Testing a transformer

[stephkre]

I have a mag lt1919wdb and it has the problem with it turning off after about 5 seconds. I have checked the caps they are good I am going to replace them anyways. I have two transformers on the inverter side of the board. This is where I need someones help. By the inverter on the same side as all the caps are on there look to be burnt spots. I need help in testing a transformer to see if it is bad and if it is where a good place to buy one. The transformer has seven leads coming off one side and two leads coming off the other side with the capacitor in between them. If someone could tell me in detail so I can make sure transformer is checked right it would help me.

[PlainBill]

Re: Testing a transformer

There are three tests possible.

The first is quick and easy. With the transformers exposed, turn the monitor on in a darkened room. Look for any signs of arcing on the transformers.

A more tedious test is to check the resistance between each pin of the transformer according to the following pattern. (This is tedious, but will detect an open winding. It will not detect a shorted turn.) Number the pins from 1-9 in any sequence that makes you happy. Check the resistance from pin 1 to pins 2, 3, 4, 5, 6, 7, 8, 9. Now check the resistance from pins 2 to pins 3, 4, 5, 6, 7, 8, 9. Continue with pin 3 to 4, 5, 6, 7, 8, 9; followed by 4 to 5, 6, 7, 8, 9; and so on. Most of the readings will show an open circuit, a few will appear as a dead short. Compare the readings on one transformer to the reading on the other.

A ring tester, such as the Anatek Blue Ring Tester is the best way to find a transformer with shorted turns.

PlainBill

[EGuevarae]

Re: Testing a transformer

I have a mag lt1919wdb and it has the problem with it turning off after about 5 seconds. I have checked the caps they are good I am going to replace them anyways. I have two transformers on the inverter side of the board. This is where I need someones help. By the inverter on the same side as all the caps are on there look to be burnt spots. I need help in testing a transformer to see if it is bad and if it is where a good place to buy one. The transformer has seven leads coming off one side and two leads coming off the other side with the capacitor in between them. If someone could tell me in detail so I can make sure transformer is checked right it would help me.

Why do you think the T's are the problem? Just for the burn spots? Why don't you post a picture of the area so one of our seniors can see and suggest accordingly?

[stephkre]

Re: Testing a transformer

Ok. I'm sorry I was going to post pictures but I was not where the monitor was when I posted this. Here is the deal I have a burnt spot as shown in the photos. Also, I forgot to mention it looks as if the screen is not as bright as my other monitor. I am hooking my the broken monitor to my laptop and using them together and I can see that the broken monitor is darker even when it has its brightness all the way up. Therefore my other question is could the tubes be a problem. The hole screen is darker not just in one area.

[PlainBill]

Re: Testing a transformer

Ok. I'm sorry I was going to post pictures but I was not where the monitor was when I posted this. Here is the deal I have a burnt spot as shown in the photos. Also, I forgot to mention it looks as if the screen is not as bright as my other monitor. I am hooking my the broken monitor to my laptop and using them together and I can see that the broken monitor is darker even when it has its brightness all the way up. Therefore my other question is could the tubes be a problem. The hole screen is darker not just in one area.

Take a look at the bottom of the board at that point. Two will get you one that there is a power FET (probably in an 8 pin smd package) on the bottom of the board. It is overheating because the caps for the inverter are bad - notice the bulging tops.

What is the capacitance, voltage, and brands of the caps on this board?

PlainBill

[EGuevarae]

Re: Testing a transformer

Take a look at the bottom of the board at that point. Two will get you one that there is a power FET (probably in an 8 pin smd package) on the bottom of the board. It is overheating because the caps for the inverter are bad - notice the bulging tops.

What is the capacitance, voltage, and brands of the caps on this board?

PlainBill

I agree with PlainBill. The first step is to get good caps in there. That's why pictures are requested. Post the cap's specs and in no time replacements would be suggested.

[stephkre]

Re: Testing a transformer

Ok I know what yall are talking about. The reason I ruled that out is because I took the caps off the board and did the resistance test on them and they climbed to infinite resistance or 0L on my meter. I said I was going to replace them anyways I am just waiting on parts. Ill let yall know in a week. Would that cause a dimmer screen. The caps are xunda 220uF 25V 105degs which you see in the pictures. 4 680uF 25V suscon, 1 more xunda 220uF 25V 105degs, 2 470uF 25V xunda 105degs. and a 10uF 25V 105deg. If yall want to give me anymore info on monitors or boards or testing in general I would love it. I am studing electrical and computer engineering and this is my side hobby I just picked up. Thank Yall.

[EGuevarae]

Re: Testing a transformer

Ok I know what yall are talking about. The reason I ruled that out is because I took the caps off the board and did the resistance test on them and they climbed to infinite resistance or 0L on my meter. I said I was going to replace them anyways I am just waiting on parts. Ill let yall know in a week. Would that cause a dimmer screen. The caps are xunda 220uF 25V 105degs which you see in the pictures. 4 680uF 25V suscon, 1 more xunda 220uF 25V 105degs, 2 470uF 25V xunda 105degs. and a 10uF 25V 105deg. If yall want to give me anymore info on monitors or boards or testing in general I would love it. I am studing electrical and computer engineering and this is my side hobby I just picked up. Thank Yall.

That's not a reliable test. Besides, if they are bulging, they are bad, and bad caps can lead to weird things : no power at all, failing inverters, overheating, shorting, etc.

Your dim screen might be the lamps, might be the caps, the FETs, the transformers, an act of God, bad luck, etc. - you just have to start from the basic (it sounds obvious, but sometimes, we don't do it).
Bulged caps? change them.
Then recheck those FETs, looking the datasheet and seeing S-G-D readings. Varying voltage input to FETs from failing PSU (from caps) can behave erratically.

[stephkre]

Re: Testing a transformer

Ok I have a question how much would it affect things if I put a cap with 85deg rating compared to what was in there a 105deg for the time being till my new ones come in. Also how do I test a transistor I have no info on it all it says on the fet is 3055 and 6j is there any documentation you can point me to. Where would be a good and not to expensive place to get parts that has low shipping. thank you

[PlainBill]

Re: Testing a transformer

Ok I have a question how much would it affect things if I put a cap with 85deg rating compared to what was in there a 105deg for the time being till my new ones come in. Also how do I test a transistor I have no info on it all it says on the fet is 3055 and 6j is there any documentation you can point me to. Where would be a good and not to expensive place to get parts that has low shipping. thank you

Quick and simple - don't do it. Most of the caps on this board are low ESR caps. Using standard caps will probably work for the short term, but in a critical application it's a bad idea to take the chance.

The 'sticky' threads at the top of the forum have lists of known good and known bad cap manufacturers. Guess which list these caps are on. My recommendation in cases like this is to order replacements for all caps (even those which look perfect). I prefer to use Panasonic FC series from Digi-Key. The caps should coat about $10 (total), Digi-Key has an option of shipping the order by First Class mail which would give delivery in 2-3 days for about $2.00.

Even though the second monitor is working well, you might want to order enough caps to repair both monitors.

PlainBill

[EGuevarae]

Re: Testing a transformer

Even though the second monitor is working well, you might want to order enough caps to repair both monitors.

PlainBill

I agree.
Preemptive action.
And practice.
And savings (more caps, less cost per unit & shipping).

And as of teh 85c caps, it is a bad idea. Not taking into account ESR, if you see how much "space" and "air circulation" those caps have, besides being close to FETs and transistors and other sources of heat, it is not a good idea. They will work (maybe, depending on the difference between the original and the replacement caps specs) but will die a quick, hot and painful dead

[stephkre]

Re: Testing a transformer

How am I suppose to know the esr on my caps. I dont have a meter and I couldnt really find it on the internet.

[PlainBill]

Re: Testing a transformer

How am I suppose to know the esr on my caps. I dont have a meter and I couldnt really find it on the internet.

With the exception of the large (100 µf, 450V) cap and possibly one other, all should be low ESR types.

Again, refer to the 'stickies' (or post side views of a couple of the caps). They will tell you how to decode the numbers on the caps. Basically, you have to locate the series identifier (usually two or three letters, probably preceding the temperature rating), and check the manufacturer's web site.

Note that many of the caps with 'exotic' names (Luxon, Chhsi, CapXon) probably didn't meet specs when the left the factory and they didn't improve by cooking inside a hot monitor for a few years.

If you are determined to repair monitors, motherboards, or power supplies professionally, an ESR meter is a must. If you are doing it as a hobby, do what I did at first - replace 'em all. Certain brands I replace even though the ESR meter swears they are good.

PlainBill

[EGuevarae]

Re: Testing a transformer

With the exception of the large (100 µf, 450V) cap and possibly one other, all should be low ESR types.

Again, refer to the 'stickies' (or post side views of a couple of the caps). They will tell you how to decode the numbers on the caps. Basically, you have to locate the series identifier (usually two or three letters, probably preceding the temperature rating), and check the manufacturer's web site.

Note that many of the caps with 'exotic' names (Luxon, Chhsi, CapXon) probably didn't meet specs when the left the factory and they didn't improve by cooking inside a hot monitor for a few years.

If you are determined to repair monitors, motherboards, or power supplies professionally, an ESR meter is a must. If you are doing it as a hobby, do what I did at first - replace 'em all. Certain brands I replace even though the ESR meter swears they are good.

PlainBill

See it this way :
If bulged -> Change it
If known Bad brand/series -> change it (even if it looks and IS OK)

The best way would be to get an ESR meter, test them and change the bad ones. But if this is either a hobby, just an attempt to repair your own unit, or you'll be doing it non-professionally, just replace them all. We are not talking of hundreds here (not even dozens in most cases...).
If you post the brand, capacitance, voltage rating and if possible series, replacement parts would be suggested.

[stephkre]

Re: Testing a transformer

Ok so I got my caps in today and I replaced them all on the board with the same kind but panasonics. However, the problem still persist. Now the monitor seems to stay on for less time. I was wondering how to test those transistors on the other side of the board. Do I have to take it of the board and what do I do. The transistors are sitting right below the burnt marks. I was also wondering if it doesnt cost much I might as well replace them but I dont know what size or anything all it says on the transistor is 550E 6J the board says 200-000-170DTLBMH and PI190DTLBMH. I still would like to test the old ones though.
Thank You.

[Bobdee]

Re: Testing a transformer

Ok so I got my caps in today and I replaced them all on the board with the same kind but panasonics. However, the problem still persist. Now the monitor seems to stay on for less time. I was wondering how to test those transistors on the other side of the board. Do I have to take it of the board and what do I do. The transistors are sitting right below the burnt marks. I was also wondering if it doesnt cost much I might as well replace them but I dont know what size or anything all it says on the transistor is 550E 6J the board says 200-000-170DTLBMH and PI190DTLBMH. I still would like to test the old ones though.
Thank You.

cant find a datasheet for those transistors
suggest that you set your meter on diode check (2K) and measure between pins, 1+2 1+3 2+3 then reverse probes on both transistors,
the readings off the first transistor should be the same as the second transistor or very close,
if you find that they are the same, then both transistors are working,
if diffrent then the bad one will show a short (probably)
the reason being,
they are mirrored parts, so one or both must be working for you to have the 2sec image
bob

can you post pics of both sides of the full board this will help

[stephkre]

Re: Testing a transformer

Ill post pics of the transistors tomorrow I am currently not by the monitor right now. I was wondering do I have to take the transistors off the board when I am testing them. Something else I rememober is that the middle leg is not attached to the board.
Thank you for the info. If anyone else can help with identifing the transistors I would be happy.
Thank you.

[EGuevarae]

Re: Testing a transformer

Something else I rememober is that the middle leg is not attached to the board.

Middle leg is the same as the back of the transistor. It is connected, only in a different way than the other two.

[Bobdee]

Re: Testing a transformer

Ill post pics of the transistors tomorrow I am currently not by the monitor right now. I was wondering do I have to take the transistors off the board when I am testing them. Something else I rememober is that the middle leg is not attached to the board.
Thank you for the info. If anyone else can help with identifing the transistors I would be happy.
Thank you.

No need to remove transistors from board to test (they may be mosfets but test same way as I described)
The middle leg cut off makes no difference, just use stump (the backplate can be measured as middle leg as they have continuity but best use stump)
The first board number that you gave (200-000-170DTLBMH) is the one to search with, you will find that this PSU/Inverter board is used in several monitors and LCD TVs when you Google (that can be great help sometimes when searching for info)
Havent had any luck yet finding datasheet, will keep looking
Get the pics up of board, both sides of full board and close ups of Inverter transistors so that we can see if any Logos
bob

[stephkre]

Re: Testing a transformer

Here is the pictures of the other side of the board where the transistors are.