Mag 780 LT776S Not working....cap question on inverterboard

[Welchs101]

Hi,

I have a Mag780 LT776S that is not working. I have been working on it off/on for a while now.

When you plug monitor in green led light comes on but nothing happens so i measured several of the caps on the board and found a lot of them were "bad". I borrowed a friends ESR meter to measure caps.

Because so many were "bad" i decided to replace all but 3 or 4 of them. Because the original caps were sooooo small i had to solder the caps on the back of the board. See back of board pic.

I also replaced the 150uF cap that was on the inverter board. But again i have to "lay-it on its side" because the original was so small.

I put it back together hoping that i fixed it ........well i didnt. However, this time when i powered it on an image appeared for a sec and then disappeared. So i did something because before i did not even get an image for a sec.

I measured the connector voltages with the inverted connected and disconnected and they were the same.

Connector voltages:
Pin1: 0V DC
Pin2: 0V DC
Pin3: 0V DC
Pin4: 0V DC
Pin5: 12.27V DC
Pin6(red): 12.27V DC

I started probing around the inverter board and i found something .....I think? See pic called "Inverterboard_cap_questionable_2". I drew a red box around the questionable cap.

When i measure the resistance of this cap i get ~ 1.2kohm on one setting and on the 40ohm setting (see enclosed meter pic) i get ~ 250ohms.

So i think this part is bad.......does this sound like a valide conclusion? Also, why do i get such drastically different readings using just the "ohm" and then the "40ohm" on my meter.

If you all do think the part is bad........how do i know what to replace it with?

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

ok. SO i removed the questionable cap. After i removed the cap i measured it. Surprisingly .......the cap measured in the Mega ohm range.

I measured where the cap (now removed) once was and it too was in the mega ohm range.

So i think i measured the resistance with the mainboard and inverter powered up.

Ok, so i think the cap is ok.

My question now is ..........what should i check on either the main board or the inverter board to help figure out whats going on.

Please help if you can

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

As i said in an earlier post. The only voltages i measure at the connector are 12.27V DC. Should there also be a 5V DC voltage present?

Also, if anyone has any ideas on what to do.....i would really appreciate it!!!

[EGuevarae]

Re: Mag 780 LT776S Not working....cap question on inverterboard

Should there also be a 5V DC voltage present?

Yes. At least one. The 12v is the power to the inverter. There should be a 5v line that is the BL_ON (BackLight-ON) signal, and maybe another variable one that is the BRI (brightness) signal.
Did you measure with the lamps connected and a valid VGA input?

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

I did not have the lamps connected nor did i have a valid VGA cable connected.

Let me try and connect lamps and see what i get.

But in the mean time do you have any ideas on other things to try?

[seanc]

Re: Mag 780 LT776S Not working....cap question on inverterboard

Try replacing the remaining small caps on the power input board. What values are they?
Will they definitely not fit on the correct side of the board?

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

ok. With lamps and valid vga connected i get the following:

Connector voltages:
Pin1: 0V DC
Pin2: 0V DC
Pin3: 0.141V DC
Pin4: 4.14V DC
Pin5: 12.27V DC
Pin6(red): 12.27V DC

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

seanc,

The small caps your referring to ........are these the 4 small black caps on the front side of the board? The only reason i did not replace them is because the ESR value was ok and it would have been hard to put them on the back with all the other caps.

yes the back is the only place i could put them........just did not appear to be enough room on the front with ALL the other components.

[seanc]

Re: Mag 780 LT776S Not working....cap question on inverterboard

Unless they're too tall, I reckon they'd fit fine on the top side just fine.

The reason I asked about the remaining caps, is because I had a board with an external PSU that would display a green LED but nothing else.
3 small caps in front of the power jack, one was very high ESR. I replaced all 3 just to be sure.

[sofTest]

Re: Mag 780 LT776S Not working....cap question on inverterboard

Did you test all the new caps before you put them on the board? Isn't there a second small cap on the Sampo inverter? The ones I've seen had one, always very high ESR. I've not seen this particular inverter model though. The new caps you are using, are almost all GP (except the KYs). Were the originals also GP?

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

good questions. Let me check.

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

Enclosed are two pics of the type of capacitors that were in the board originally. The black caps.......dont see a manufacturer.....parts are too small i guess to put this on there. I did take a pic of the writing that was on it.....47uf/16V. Not sure if its general purpose or not.

The blue cap is a unicon cap. again, not sure if its general purpose or not.

Regarding other questions: I guess i could replace the additional ones but when i measure the voltages across them i seem to get voltages that one might expect in the circuit 4.97 and 3.3 volts DC. But your right i should replace all just to be sure.

Did i measure all the new caps...........no. I measured some but not all.

[EGuevarae]

Re: Mag 780 LT776S Not working....cap question on inverterboard

ok. With lamps and valid vga connected i get the following:

Connector voltages:
Pin1: 0V DC
Pin2: 0V DC
Pin3: 0.141V DC
Pin4: 4.14V DC
Pin5: 12.27V DC
Pin6(red): 12.27V DC

Pin 4 is the BL_ON signal. If you remove the VGA cable it should drop in a few seconds. Pin 3 must be BRI.
If you have the VGA cable connected can you see any image in the panel (assuming the panel is connected)? The lamps turn on?
If you have image but no lamps, your inverter is shot.
As for the caps, I know they won't fit in that reduced space. I had a Proview 568 that I fixed using PCBONEZ advice but I had to take the caps from a broken screen unit or so I remember : I couldn't find replacement caps that size .....

[sofTest]

Re: Mag 780 LT776S Not working....cap question on inverterboard

What's the ESR of the 2.2uF 50V SMD-cap on the inverter?

I just found my notes on a ViewSonic VE175b, with a Sampo inverter. The 150uF I replaced with a Panasonic FM (could not find any data on the original NKCon, and it had no capacitance nor ESR when measured) and the 2.2uF SMD I replaced with with a Panasonic FC (the original had an ESR of 95!).

[PlainBill]

Re: Mag 780 LT776S Not working....cap question on inverterboard

I did not have the lamps connected nor did i have a valid VGA cable connected.

Let me try and connect lamps and see what i get.

But in the mean time do you have any ideas on other things to try?

This looks like the classic 'two seconds to black' scenario. If the control and brightness signals do not change when the monitor goes black, that confirms it. Troubleshoot accordingly.

PlainBill

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

What is troubleshoot accordingly mean for this scenario? Do you have a link?

[EGuevarae]

Re: Mag 780 LT776S Not working....cap question on inverterboard

What is troubleshoot accordingly mean for this scenario? Do you have a link?

Look around the forums. The problem is all around the place.

[severach]

Re: Mag 780 LT776S Not working....cap question on inverterboard

Video boards don't see the heat that power supply boards do so few caps go bad. What's the model number of that meter? It doesn't look like an ESR meter to me. A regular ohm meter cannot measure ESR.

Rather than arbitrarily number the caps you should have written the uF value on the board. That would have been useful the world over and no more difficult for you to match the right parts.

Non polar SMD ceramic capacitors like the one in the red box almost never go bad and are so small that an ESR meter can't effectively measure them. All you can do is ensure that the capacitance is right and many common meters have a capacitance scale that will measure that part. Not really necessary though.

I hope it's a trick of the light but a black capacitor in the top of the 3rd picture looks already blown. Too much heat from the solder gun can blow them.

Now that you have two seconds out of the back light test each inverter channel. Plug only one lamp set and power it on. If you find that one channel lights the display and the other doesn't then swap bulbs and channels until all of one or the other has been verified good. Since you're testing 2 bulbs at a time watch closely to see if one bulb pair is dimmer than the other or flickers.

[Welchs101]

Re: Mag 780 LT776S Not working....cap question on inverterboard

ok. So two of the lamps were bad. Both lamps came from the same lamp assembly. I replaced the two lamps and lcd comes on for about 2secs now but then goes off.

You can see the image on the screen after it goes blank if you shine a flashlight on the screen.

severach, your right the black cap in the pic looks blown but its not. Just an artifact of the pic. Also, your right the meter in the pic is not an esr meter. I borrowed an esr from a friend. I need to borrow it again.

After replacing the bad bulbs i tested both lamp assemblies with a bulb tester (which is nothing more than another monitor) and they seem to be good.

Since i dont have an esr meter right now i cant measure any of the caps on the inverter board or main board.

I guess this one is going in the trash (or used as parts) unless someone has
some good ideas.

Thanks for the ideas already given.

If you have any ideas please let me know. Getting very frustrated with this lcd.

[ecking767]

Re: Mag 780 LT776S Not working....cap question on inverterboard

Just test the inverter alone using an external power supply. The process of elimination. You can use a computer power supply +12vdc and +5vdc. this is not hard to do!!!