Magnavox 32MF301B/F7 blowing fuses

[HittingSmoke]

My GF's parents were going to throw away this TV claiming it was fried by a power surge. I decided to bring it home and see if it was just a blown fuse.

I changed out the 125V/4A fuse and it immediately blew as soon as I plugged the power in.

Here are some pictures of the power supply. Sorry for the picture quality. The flash washed it out too much so I had to use HDR mode. My hands aren't quite steady enough for it:

http://imgur.com/a/ho0li (The first image appears currupted but if you open the full resolution version it shows up properly)

The MOSFET on that metal heat sync has k10a50d written on it. Based on other threads I found here when searching I checked that first. Resistance readings on my Fluke meter set to Auto are:

1-2: 405.9
1-3: 405.6
2-3: 1.8

Do those readings indicate a bad MOSFET or is there something else I should be checking next?

[budm]

Re: Magnavox 32MF301B/F7 blowing fuses

There should some kind of letter after the number as display on the meter, I.E. 1.8 Ohms?, 1.8 K OHMS?

[retiredcaps]

Re: Magnavox 32MF301B/F7 blowing fuses

Resistance readings on my Fluke meter set to Auto are:

1-2: 405.9
1-3: 405.6
2-3: 1.8

Do those readings indicate a bad MOSFET or is there something else I should be checking next?

2-3 is shorted it if reads 1.8 ohms. If it is 1.8K or 1.8M ohms, then it is not shorted. Units matter when reporting your readings.

[HittingSmoke]

Re: Magnavox 32MF301B/F7 blowing fuses

There should some kind of letter after the number as display on the meter, I.E. 1.8 Ohms?, 1.8 K OHMS?

2-3 is shorted it if reads 1.8 ohms. If it is 1.8K or 1.8M ohms, then it is not shorted. Units matter when reporting your readings.

Ahh, yes, sorry. It's 405.9 k and 405.6 k. The 1.8 reading is just ohms and it seems to fluctuate between 1.6 and 4ish.

So it needs to be replaced. Is this something I can pick up at Radio Shack or do I need to order a specific matching part?

[retiredcaps]

Re: Magnavox 32MF301B/F7 blowing fuses

The 1.8 reading is just ohms and it seems to fluctuate between 1.6 and 4ish.

Desolder the component and re-measure it out of circuit to verify it is shorted. Another component on the board could be causing a false reading.

[budm]

Re: Magnavox 32MF301B/F7 blowing fuses

One thing to watch out if the MOSFET shorted out, it may also damage the IC and the related components that drive the Gate of the MOSFET.

[HittingSmoke]

Re: Magnavox 32MF301B/F7 blowing fuses

Desolder the component and re-measure it out of circuit to verify it is shorted. Another component on the board could be causing a false reading.

Reading are very different after desoldering. 2-3 is 340K. 1-2 and 1-3 both read OL.

One thing to watch out if the MOSFET shorted out, it may also damage the IC and the related components that drive the Gate of the MOSFET.

I did some arbitrary measuring of resistors around the MOSFET and I think at least one of them is shorted as well. The resistor circled here right behind the MOSFET heat sync is reading 0.5 ohms.

[retiredcaps]

Re: Magnavox 32MF301B/F7 blowing fuses

Reading are very different after desoldering. 2-3 is 340K. 1-2 and 1-3 both read OL.

That mosfet is not shorted and likely good. Measure the pads 2-3 on the pcb. What is the reading?

The resistor circled here right behind the MOSFET heat sync is reading 0.5 ohms

Resistors generally fail open. What is the color code of that resistor? Look it up using

http://www.hobby-hour.com/electronic...calculator.php

[HittingSmoke]

Re: Magnavox 32MF301B/F7 blowing fuses

That mosfet is not shorted and likely good. Measure the pads 2-3 on the pcb. What is the reading?

Readings on the PCB are pretty much what they were before removing the MOSFET. 2-3 is 1.8 ohms.

Resistors generally fail open. What is the color code of that resistor? Look it up using

http://www.hobby-hour.com/electronic...calculator.php

I believe it's 0.18Ω 5% but I'm colorblind so I may not be getting the colors perfect. I dug out a better camera and got a macro shot of the resistor. I'm not certain if the first one is red or brown. I can get better shots of anything needed now as well.

[retiredcaps]

Re: Magnavox 32MF301B/F7 blowing fuses

I believe it's 0.18Ω 5% but I'm colorblind so I may not be getting the colors perfect. I dug out a better camera and got a macro shot of the resistor. I'm not certain if the first one is red or brown.

Brown or red, for this case, doesn't really matter. Your multimeter leads are likely 0.2 ohms when shorted so 0.5 ohms reading is "close enough".

Check your bridge rectifier and big filter cap to see if they are shorted. Make sure the big filter cap is discharged first.

[retiredcaps]

Re: Magnavox 32MF301B/F7 blowing fuses

PS. Post your pictures here using the manage attachment function. I, personally, for a number of reasons, will not look at pictures hosted anywhere else.

[HittingSmoke]

Re: Magnavox 32MF301B/F7 blowing fuses

Brown or red, for this case, doesn't really matter. Your multimeter leads are likely 0.2 ohms when shorted so 0.5 ohms reading is "close enough".

Check your bridge rectifier and big filter cap to see if they are shorted. Make sure the big filter cap is discharged first.

Am I correct in assuming this chip is the bridge rectifier? I found a tutorial for testing them but I'm not certain exactly where it is.



I have two large caps and if I'm using my meter correctly, it says they're both bad. According to the capacitance test outlined in my manual the caps aren't charging at all.

EDIT: Uploaded the picture in an attachment. I can't find anything on this PCB that looks like a bridge rectifier.

[mcnaryxc]

Re: Magnavox 32MF301B/F7 blowing fuses

Am I correct in assuming this chip is the bridge rectifier? I found a tutorial for testing them but I'm not certain exactly where it is.

Ya, but seeing how I see four very large diodes all close to each other, I'm thinking the power supply designers decided to use four large diodes instead of an all-in-one bridge rectifier. I'd check those for shorts. And I would check those two big capacitors for shorts also.

[budm]

Re: Magnavox 32MF301B/F7 blowing fuses

Do you have the chassis number of the TV? It will be something like 32MF301B FL9.2 (FL92 is the chassis number), I want to look up the service manual.

[HittingSmoke]

Re: Magnavox 32MF301B/F7 blowing fuses

Ya, but seeing how I see four very large diodes all close to each other, I'm thinking the power supply designers decided to use four large diodes instead of an all-in-one bridge rectifier. I'd check those for shorts. And I would check those two big capacitors for shorts also.

Well damn. It appears all four of those diodes are shorted. 0.55V going both directions on all of them.

If I'm reading the caps correctly they're shorted as well. Both are a steady 1.8 ohms. They should climb in resistance if they're good, no? I also did the capacitance test using the setting designed for it on my meter and it reports they do not charge.

Given so many failed parts it's probably safe to assume there are more? I left this out in the OP but now I feel I should note, these fuses did not fail gracefully, even the second one which was plugged into a good circuit. It failed with a bright flash of light and the inside of the glass is smoked as a result.

Is this a lost cause for a novice like me or is there more testing to be done?

[HittingSmoke]

Re: Magnavox 32MF301B/F7 blowing fuses

Do you have the chassis number of the TV? It will be something like 32MF301B FL9.2 (FL92 is the chassis number), I want to look up the service manual.

Is that the /F7 in the model number? That's the only thing I can see which resembles what you're describing.

[budm]

Re: Magnavox 32MF301B/F7 blowing fuses

No, it should be on the model number tag. But now I remember helping people troubleshooting this board before, please read this post:
https://www.badcaps.net/forum/showth...light=LC320EM2
When the MOSFET shorted out, a lot of other components will usually get damaged also.

[HittingSmoke]

Re: Magnavox 32MF301B/F7 blowing fuses

No, it should be on the model number tag. But now I remember helping people troubleshooting this board before, please read this post:
https://www.badcaps.net/forum/showth...light=LC320EM2
When the MOSFET shorted out, a lot of other components will usually get damaged also.

This is the model tag. Am I missing something?



Yikes, that thread is not promising. I read through the first page. It looks like I might be replacing a lot of stuff if I want this working.

When I have time I'll finish reading through it, do some more testing and report back. Thanks a lot of all the help, everyone.