Need some help with a MAG LT782s LCD

[Psyfer]

Hello to all.

I have a MAG LT782s LCD monitor. I got this free from a friend's shop. This start out as a "some times it works" to a "deader than you know what".

First thing I did was research. I found this site and found that some people did a cap replacement. So, I did a cap replacement with some caps I had. The power board model is AD-1700 REV: F from Mirage Electronics dated 1,14,2005. There is also another number on the board that is 200-000-1700B VERSION: H.

Here is what I replaced.

6 x 470uF 25v with 5 x 470uF 25v and 1 x 470uF 50v (only had 5 25v and the next one up was the 50v)
2 x 10uF 50v
2 x 100uF 25v

When I replaced the six 470uF, the monitor started to work great. It worked for a day then stared the 2 seconds on then black with green LED. I can still see the screen and what is on it if you look at it in the right lighting. So I know the video is all good. I then replaced the 10uF and 100uF caps and it is still doing the 2 seconds on then black. I can post pics if needed, just ask what you need. Someone suggested that it may be bad CCFLs. I would try replacing them, but I don't have any new ones. I did try some old ones from an old cracked LCD, but it did the same thing.

Any thoughts?

[smason]

Re: Need some help with a MAG LT782s LCD

"some caps I had" doesn't really instill feelings of quality.
What brand/series did you use.
Caps in Monitor power supplies and inverter circuits need to be good quality low ESR caps.

Also, another cause of 2 seconds to black, besides bad CCFLs and wiring, is bad transformers. Measure the resistance of the transformers and compare the readings, they should be fairly close to each other. Sometimes a transformer can have a few turns shorted internally, this will cause the protection circuit to shut down. I have 2 on my "needs parts" shelf with this problem.

[sam_sam_sam]

Re: Need some help with a MAG LT782s LCD

This is on the inverter board

Check the transistors or mosfet next to the transformer to see if the are short

Check the caps near the transistors or mosfets the ones that are not electrolytic type caps I just work on a monitor that this type of cap and it was bad

[Psyfer]

Re: Need some help with a MAG LT782s LCD

Can you point to the components to test? I have links to pics of the front and back. There not that great, had to use my cell to take them since I couldn't find the digital camera. As for now, off to bed. I will check back tomorrow. Thanks for the help!!!

http://img402.*************/img402/9...0709205704.jpg
http://img843.*************/img843/6...0709210132.jpg

[PlainBill]

Re: Need some help with a MAG LT782s LCD

Can you point to the components to test? I have links to pics of the front and back. There not that great, had to use my cell to take them since I couldn't find the digital camera. As for now, off to bed. I will check back tomorrow. Thanks for the help!!!

For several reasons, the pictures should be hosted here. Attach them using 'Manage Attachments' - below the text entry area. On second thought, cell phone pictures don't really cut it. Maybe it would be better to wait until you find the digital camera.

PlainBill

[Psyfer]

Re: Need some help with a MAG LT782s LCD

Sorry for the late reply, been out of town for a couple of days installing a security system for work. Anyway, I have attached the pics here. As for the pics, I won't have the digital cam back for 2 weeks, parents have it on there vacation (I need one, lol). As I said, I took them with my cell. It has a 5 megapixel cam in it (thank you droid). I can take more if needed, just let me know. Off to bed again, but no work in the morning unless called in (I hope not).

[PlainBill]

Re: Need some help with a MAG LT782s LCD

The pictures are tantalizing - they ALMOST show enough to trace the circuits. Still, they are an improvement over most cell phone pics and a vast improvement over n pictures.

You mention having a monitor with a cracked screen available. Here's what I'm going to suggest. Position the two monitors so you can hook leads from the CCFLs in the other monitor to the connectors on this power supply / inverter. One at a time, substitute a ccfl from the other monitor for a ccfl in this monitor. As a verification, hook the CCFLs in this monitor to the inverter outputs of the other monitor. While doing this test, CCFLs must be connected to both connectors of the pair (upper or lower) being tested.

The object is to see if a pattern can be discerned - for example, if any CCFL hooked to a particular output will never light, the conclusion is something is wrong with that output. If the test monitor is stable with all CCFLs, but this monitor shows two seconds to black no matter what CCFLs are used, it's the power supply / inverter on this monitor.

OK, after doing a bunch of typing, I see you have done at least half the test.

One thing stands out - it worked for a while after replacing the caps. What brand and series of caps did you use?

PlainBill

[Psyfer]

Re: Need some help with a MAG LT782s LCD

6 x 470uF 25v with 5 x 470uF 25v and 1 x 470uF 50v (only had 5 25v and the next one up was the 50v)

That is what I replaced before I started replacing some of the inverter side components. I didn't replace the 220uF 25v because I didn't have one. As for the cracked LCD, it only has one CCFL in it (one connecter). It came from a HP laptop. I have noticed that a lot of the laptop LCD screens that I have replaced just use one CCFL instead of two.

I have another LCD here that I fixed a wile back. It got hit with a surge or something and blew the inverter to pieces. I was going to use the CCFLs out of it, but there burned too. Lucky, that LCD still works. I could fix the inverter, nor could I find one to replace it with. So I bought one of those PC case light mods for $10. It is not as bright as it should be, but its bright enough for a shop monitor.

[PlainBill]

Re: Need some help with a MAG LT782s LCD

Repeating: One thing stands out - it worked for a while after replacing the caps. What brand and series of caps did you use? Using general purpose caps has been known to result in 'two seconds to black',

PlainBill

[Psyfer]

Re: Need some help with a MAG LT782s LCD

Here are some more pics. I found a neighbor with a digital cam and took some new pics. Its only a 6mp but better than my phone.

Had to use imageshack due to the resolution being to big.

http://img29.*************/img29/953/back2mzf.jpg
http://img163.*************/img163/2733/front2v.jpg

[Psyfer]

Re: Need some help with a MAG LT782s LCD

I could swear that I posted the brands here. Anyway, here is what I used.


5 x Fuhjyyu 470uF 25v and 1 x Taicon 470uF 50v (only had 5 25v and the next one up was the 50v)

2 x Chang 10uF 50v

2 x Jaro 100uF 25v

[PlainBill]

Re: Need some help with a MAG LT782s LCD

I could swear that I posted the brands here. Anyway, here is what I used.


5 x Fuhjyyu 470uF 25v and 1 x Taicon 470uF 50v (only had 5 25v and the next one up was the 50v)

2 x Chang 10uF 50v

2 x Jaro 100uF 25v

Those are the brands, but you didn't list the series. One, Fuhjyyu is the Yugo (or worse) of the capacitor world.

PlainBill

[Psyfer]

Re: Need some help with a MAG LT782s LCD

Yes, that is the brands. There are no series that I can find except for the 470uF which doesn't mater. I'm getting ready to buy new ones but a friend has some 470uF that are JACKET brand and said he would sell them to me for $5 or $6. I have uploaded a pic of them (pic is from ebay, but they are identical). Is that a good brand? If not I have been looking on eBay for some more. I have told myself if I get to $25 or $30 in parts, I'm done trying to fix this. Not going to spend half the price of the monitor to fix it when I could spend that and some more to get a new one. Anyway, thanks for the help so far.

[Psyfer]

Re: Need some help with a MAG LT782s LCD

Never mind the JACKET brand. Just got some Panasonic FM. I will post back when I get them installed.

[Psyfer]

Re: Need some help with a MAG LT782s LCD

All right, got the new caps in and soldered. I have replaced all the following caps.


power side
5 x 470uF 25v
1 x 220uF 25v

inverter side
2 x 10uF 50v
2 x 100uF 25v

It is still doing the 2 seconds on then black screen with green LED. It will come on when changing resolution then do the same thing. Any ideas?

[PlainBill]

Re: Need some help with a MAG LT782s LCD

All right, got the new caps in and soldered. I have replaced all the following caps.


power side
5 x 470uF 25v
1 x 220uF 25v

inverter side
2 x 10uF 50v
2 x 100uF 25v

It is still doing the 2 seconds on then black screen with green LED. It will come on when changing resolution then do the same thing. Any ideas?

Dang!!!

There are two thing I ould like you to try.

The first is safe. Measure the resistance between the two points circled in black. Measure the resistance between the two points circled in yellow. These are the secondaries of the transformers. They should match within 3%.

The second gives me the heebie jeebies. Hook up the boards and connect one pair of CCFLs to the output connector. Leave the other pair disconnected. Turn on the monitor. Does the backlight come on? Turn off the monitor, disconnect the first pair of CCFLS, connect the second pair, and turn on the monitor again. Does the backlight come on?

If the backlights come on at the same brightness in each case (I understand it will be difficult to judge brightness), repeat the test, but hook up only one CCFL at a time. You are looking for one (or more) CCFL that does not light.

PlainBill

[Psyfer]

Re: Need some help with a MAG LT782s LCD

It's been a wile and I haven't forgot about this. I was on vacation. As of now, I don't have the time to work on this monitor. I should have some time next week thou and will post my findings. Thanks for the help so far, I hop I get this thing working. My HP monitor has a half inch blob of dead pixels in it due to something hitting it. It still works, but is annoying the heck out of me. It is now a shop monitor for PC repairs (system reloads mainly) and what not.

[Psyfer]

Re: Need some help with a MAG LT782s LCD

Well, I got some time to try what you said. I tested the points and they both were really close, not enough to matter. I tried testing the CCFLs and they all looked the same brightness. I went a step further and tried every CCFL on every plug, first just one then both. They all did the same thing, power on with green led then 2 seconds the screen goes black with green led. I can change the resolution and the screen with come on again for 2 seconds then goes black with green led. Is this the CCFL driver side of the board going in to a protective mode or something? If so, is there a way to disable it where it is always on?

[PlainBill]

Re: Need some help with a MAG LT782s LCD

Well, I got some time to try what you said. I tested the points and they both were really close, not enough to matter. I tried testing the CCFLs and they all looked the same brightness. I went a step further and tried every CCFL on every plug, first just one then both. They all did the same thing, power on with green led then 2 seconds the screen goes black with green led. I can change the resolution and the screen with come on again for 2 seconds then goes black with green led. Is this the CCFL driver side of the board going in to a protective mode or something? If so, is there a way to disable it where it is always on?

Yes, this is a case of protection shutting down the inverter. Disabling it is not advisable.

I grew up on a farm. Farm tractors have a power take off (PTO) that allows connecting a drive shaft that powers whatever implement (mower, corn or grain binder, hay rake, etc. The drive shaft is enclosed in a protective housing. In spite of the housing, plant stems will wind up tangled around the drive shaft, and will cause problems. Clearing that is a real pain because first you must remove the protective housing. Some farmers would leave the housing off to save time. The lucky ones who tried that would be assisted by their neighbors while they learned to do the chores one-handed. The unlucky ones wound up in a coffin.

The smart approach is to figure out WHY the protection is shutting down the inverter, and fixing it. A common problem is a CCFL that is near the end of it's life. Substituting a known good CCFL can identify the defective one.

PlainBill

[Psyfer]

Re: Need some help with a MAG LT782s LCD

Yes, this is a case of protection shutting down the inverter. Disabling it is not advisable.

I grew up on a farm. Farm tractors have a power take off (PTO) that allows connecting a drive shaft that powers whatever implement (mower, corn or grain binder, hay rake, etc. The drive shaft is enclosed in a protective housing. In spite of the housing, plant stems will wind up tangled around the drive shaft, and will cause problems. Clearing that is a real pain because first you must remove the protective housing. Some farmers would leave the housing off to save time. The lucky ones who tried that would be assisted by their neighbors while they learned to do the chores one-handed. The unlucky ones wound up in a coffin.

PlainBill

LOL. Well, that sucks. Do you think its the CCFLs? If so, where do I get some new ones, eBay?