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

[PlainBill]

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

ok. So do you think i should replace the A7 diodes? Or continue searching for other issues on this board?

When i made the measurement in the previous post i was thinking that the low resistance equaled the resistors next to the transistors.....

I wasn't thinking so much about replacing them as swapping them around. If the side with the lower return voltage moves, we know we have identified the faulty component.

I may have gotten ahead of myself. Please establish some information. There are two transformers, each driving two CCFLs. There are two return lines from the CCFLs; one shows a higher voltage than the other. There are two diode / resistor circuits. One of them shows a slightly lower resistance than the other. Is the one with the lower resistance also the one that has a lower return voltage?

PlainBill

[Welchs101]

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

Are these the resistances your referring too:

A7 Diode1:
Pin1 (red probe) to Pin3 (black probe):0.971kohm
Pin1 (black probe) to Pin3 (red probe):0.984kohm

A7 Diode2:
Pin1 (red probe) to Pin3 (black probe):0.991kohm
Pin1 (black probe) to Pin3 (red probe):0.991kohm


if so then referr to the figure.

The diode => A7 Diode1 is on the same side of the board as Vreturn-top which had a voltage of 2.15V.

The diode => A7 Diode2 is on the same side of the board as Vreturn-bottom which had a voltage of 3.01V.


also recall that the turn on voltages for diodes 1 and 2:

A7-Diode1:
Pin1-Pin3 diode voltage:0.45V
Pin3-Pin2 diode voltage:0.59V


A7-Diode2:
Pin1-Pin3 diode voltage:0.52V
Pin3-Pin2 diode voltage:0.59V


Summary:

Diode1:
Pin1-Pin3 diode voltage:0.45V
Pin1 (red probe) to Pin3 (black probe):0.971kohm
A7 Diode1 is on the same side of the board as Vreturn-top which had a voltage of 2.15V


Diode2:
Pin1-Pin3 diode voltage:0.52V
Pin1 (red probe) to Pin3 (black probe):0.991kohm
A7 Diode2 is on the same side of the board as Vreturn-bottom which had a voltage of 3.01V

[PlainBill]

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

Are these the resistances your referring too:

A7 Diode1:
Pin1 (red probe) to Pin3 (black probe):0.971kohm
Pin1 (black probe) to Pin3 (red probe):0.984kohm

A7 Diode2:
Pin1 (red probe) to Pin3 (black probe):0.991kohm
Pin1 (black probe) to Pin3 (red probe):0.991kohm


if so then referr to the figure.

The diode => A7 Diode1 is on the same side of the board as Vreturn-top which had a voltage of 2.15V.

The diode => A7 Diode2 is on the same side of the board as Vreturn-bottom which had a voltage of 3.01V.


also recall that the turn on voltages for diodes 1 and 2:

A7-Diode1:
Pin1-Pin3 diode voltage:0.45V
Pin3-Pin2 diode voltage:0.59V


A7-Diode2:
Pin1-Pin3 diode voltage:0.52V
Pin3-Pin2 diode voltage:0.59V


Summary:

Diode1:
Pin1-Pin3 diode voltage:0.45V
Pin1 (red probe) to Pin3 (black probe):0.971kohm
A7 Diode1 is on the same side of the board as Vreturn-top which had a voltage of 2.15V


Diode2:
Pin1-Pin3 diode voltage:0.52V
Pin1 (red probe) to Pin3 (black probe):0.991kohm
A7 Diode2 is on the same side of the board as Vreturn-bottom which had a voltage of 3.01V

OK, that matches what I was thinking. I suspect Diode 1 is allowing current to flow to ground when it is reverse biased. That means the sense voltage doesn't go high enough, and the inverter controller thinks the lamp isn't drawing enough current.

PlainBill

[Welchs101]

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

So, i guess i need to replace diode1.

[PlainBill]

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

So, i guess i need to replace diode1.

If you have them on hand, do so. In many cases we don't, which is why I suggest swapping from one side to the other.

If you order the parts and a week later discover it doesn't make any difference, I'll feel guilty. If you swap the two parts and nothing changes I'll sooth my conscience by telling it you needed the practice.

PlainBill

[Welchs101]

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

BILL! YOU ARE THE MAN!!!!!!!!!!!!!!!!

I replaced A7 ( I happen to have one.....that does not happen often) and the monitor TURNED ON!

I then looked at the screen and noticed that the right side of the screen was strange. Its like the rt-most part of the screen is just not displaying.

Do you have any ideas what might have caused this?


I learned alot on this one. I will have some questions for you but first if anyone knows why the screen on the rt side is not showing let me know.

[ecking767]

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

YOU Have a broken flex cable.

[Welchs101]

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

is this something that can be fixed?

[kc8adu]

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

look at the ribbon above the black section.see if it is unplugged.or worse torn loose.some are bonded and others have sockets.

[Welchs101]

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

Yep! Its not fixable. Cant seem to put the ribbon cable back into the lcd screen.

But i learned a lot trying to fix this thing.



Bill, i did have a few questions. How did you know the diode may be bad? I did not think the resistances were that different.......just trying to learn.

[PlainBill]

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

Yep! Its not fixable. Cant seem to put the ribbon cable back into the lcd screen.

But i learned a lot trying to fix this thing.



Bill, i did have a few questions. How did you know the diode may be bad? I did not think the resistances were that different.......just trying to learn.

Well, first of all, I didn't KNOW, I suspected. Without going through a lot of detail, the 'good' diode and it's resistors gave very consistent readings. The resistance across the diode was identical in both directions, the voltage drop across the diodes were identical. On the bad side the voltage drop across the diodes were different, and the resistance across the diode was different in each direction. A DMM's resistance function is designed using a voltage so low it won't cause a diode to conduct. The fact the resistance was different in each direction indicated a problem in that area.

PlainBill

[Welchs101]

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

Thanks.

Question:
When you measure the resistance of a "diode" dont you get different readings in different directions?

Because of this i thought the good diode was actually the bad one. Your instincts were "spot on". Just trying to pick your brain......hope it does not hurt.

[PlainBill]

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

Thanks.

Question:
When you measure the resistance of a "diode" dont you get different readings in different directions?

Because of this i thought the good diode was actually the bad one. Your instincts were "spot on". Just trying to pick your brain......hope it does not hurt.

A lot will vary with the meter. With a VOM you will notice a big difference in reading because they use a high enough voltage to cause the diode to conduct in the forward direction.

A DMM usually uses a very low voltage for the resistance tests and will not cause the diode to conduct, and the leakage current will be very low. The same applies for a resistance check in the reverse direction. Again, the current will be so low that the meter says it is 'out of range'.

The diode test was another clue. The A7 diode is actually two diodes in series, with a terminal connected the the junction between the two. I would expect them to have the same forward voltage drop. Instead the two diodes in the 'bad' diode had slightly different forward voltage drops.

PlainBill