of CN2. Since the vast majority of the voltage is already dropped across C8, no significant changes to the behavior of the inverter will occur if a short is placed across C13 EXCEPT that the overvoltage protection is disabled.

Back to your board. C403 is the equivalent of C8. Other equivalents appear to be C406, C410, and C413. I'm not sure of the other equivalents for C13, but you are smart and can figure it out.

Double check that one end of C415 is tied to ground. If it is, place a short jumper across C415, then test the inverter. There is one chance in 8 of the CCFLs staying on. If they don't, go to the next C13 equivalent and short across it, leaving the first short in place. Test the inverter again. If you short across all C13 equivalent capacitors and the CCFLs still won't stay on we know the problem is not the OVP. If the inverter stays on after shorting across a particular capacitor you have found a suspect. Now go back and remove the shorts one at a time starting with the one on C415. Test after each short is removed.

IF the inverter works only if a particular capacitor is shorted, you have identified the sense circuit that is causing the problem. Possible causes would be a bad CCFL, bad wire to the CCFL, bad connector, bad capacitor, or bad diode. The C13 equivalent is the prime suspect, however.

PlainBill

[sceva]

Re: LG Flatron 1950s-sn 2 sec to black

Which large cap? Do you mean C101? That one almost never fail...

J is jumper.
JR is ... "Who shot JR"? OK, maybe a jumper resistor
Q is transistor

BTW, those CCFL sets you used to test, were they from another LCD panel or are they for computer mod?

Sorry, I meant the caps like C401 and C403.

Who shot JR? I think it was his wife?

The ccfl's come out of another lcd. I use them for testing.

C403 is the equivalent of C8. Other equivalents appear to be C406, C410, and C413. I'm not sure of the other equivalents for C13, but you are smart and can figure it out

I see how you came up with C406/10/13, but does C402 and C403 combine to make the equivalent of C8, or does my board have C8 and C8a?

If you short across all C13 equivalent capacitors...

You gave me C404 & C415. I came up with C407 &C417, C414 & C418, and C411 & C419. One side of each of them is ground.
What is OVP? Over Voltage Protection?

I will report my findings.




Thanks.

[PlainBill]

Re: LG Flatron 1950s-sn 2 sec to black

Sorry, I meant the caps like C401 and C403.

Who shot JR? I think it was his wife?

The ccfl's come out of another lcd. I use them for testing.


I see how you came up with C406/10/13, but does C402 and C403 combine to make the equivalent of C8, or does my board have C8 and C8a?

You gave me C404 & C415. I came up with C407 &C417, C414 & C418, and C411 & C419. One side of each of them is ground.
What is OVP? Over Voltage Protection?

I will report my findings.

Thanks.

It looks like they are using two caps in parallel, probably to achieve an uncommon value. OVP is Over Voltage Protection. Sometimes OLP (Open Lamp Protection is used.

PlainBill

[sceva]

Re: LG Flatron 1950s-sn 2 sec to black

IF the inverter works only if a particular capacitor is shorted, you have identified the sense circuit that is causing the problem. Possible causes would be a bad CCFL, bad wire to the CCFL, bad connector, bad capacitor, or bad diode. The C13 equivalent is the prime suspect, however.

I shorted C415 and BAM - the ccfl's came on bright and stayed on. I couldn't believe I was that lucky, and just to make sure, I removed the jumper and put it onto C404, and the problem re-appeared. And just to really make sure, I put the jumper onto C419 and again 2 sec to black. So the problem is in the C415 circuit. I tested the connector - good, then swapped the ccfl's, and no change. My next thought would be to replace the cap I shorted over, except I have no idea how to figure out what to use to replace it. Any ideas?

One other thing, did you see something in the pics, or know something from experience that made you recommend C415 - or just lucky? Thanks.

[PlainBill]

Re: LG Flatron 1950s-sn 2 sec to black

I shorted C415 and BAM - the ccfl's came on bright and stayed on. I couldn't believe I was that lucky, and just to make sure, I removed the jumper and put it onto C404, and the problem re-appeared. And just to really make sure, I put the jumper onto C419 and again 2 sec to black. So the problem is in the C415 circuit. I tested the connector - good, then swapped the ccfl's, and no change. My next thought would be to replace the cap I shorted over, except I have no idea how to figure out what to use to replace it. Any ideas?

One other thing, did you see something in the pics, or know something from experience that made you recommend C415 - or just lucky? Thanks.

Luck!!! - we don't need no stinking luck!!! I choose C415 because that was the only circuit I could see clearly.

It looks like the two capacitors are in parallel. If that is the case there are several possibilities. C415 could be bad; C404 could be bad, or some other component may be bad. If you have a DMM with a capacitance function, check the capacitance of C415 / C404. Compare them to the other capacitance pairs. if all of them read the same, it's time to look elsewhere. The next suspect is C403 - the blue cap. Try swapping it with C406, then see if the problem moves. If it doesn't, D401 is a suspect. And so on. Generally this circuit is rather simple. All of the voltage dividers are 'or-ed' together by D401, etc. Each of them have the capability of pulling the OVP pin high enough to to shut down the inverter.

PlainBill

[sceva]

Re: LG Flatron 1950s-sn 2 sec to black

My dmm has a diode setting, but not capacitance. I went ahead and tested C415, C404, and D401, and the readings were similar to the other components on the other circuits. I then took my esr meter and tested C415, C404, and the similar parts on the the other circuits. The other 3 circuits showed 95 to 97 on each one. C415 and C404 were showing "-", which is the same as when the probes are not connected to anything.

I either don't have the right tools or skills to remove those small devices very well, so I opted to swap C402 with C406, and then C401 with C405 just in case, as I don't have any problem with thru hole soldering. But neither of those helped.

So I tried to swap C415 with C419 and see what happens. To remove a small component I use 2 irons, put a little extra solder on each end of the component, and then apply both irons at the same time and pick up the cap. I put the 'C419' cap at c415, and put a jumper where C419 was. Turned it on and 2 sec to black. So this should say that the original from C415 should be ok? Well, maybe. In my soldering, I have overheated both caps and each of them have lost the metal thing on the end that you solder to the board... It is possible I could have overheated the one from C419 while moving it to C415...

Any ideas on how to get replacements - I have no idea what they are, as they have no markings on them?

Or any tips on soldering?

btw, I put jumpers back onto C415 and C419 and it works with those in place.

[PlainBill]

Re: LG Flatron 1950s-sn 2 sec to black

My dmm has a diode setting, but not capacitance. I went ahead and tested C415, C404, and D401, and the readings were similar to the other components on the other circuits. I then took my esr meter and tested C415, C404, and the similar parts on the the other circuits. The other 3 circuits showed 95 to 97 on each one. C415 and C404 were showing "-", which is the same as when the probes are not connected to anything.

I either don't have the right tools or skills to remove those small devices very well, so I opted to swap C402 with C406, and then C401 with C405 just in case, as I don't have any problem with thru hole soldering. But neither of those helped.

So I tried to swap C415 with C419 and see what happens. To remove a small component I use 2 irons, put a little extra solder on each end of the component, and then apply both irons at the same time and pick up the cap. I put the 'C419' cap at c415, and put a jumper where C419 was. Turned it on and 2 sec to black. So this should say that the original from C415 should be ok? Well, maybe. In my soldering, I have overheated both caps and each of them have lost the metal thing on the end that you solder to the board... It is possible I could have overheated the one from C419 while moving it to C415...

Any ideas on how to get replacements - I have no idea what they are, as they have no markings on them?

Or any tips on soldering?

btw, I put jumpers back onto C415 and C419 and it works with those in place.

As far as removing SMT components, get some Chip-Quik and a roll of Solder-Wick. Chip-Quik makes removing SMT components a snap. Use the solder wick to clean up the area before soldering in a replacement part. (You can get by without the solder wick, but it's a PITA.

As far as the value of the caps you need, is there any marking on the blue cap? From that we might be able to calculate the value of the SMT caps. Another idea would be to borrow the use of a meter with capacitance measuring capability, or find a service manual for the monitor.

PlainBill

[retiredcaps]

Re: LG Flatron 1950s-sn 2 sec to black

Or any tips on soldering?

I like this youtube video on SMD soldering ...

http://www.youtube.com/watch?v=3NN7UGWYmBY

[sceva]

Re: LG Flatron 1950s-sn 2 sec to black

I like this youtube video on SMD soldering ...

http://www.youtube.com/watch?v=3NN7UGWYmBY

I saw that a while ago - great video! I even ordered some items from the guy, but at the time I hadn't had much of a need for soldering SMD's and didn't get the chip-quick. I guess now is the time!

[sceva]

Re: LG Flatron 1950s-sn 2 sec to black

I am ordering some chip-quick. I could not find the service manual. The small blue cap @ C403 says:
NPO
1DJ
6KU or maybe 6KV

The larger blue cap at C402:
E
222M
3KV.

When I get the chip-quick I will swap a couple of the small caps from another circuit and see what happens. Hopefully it is one of the small caps. I have been tempted to get one of those devices that check esr and capacitance, but like the esr70 from Anatek is a bit pricey for what I am doing. I already have a blue esr meter, any ideas of a inexpensive tester for capacitance? Thanks.

[sceva]

Re: LG Flatron 1950s-sn 2 sec to black

Chip-quick is nice, but a little hard to clean up. (it may be my cheap solder wick...)

I did come across a dmm with cap tester, only up to 20u, but seems to be enough for this. In the two locations where I have not removed a cap, 414/418 and 407/417, I get a reading of 16nf and 16.5nf respectively.
At 411/419, I have 419 removed, and the reading for 411 is 5.8nf.
At 404/415, I have 415 removed, and the reading for 404 is .3nf.

Since the pairs of caps are soldered in parallel I am not sure how to compare the readings. I think 404 is bad, and maybe 411. I am hoping someone can help, otherwise I will have to remove one of the pairs that are still working, which I loathe to do (still a beginner with smd's.)

I have a bunch of old boards I might be able to get some spares off of, but not exactly sure what I need. Thanks.

[sceva]

Re: LG Flatron 1950s-sn 2 sec to black

I moved the caps from 407/417 to 404/415 and moved the jumper to 407 and it worked. I think we can rule that one or both of the caps at 404/415 were bad.

I tested the capacitance of the two good caps off the board, and one was 2.1 nf and the other 15.0 nf. When soldered onto the board they read about 17.4 nf. This is a little different from what I got previously - 16.5 nf, and higher than 414/418 which is 16.1 nf.

I looked thru some old boards and could not find a 15 nf and a 2 nf, but did find a 17 nf. I tried it and the lights stayed on, but the display was not 100%. I don't know how to describe it exactly, but it had what looked like rolling horizontal lines going down the screen. Kind of like when you used to be able to mess with the fine tuning controls on a tv. So I guess I need to find two similar caps. I could not see any identifying marks on them. Anyone have an idea? Thanks.

[sceva]

Re: LG Flatron 1950s-sn 2 sec to black

I am hoping to place an order tomorrow, and would love to include these parts if anyone could help me in determining what to order. Thanks.

[retiredcaps]

Re: LG Flatron 1950s-sn 2 sec to black

I am hoping to place an order tomorrow, and would love to include these parts if anyone could help me in determining what to order. Thanks.

I'm going to GUESS (don't order until you get confirmation from someone like PlainBill)

15nf

http://search.digikey.com/scripts/Dk...=709-1340-1-ND

2.2nf

http://search.digikey.com/scripts/Dk...=709-1207-1-ND

minimum order is 10 for both.

Also found this

http://en.wikibooks.org/wiki/Electro...SMT_capacitors

SMD/SMT capacitors

"These are often not even labeled. If they are labeled, it is the same system as SMT resistors, but representing pF. Polarized capacitors are marked with a stripe on the positive end of the package, though some electrolytic SMT capacitors are marked on the negative end of the package. They are rectangular, and often black or tan colored. The black capacitors are easily mistaken for diodes due to the white stripe on one end."

PS. I looked at a couple of my boards and see tan colored SMD caps, but no markings at all. One thread I found stated the only way to know was to use a capacitance meter.