Mag LT916s Model 900p 19" LCD dropping like flies

[thisisbrian]

Re: Mag LT916s Model 900p 19" LCD dropping like flies


Are the 2 100uf caps polarized? If so what direction do the leads face?

I'm assuming positive side leads face the positive side leads of the 470uf... Please correct me if I'm wrong.

[EGuevarae]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

Are the 2 100uf caps polarized? If so what direction do the leads face?

I'm assuming positive side leads face the positive side leads of the 470uf... Please correct me if I'm wrong.

First of all, Welcome to the forums.
Second : get rid of ALL the caps in the picture : if they look OK now, they won't be later (bad brand).
Third : The caps are polarized. The white line you see in the side is the negative (it even has the - sign, as you can see in the smaller cap in the picture - C219). This pin aligns (normally) with a mark on the silkscreen in the board that can either be a lined pattern (like the one underneath C219), a small bold line in the outer circle where the cap goes, or just a "-" sign. Sometimes the negative is not marked, but on the opposite side you'll see a (+) sign, so you know where the (-) will be.
Just make sure you put them in the right direction, or else they will explode (literally)

[thisisbrian]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

I'm talking about the 2 little ones, I just talked to my EE professor and found out the they are not capacitors, but in fact inductors. If you take a close look inside you can see the coil.

This is why I'm not an EE major, LOL.

Thanks for the welcome, BTW!

[EGuevarae]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

I'm talking about the 2 little ones, I just talked to my EE professor and found out the they are not capacitors, but in fact inductors. If you take a close look inside you can see the coil.

This is why I'm not an EE major, LOL.

Thanks for the welcome, BTW!

Anyway, get rid of those bad caps, and leave the inductors alone ...

[thisisbrian]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

Definitely! That picture wasn't mine, my capacitors are blown to smithereens.

I replaced the 6 470uf, 1 220uf and 2 100uf. What was once a person's trash they gave away for free is now my treasure.

Thanks to this forum!

[EGuevarae]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

Definitely! That picture wasn't mine, my capacitors are blown to smithereens.

I replaced the 6 470uf, 1 220uf and 2 100uf. What was once a person's trash they gave away for free is now my treasure.

Thanks to this forum!

What brand/type of caps did you use? You don't want to recap with crap brands, unless you want t do it over and over again.....
Pay special attention to the temp (stick to 105C, as these power supplies have no ventilation and can get real hot in there ...).

[thisisbrian]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

I agree! Caps are all Nichicon rated at 105C, the voltage was upped a bit to 35V for the 220 uf and 470 uf and 50V for the 100 uf because this is all I could find.

They're free at the EE stockroom at my school, just gotta dig around for the good ones. I made sure to test them if they're good too.

I'm a cheapass, so when it comes to saving money, I make sure to research.

Cheers!

[Schno009]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

Hello,

I have a Packard Bell Callisto monitor for repair. It has pretty much identical circuit boards as the Mag's in this topic. The Xunda caps were bulged and the screen made the known noises and worked for a sec wereafter it would quit with a lot of flickering. I've replaced the 470's and the 220 with new Low-ESR's. Now, the screen itself works fine, but the backlight falls away after about a second or so. The screen functions fine, you still see some *very* vague image under an angle.

What could be the problem? More rotten caps in the inverter maybe? I didn't replace them because they didn't seem bulged or leaking to me, because they didn't have the popular vents on top of them... Furthermore there were no burnmarks and the PSU is completely silent, no noise anymore. Could it be a failed CCFL? Seems unlikely to me bedcause the screen hasn't been used much...

[EGuevarae]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

Hello,

I have a Packard Bell Callisto monitor for repair. It has pretty much identical circuit boards as the Mag's in this topic. The Xunda caps were bulged and the screen made the known noises and worked for a sec wereafter it would quit with a lot of flickering. I've replaced the 470's and the 220 with new Low-ESR's. Now, the screen itself works fine, but the backlight falls away after about a second or so. The screen functions fine, you still see some *very* vague image under an angle.

What could be the problem? More rotten caps in the inverter maybe? I didn't replace them because they didn't seem bulged or leaking to me, because they didn't have the popular vents on top of them... Furthermore there were no burnmarks and the PSU is completely silent, no noise anymore. Could it be a failed CCFL? Seems unlikely to me bedcause the screen hasn't been used much...

What caps does the inverter have? They might be the problem. They can fail sometimes without visual indication (I've replaced Hermei and Teapo caps on inverters that looked good but were bad). Also, could be the CCFLs themselves.

[Schno009]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

I think they were Xunda caps.... Terrible according to this topic...

EDIT: The replacements for the larger caps are Samwha capacitors, readily availible in Holland and experts said that they are fairly good caps...

[EGuevarae]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

I think they were Xunda caps.... Terrible according to this topic...

Change them (if in the inverter) and test from there.

[mladen82]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

I don't understand how IC TL494C is used like inverter ic in this mirage electronics board, as TL494C is an SMPS PWM controller

[PlainBill]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

I don't understand how IC TL494C is used like inverter ic in this mirage electronics board, as TL494C is an SMPS PWM controller

What don't you understand? A SMPS uses a PWM controller to drive a transistor (or two) to drive a transformer at a high frequency. The output voltages are rectified, and one of them (usually +5V in a PC power supply) is fed back to the control input to maintain regulation by varying the duty cycle.

An inverter uses a PWM to drive a transistor (or two) to drive a transformer at high frequency. The output is used to drive a CCFL. The control input varies the duty cycle and thus the brightness.

PlainBill

[mladen82]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

idea is ok, but this ic is not constructed for inverter application, even this ic doesnt have any protection for overvoltage or over curent from CCFL

[Wizard]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

TL494 *does* have cut out pin but not what you think and is *documented* in the document sheet for TL494.

This is how over current protection worked in SMPS is now used same way for TL494 in a lamp inverter using sense resistors and transistor network to this pin. Either current sensing transformer (as it is in SMPS if TL494 is on cold side and need to measure current on hot side.) or a sense resistor (simply measuring voltage drop across resistor as done in other cold-side DC to DC PSUs and lamp inverters).

TL494 is produced for so long (over 20 years!) and dirt cheap but does job well but there are much better ICs out there that is more reliable and fewer count and doesn't blow the drivers as easily due to better design.

One CAN do a 558 dual timer IC as a PWM but requires more components to set up overcurrent & overvoltage trip circuit and not as sensitive enough and not so reliable due to tolerances and quality of components, layout of circuit. And bit harder to add external control to adjust PWM. PLUS big issue with 555 based ICs is extreme sensitivity to noise and thermal, real "loud" electrical noise IC generates when swinging voltages due to old 555 design.

Biggest reason is TL494 and 555 based are not equipped to handle high KHz and trigger transistors *cleanly* for reliablity especially for newer efficient inverters over 50-100 plus KHz. Newer SMPS's and inverters are trending to high KHz designs to extract more efficiency.

Cheers, Wizard

[mladen82]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

TL494 *does* have cut out pin but not what you think and is *documented* in the document sheet for TL494.

This is how over current protection worked in SMPS is now used same way for TL494 in a lamp inverter using sense resistors and transistor network to this pin. Either current sensing transformer (as it is in SMPS if TL494 is on cold side and need to measure current on hot side.) or a sense resistor (simply measuring voltage drop across resistor as done in other cold-side DC to DC PSUs and lamp inverters).

TL494 is produced for so long (over 20 years!) and dirt cheap but does job well but there are much better ICs out there that is more reliable and fewer count and doesn't blow the drivers as easily due to better design.

One CAN do a 558 dual timer IC as a PWM but requires more components to set up overcurrent & overvoltage trip circuit and not as sensitive enough and not so reliable due to tolerances and quality of components, layout of circuit. And bit harder to add external control to adjust PWM. PLUS big issue with 555 based ICs is extreme sensitivity to noise and thermal, real "loud" electrical noise IC generates when swinging voltages due to old 555 design.

Biggest reason is TL494 and 555 based are not equipped to handle high KHz and trigger transistors *cleanly* for reliablity especially for newer efficient inverters over 50-100 plus KHz. Newer SMPS's and inverters are trending to high KHz designs to extract more efficiency.

Cheers, Wizard

thanks for good explanation, seems that mirage electronics engineers are cost-effectively with big knowledge

[Deflashing]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

Definitely! That picture wasn't mine, my capacitors are blown to smithereens.

I replaced the 6 470uf, 1 220uf and 2 100uf. What was once a person's trash they gave away for free is now my treasure.

Thanks to this forum!

I am not an EE, however I did open up my Mag LT that was starting to shut down. We put a fan in the back of it to cool it down and to help it stay on longer.

I do not have the capacity, tools, time or where with all to replace the faulty unit. I was actually hoping to swap out the board. Do you know of anyone, including yourself that could do this for me, and at what cost?

Thanks

[severach]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

Search eBay for "repair your LCD"

[Nama]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

Hello, I would first off like to say you are all awesome! And thank the owners, administrators and users of Badcaps for putting this wonderful site together.

This site has helped in more ways than i could imagine. I just finished replacing the "badcaps" on this mag. everything went as planned it is now on and looks perfect. I do thought have one problem if you lot would be so kind to guide me through it. Since replacing the caps my backlight is on 24/7 even if the power is off. I am decent with a soldering iron and dont think i crossed anything i shouldn't have,,,, any suggestions besides putting on a on off switch?..

Thanks in advance, Jacob. Denver, Co.

[PlainBill]

Re: Mag LT916s Model 900p 19" LCD dropping like flies

Hello, I would first off like to say you are all awesome! And thank the owners, administrators and users of Badcaps for putting this wonderful site together.

This site has helped in more ways than i could imagine. I just finished replacing the "badcaps" on this mag. everything went as planned it is now on and looks perfect. I do thought have one problem if you lot would be so kind to guide me through it. Since replacing the caps my backlight is on 24/7 even if the power is off. I am decent with a soldering iron and dont think i crossed anything i shouldn't have,,,, any suggestions besides putting on a on off switch?..

Thanks in advance, Jacob. Denver, Co.

Trace the backlight control signal. It originates on the logic card and enters the power supply / inverter at the ten pin connector. It's labeled BLON. The most likely cause is a solder short, but a bad transistor could also do it.

PlainBill