Magnavox 42MF437B/37 Blowing Fuses after "Professional" Repair

[briankb]

Re: Magnavox 42MF437B/37 Blowing Fuses after "Professional" Repair

I removed the bridge rectifier and performed the test as shown in the video. My results were the same as the video so I assume this bridge rectifier is good.

What should I check next?

[Mr Bill]

Re: Magnavox 42MF437B/37 Blowing Fuses after "Professional" Repair

Are there any other mosfets on the board (Q)? You might just need to replace the W26xxx and the fuse and that'll be it. If you order the mosfet get the few remaining caps the "pro" didn't do before. All but the big 450v one. From your picture in post #1 i can see at least 1 capxon and 2 lelons.

[briankb]

Re: Magnavox 42MF437B/37 Blowing Fuses after "Professional" Repair

There is a small one without a heat sink (I think) at Q908 (right of the 450v cap). I flipped the board over and tested what I think are pins 2-3 and I get a reading in both directions.

Isn't the 450v cap a lelon? What's the reason for not replacing it as well? Just curious so I can learn more about these things.

[Mr Bill]

Re: Magnavox 42MF437B/37 Blowing Fuses after "Professional" Repair

Q908 you'll test like you did the W26xxx mosfet. Meter in the ohm setting, 1-2, 2-3, 1-3. Anything under 30 ohms is considered shorted. I think your trying to test it like a diode. The large cap doesn't endure the stress the little one do and they rarely fail (but can).

[briankb]

Re: Magnavox 42MF437B/37 Blowing Fuses after "Professional" Repair

You were right I did test it like a diode, I did it again and it failed.

I will double check these tomorrow and check what other caps I need for a order.

Thanks for your considerable help!!!

Were all of these tests just from experience or is there a method to tracing these things down? Or do you just test all of these diode,mosfets, ect... until you find something fail?

[briankb]

Re: Magnavox 42MF437B/37 Blowing Fuses after "Professional" Repair

thanks to this thread and wikipedia I understand what a bridge rectifier does and how it works. very cool. two men enter one man leaves :-)

http://en.wikipedia.org/wiki/Diode_bridge

[retiredcaps]

Re: Magnavox 42MF437B/37 Blowing Fuses after "Professional" Repair

Were all of these tests just from experience or is there a method to tracing these things down? Or do you just test all of these diode,mosfets, ect... until you find something fail?

When a fuse blows, there are certain main suspects that need to be checked. Namely, the bridge rectifier, the power mosfets and diodes. If any are shorted, they will cause the fuse to blow.

[budm]

Re: Magnavox 42MF437B/37 Blowing Fuses after "Professional" Repair

Schematics, it should match your 47".
Do you have the chassis number as printed on the tag on the back of the TV?

[briankb]

Re: Magnavox 42MF437B/37 Blowing Fuses after "Professional" Repair

Thanks for that schematic! It looks just like my board. I replaced Q908 and the mosfet just above R927 in the photo. The fuse did not pop as far as I could tell, but I still need to pull the fuse and test it. It's a slow blow white ceramic fuse and I'm not sure if those show visual signs if blown.

[tom66]

Re: Magnavox 42MF437B/37 Blowing Fuses after "Professional" Repair

Usually ceramic fuses don't burst or show any signs of failure (they are much safer than glass fuses which can burst), they just test open. However, if it did blow, there would usually be an audible pop and bright flash from the fuse.

[briankb]

Re: Magnavox 42MF437B/37 Blowing Fuses after "Professional" Repair

Thanks for that bit of info. Because it didn't blow on the test table I installed it back into the TV. I will pull it out again and continue testing in a few weeks.

[Anton johnson]

Re: Magnavox 42MF437B/37 Blowing Fuses after "Professional" Repair

I have e same tv and problem but the repair guy said that I needed a new power supply board. My question is that I have a dark shadow in the background, like the backlight is blown in some places, what could be the cause of it

[tom66]

Re: Magnavox 42MF437B/37 Blowing Fuses after "Professional" Repair

That is MURA due to LCD panel fault. Not repairable, and nothing to do with the power supply.
e.x. http://cdn.avsforum.com/f/f6/f6b2a69...ach152841.jpeg

Only applies on TVs using cheap ChiMei and Chungwha panels, which a lot of manufacturers use to cut costs.