Magnavox 32MF338B/F7 has no power, blown fuse

[SurrealMustard]

This Magnavox 32MF338B 32" LCD television recently stopped working. No power light, nothing. Further inspection revealed the fuse by the mains input on the MUT board was blown. I can't see any obviously damaged components on either that or the MTS board. It's kind of tricky because the power supply stuff looks like it is split across the MUT and the MTS. Any ideas as to how to nail down where the problem lies?

[budm]

Re: Magnavox 32MF338B/F7 has no power, blown fuse

Disconnect the red/black cable between the main power supply/inverter board and the sub power supply board, then check the resistance between the Source and Drain pin of the power MOSFET mounted on the heatsink and the main rectifier diodes of the main power supply/inverter board (where the main AC fuse is) to see if it shows low resistance.
Then check the S and D pin of the sub-power supply board power MOSFET.

[CapLeaker]

Re: Magnavox 32MF338B/F7 has no power, blown fuse

Don't forget to look at the back side for SMD diodes that might be shorted at the secondary.

[SurrealMustard]

A closer look

Disconnect the red/black cable between the main power supply/inverter board and the sub power supply board, then check the resistance between the Source and Drain pin of the power MOSFET mounted on the heatsink and the main rectifier diodes of the main power supply/inverter board (where the main AC fuse is) to see if it shows low resistance.
Then check the S and D pin of the sub-power supply board power MOSFET.

I checked the four diodes at the bottom of the board, and the one at the bottom right does indeed appear to be no good, while the other three test normally. I was able to rule out the possibility of problems on the MTS board by checking with a second, working MUT board on the television.

Finally, I tried to test the MOSFET. I attempted the instructions shown in the "how to test MOSFET" videos online where you put the black lead on the right leg of the MOSFET, the red lead on the middle one, touch the red lead to the left one, and then check the reading again on the middle one, but got nothing. I got the same thing on the working board though, so maybe I'm just not doing it right (I did look up the documentation for the component to make sure the legs were in the standard GDS order).

TL;DR? Bottom right diode was bad, MTS board was confirmed good, MOSFET test was inconclusive.

Don't forget to look at the back side for SMD diodes that might be shorted at the secondary.

I didn't see any, but I'm not real familiar with what they look like. Are the components in the picture labelled "E02", "289", and so on examples of surface mount diodes?

[CapLeaker]

Re: Magnavox 32MF338B/F7 has no power, blown fuse

There are non on that pic.

[SurrealMustard]

Diode question

I guess I should start with diode replacement. Any ideas as to how to find out what kind I need? The one in question is mounted facing downward, so I'm unable to read the markings on it.

[CapLeaker]

Re: Magnavox 32MF338B/F7 has no power, blown fuse

Which diode is bad?

[CapLeaker]

Re: Magnavox 32MF338B/F7 has no power, blown fuse

Btw before you label a component bad, always take it out of circuit or lift a leg and measure again.

[SurrealMustard]

Follow up

Which diode is bad?

The one in the bottom right of the cluster of diodes toward the bottom of the board.

Btw before you label a component bad, always take it out of circuit or lift a leg and measure again.

I will have do that and check it again when I get the chance.

[budm]

Re: Magnavox 32MF338B/F7 has no power, blown fuse

"The one in the bottom right of the cluster of diodes toward the bottom of the board." IN which pictures? The two pictures in POST 1, or in post 4?

[SurrealMustard]

Components

"The one in the bottom right of the cluster of diodes toward the bottom of the board." IN which pictures? The two pictures in POST 1, or in post 4?

The first picture in post 1; I should've been more specific.

[budm]

Re: Magnavox 32MF338B/F7 has no power, blown fuse

Those 4 rectifiers by the white power resistors (fusible resistors) are the bridge rectifier for primary side of the circuit.
So the power MOSFET on the heatsink next to the resistors is OK, no low resistance between S and D for sure because that is the commonly failed mode on this two transistor cheaply made switching power supply.
There are many parts that will fail in the primary side of the circuits.
Service manuals:
http://elektrotanya.com/?q=showresul...at2=schematics

[budm]

Re: Magnavox 32MF338B/F7 has no power, blown fuse

Power supply diagrams.

[CapLeaker]

Re: Magnavox 32MF338B/F7 has no power, blown fuse

Lets say, if only that diode and the fuse blew, than you're lucky. My experience with these types of sets has been that there is much more gone than one diode and a fuse. Btw, I'd like to see a shot of the whole power supply / mainboard combo of the back side.

[SurrealMustard]

Picture

Lets say, if only that diode and the fuse blew, than you're lucky. My experience with these types of sets has been that there is much more gone than one diode and a fuse. Btw, I'd like to see a shot of the whole power supply / mainboard combo of the back side.

Hopefully we can get to the bottom of it. Here's a photo of the entire bottomside of the MUT board in the attachment.

[CapLeaker]

Re: Magnavox 32MF338B/F7 has no power, blown fuse

Make yourself a new jig, you might need it. Basically it is a light bulb in SERIES with the power cord. So if there is still something shorted on the primary, all that is going to happen is that the light bulb glows bright. I used a 100W light bulb. Remember to hook it up in series! You also could solder a couple of wires on to the bulb and jam that with the blown fuse into the fuse socket. This way you prevent any further damage and the new parts you just put in wont blow right away up again.

http://www.repairfaq.org/samnew/tvfaq/tvtslbt.htm

On the backside of the board, measure the 4 diodes on top center for shorts. Other than that, you have to start lifting stuff at the primary until the short is gone (that is the primary objective right now), then verify other components if they are still good.

[budm]

Re: Magnavox 32MF338B/F7 has no power, blown fuse

This is one of the version I made:
https://www.badcaps.net/forum/showpo...&postcount=112

[CapLeaker]

Re: Magnavox 32MF338B/F7 has no power, blown fuse

This is one of the version I made:
https://www.badcaps.net/forum/showpo...&postcount=112

Looks almost the same as mine!

[SurrealMustard]

Woah

That looks pretty advanced. I don't know if I have the spare resources or know-how to build something like that. Is there any other way to find the bad components?

[SurrealMustard]

The moral of the story

I guess I'll keep my eyes open for a power supply then. I bought the very last one available from the very last store that had it in stock a week or two ago only to find that it was broken. When I asked "Electron Resort" for a refund, they said, "fill out this form and tell us exactly what components are broken. Then, you can pay your own return shipping and give us a restocking fee". WHAT?! Why should I have to diagnose their bad item? Why should I have to pay return shipping? More importantly, why is there a "Restocking fee" for something that shouldn't be restocked or stocked at all? Thankfully, payment was made through PayPal, and as annoying as PayPal can be, they can be remarkably effective at taking care of crooks like "Electron Resort". We now have a full refund and Electron resort once again has a broken MUT board for sale to scam a few more people who aren't familiar with the power of PayPal.

TL;DR? DON'T EVER SHOP AT AN ONLINE STORE CALLED "ETVPART.com" OR "ELECTRON RESORT"!!


With that out of the way, does anyone know where I can get a new MUT board for this thing since we can't nail down exactly what components are bad?