Troubleshooting Mag LT865B

[Welchs101]

Hi,

A friend gave me his old LCD monitor to try and fix for him.

Problem: When you plug in monitor ...... nothing happens. The "LED on" light does not even come on. This LCD has as its input-power source a 12V DC power cord.

I opened it up and found several bad capacitors. Replaced those and still nothing happens when i turn it on.

NOte: I did not replace ALL the caps on the board but i replaced the ones that had a slight "bulge" in the top of the cap.

I am enclosing a picture of the board.

If anyone has ANY ideas what to try and figure this out please let me know.

[PincushionMan]

Re: Troubleshooting Mag LT865B

We need some more information:

1) What kind of caps are on your board? If they fall into the bad cap list, you'll probably want to replace them all. Also, some may fail with no visible indicators, so keep that in mind.

2) You mentioned a 12V DC power supply. Have you tested the output with a Voltmeter? It is quite possible there are some bad caps in there as well - that's what I've heard anyway. But I wouldn't crack it open until a senior member tells you that you should, or the output is not 12V DC.

Post back with that, and perhaps we'll find something that you missed.

Good luck!

[PincushionMan]

Re: Troubleshooting Mag LT865B

Last thing:

Are there any fuses near the 12V input? If your 12V DC power is good, then check for open fuses on your power board, near the input jack. Usually, they have an F designator on the silkscreen. Some look like a small black button (radial type), others look like little green or purple resistors (axial type). I'm sure there's more types than I know about.

Hmm, I don't see any in your pictures, but I'm no expert. It's possible the fuse may be in the power brick. But, please, don't crack it open on my say so, and definitely check the 12V output first.

I recommend having a multimeter for situations like this one. I've had good luck with Fluke multi-meters, but they're pricey. There may be others that are recommended, I haven't checked the EE threads.

[Welchs101]

Re: Troubleshooting Mag LT865B

Thanks for the input.

The capcitors are manufactured by "TEAPO".

I did check the output of the dc power cord and its 12.32volts......so i think thats ok.....right?

I also checked the fuses and they are still shorts.

[Welchs101]

Re: Troubleshooting Mag LT865B

I found a schematic.......supposedly for this monitor. see enclosed

But to be honest .....not sure how to use it to figure out my problem.

Note: When i plug monitor into an outlet nothing happens. The LED-ON light does not even come on. Does this information help?

[Welchs101]

Re: Troubleshooting Mag LT865B

For some reason i cant see the schematic pdf file i attached. Can anyone else see it?

[PincushionMan]

Re: Troubleshooting Mag LT865B

Nope, I don't see the PDF. Maybe it was too large to upload? Anyone know if there's a PDF size limit?

As for the 12V, that's good news. Now for the bad news: I've heard horrible things about Teapo caps, though, so you'll probably want to replace all of them, even the little beggars. Chances are (according to some other threads here) that caps below 470uF won't really show signs of failure, they'll just quietly drop their capacitance to near nothing. If there's one thing that the crapcap companies are getting better at, it's hiding their failings.

Also, what caps did you replace the Teapo caps with? We need the voltage, capacitance, maker and model numbers. Hopefully you kept the original caps (and their locations), so we can make sure you are using equivalent caps in their place. The caps are polarized, but you sound like you know what you are doing, so I won't go into that.

Oh, and FYI, general usage caps (like those from Radio Shack and similar) are generally not sufficient, and other members (PCBONEZ, et al) recommend either Panasonic FM or FC from digi-key or mouser. However, I could be completely wrong in this case, so wait to hear from one of them.

Good luck (again)!

[Welchs101]

Re: Troubleshooting Mag LT865B

I will get the list. The caps i replaced are identified in Red. I will get the names and ratings.

[Welchs101]

Re: Troubleshooting Mag LT865B

Referring to the caps labeled in the picture:
1) Nichon 470uF/16V 85c
2) Rubycon 470uf/16v 85c
3) g-Luxon 470/16v 105c
4) rubycon 10uf/50v 85c
5) asiacon 1000uf/6.3v 105c
6) Luxon 470uf/16v 105c

Its a mix of parts that i had left over from some other electronics project. Glad i made this lists as i did not realize i put caps rated at 85c in there. But they should be good enough to see if i can get the unit working. I plan on placing an order to digikey. I was planning to buy the following caps from digikey:
****************
CAP 470UF 16V LOW Z HI REL RAD
5000 Hrs @ 105°C
0.315" Dia x 0.453" H (8.00mm x 11.50mm)
Lead spacing: 0.138" (3.50mm)
-40°C ~ 105°C
****************

[Welchs101]

Re: Troubleshooting Mag LT865B

Note: The schematic file i tried to upload is only 997KB........is this too big of a file to upload?

[Welchs101]

Re: Troubleshooting Mag LT865B

Just to be clear. The caps i listed in an earlier post are the "new" caps to replace the bad-old caps. Did not want to confuse anyone.

[EGuevarae]

Re: Troubleshooting Mag LT865B

Just to be clear. The caps i listed in an earlier post are the "new" caps to replace the bad-old caps. Did not want to confuse anyone.

You need to post better pics for us to see the components.

[Bobdee]

Re: Troubleshooting Mag LT865B

If anyone has ANY ideas what to try and figure this out please let me know.

Resolder the copper toroid next to cap one even if it looks good, they are known to cause this problem

[Welchs101]

Re: Troubleshooting Mag LT865B

Here is a better picture.



also, here is the web site where i found the schematic.
http://www.eserviceinfo.com/download...iew_AY965.html

[Welchs101]

Re: Troubleshooting Mag LT865B

I will resolder the toroid..........but can you tell me what kind of problems resoldering the toroid will solve.

[PlainBill]

Re: Troubleshooting Mag LT865B

I will resolder the toroid..........but can you tell me what kind of problems resoldering the toroid will solve.

It will solve a problem of no power getting through the toroid due to bad solder joints.

Have you tried following the troubleshooting flowchart in the service manual?

PlainBill

[Welchs101]

Re: Troubleshooting Mag LT865B

I have tried some of it. I tried the first part finding U10/P141. But i get no High/Low change in voltage. I was thinking that i should replace all the caps first and then try again. But before i can replace all the caps i will need to order some from digikey as i dont have all the necessary caps.

[Welchs101]

Re: Troubleshooting Mag LT865B

ok. SOldered toroid and replaced 97% of the many many caps on this main board. Still nothing. The power on/off LED does not even light up.

I am at a loss. Not sure how to proceed from here.

Any suggestions?

[PlainBill]

Re: Troubleshooting Mag LT865B

ok. SOldered toroid and replaced 97% of the many many caps on this main board. Still nothing. The power on/off LED does not even light up.

I am at a loss. Not sure how to proceed from here.

Any suggestions?

The troubleshooting chart told you to press the on/off button while looking for high / low transitions at U10, pin 141. What was the voltage at that pin?

Can you read schematics? If so start by checking the power supply voltages on the logic card.

If you prefer a 'divide an conquer' approach, go to U7 (Page 24 of the service manual). Check pin 22 (Gnd), 44 (Vcc), and 10 (Reset, 0 Volts). If those are correct, observe pins 2-6 as you press the corresponding pins on the front panel.

PlainBill

[Welchs101]

Re: Troubleshooting Mag LT865B

hi, to measure the transistion from low to high is very difficult for me to do..........i mean i could do it but the boards are such that its hard to physically do. Having said that i did try and measure the transition and measured "no" change in voltage.

I looked on page 24 for U7...........where exactly is this on the page. Once i find U7 i will try what you suggested.

thanks for responding.