The Magnavox 42mf337b LCD repair thread - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 28 Old 05-02-11, 03:14 PM Thread Starter
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The Magnavox Philips 42" LCD 42mf337b Power Supply repair thread

This is a 42" 720P LCD, that shares parts with Philips (Has a LG sticker on the back of the display).
Was bought at home depot I was told. Manufactured in 2007.

Other model numbers:
Magnavox 42MF337B/37
Magnavox 42MF437B/37
Philips 42PFL5432D/37
Philips 42MF337B/37
Philips 42MF437B/37


Original problem was no power. Wouldn't power back on after blowing the bed room breaker.
I took out the power supply and found the TO-247 MOSFET Q901 a shorted mess. No physical damage, but it was a dead short across all leads in all directions. And the 5 amp fuse was blown.

Spent all day looking for the STW18NK80Z MOSFET. Checked with Newark, Digi-Key, Mouser and MCM (emailed
ACME but didn't hear back). Out of stock and Discontinued Obsolete. Seem like a kinda new TV to have parts discontinued. Anyways, some place called Audio Lab (770-455-0571) said they had it in stock for $12 plus shipping. But I did'nt feel comfortable giving some place I never heard of my credit card number, plus the price was about twice what it should be with the UPS charge on top of that.

So, went looking for a sub. Mouser said the SPW11N80C3 would sub, but it was only rated for 11amp 156 watts. The OEM MOSFET is rated 19amps 350watt. So thats a no-go for me.
Digi Key said the IXFH20N80Q would sub, 20amps 360 watts, sounds good, but doesn't have the build in Zener protection the ST brand has. I didn't want to figure out what Zener was needed to be add externally, so I skipped that one, plus it was $16-18.

So, I went with a STW15NK90Z same brand as the OEM peice, same internal Zener protection, same pin out, same case. This is rated at 15A instead of 19A, But 900v instead of 800V. Both are rated 350W. And both have simular RDS .38 vs .55. Was $6 from mouser.

Now I just have to hope the MOSFET driver chip isn't damaged, or the transformer.

Looks like the power supply part number is
philips part number: 996500044559.

Last edited by jeffescortlx; 05-06-11 at 03:27 PM.
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post #2 of 28 Old 05-02-11, 09:02 PM
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Re: The Magnavox 42mf337b LCD repair thread

I would have just bought the part from audiolab. They are generally ok and for a part like this it is usually best to go with the original part.




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post #3 of 28 Old 05-03-11, 08:31 AM Thread Starter
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Re: The Magnavox 42mf337b LCD repair thread

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lcaillo wrote: View Post
I would have just bought the part from audiolab. They are generally ok and for a part like this it is usually best to go with the original part.
I agree with you that it's best to stick with original parts. I felt uncomfortable making a change. But I felt more uncomfortable giving some place I never heard of my credit card number.

I think I will be OK with the change. The two parts are identical except the amperage is a little lower, but voltage a little higher. They both have the same power rating of 350w. It's a 15 amp (OEM 19) MOSFET on a circuit with a 5 amp fuse.

I think the failure is from the lack of good heat sinking. It shares a heat sink with another semiconductor, and a insulator is required. The heat sink compound looked a little dried out.
I'd like to see it on it's own heat sink, with out an insulator. With maybe some air flow.

Last edited by jeffescortlx; 05-03-11 at 09:01 AM.
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post #4 of 28 Old 05-05-11, 06:07 PM Thread Starter
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Re: The Magnavox 42mf337b LCD repair thread

Good new's. It works with just a new MOSFET and fuse.
But the power supply has a high pitch squeal.
I suspect a bad cap. Going to sillyscope them or just measure them.
I should of known, it has capXon brand caps in it. Same brand that failed in my samsung monitor. But since it's a high pitch squeal it's probably the 3 cap's off the high frequency transformer. CapXon cap's have a rep for being junk.

Found some one else with bad caps, he has a "how to" for the fix.:

Quote:
The "TV shuts itself off" or "TV won't power on" problem in the Philips 42PFL5432D/37 HDTV is due to a faulty Power Supply Board, # 715T2432-2. To be more specific, bad electrolytic capacitors on the PSB cause failure problems. These caps most likely failed due to a lack of preservatives that protect the caps integrity.
...locate the PSB (the board that the power cord plugs into) PSB #715T2432-2...Visually inspect individual capacitors (C951, C952, C953 rated at 1500uf at 35V and C955 rated at 2200uf at 16V) for bulging or leaking. Even if they show very little sign of or even no distortion they may still be faulty...
-Kaufman605
http://www.fixya.com/support/r720858...ts_off_its_own
But, I just got done inspecting mine, look good on the oscilloscope. No physical signs of trouble, I removed them anyways and they measure out with in spec, and they can hold a charge for a long time so there is no internal resistance.
So I guess my caps are good after all.

Just some bench testing trouble shooting tips. C955 is a constant 12v supply (lower power section). You can power it up with just a power cord while disconnected from the rest of the TV.

C951, C952, C953 is "switched" (only on when the TV is on) higher power, I think it's a 15v supply. You can force it on by connecting a high resistance resistor from pin 10 on connector CN902 to 12v positive pin 3 or 4.

The MOSFET that failed in my Power Supply was the one that drove largest transformer that drove the C951, C952, C953 rated at 1500uf at 35V.

Last edited by jeffescortlx; 05-06-11 at 03:14 PM.
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post #5 of 28 Old 08-19-11, 04:13 PM Thread Starter
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Re: The Magnavox 42mf337b LCD repair thread

Well 3 months later the power supply blew again. But this time it was'nt a MOSFET.
A pair of zener diodes shorted, and caused the unit to not power up. It would pulse a high pitch sound while it trying to power up.
ZD904 & ZD902 are in series with each other, on the input of the primary side of the large transformer.
They are both a LT7216 P6KE120A.
But remember me saying there was a squeal the first time I replaced the mosfet? It was coming from C944. And it's tied to the 2 blown diodes. I have a feeling the 2 problems are related, and once I replace the diodes the squeal sound will go away.

Mouser has the P6KE120A in stock at 27 cents each. I might as well replace all the capXon caps while I'm at it.

Last edited by jeffescortlx; 08-19-11 at 04:38 PM.
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post #6 of 28 Old 08-21-11, 07:55 AM
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Re: The Magnavox 42mf337b LCD repair thread

Caps are relatively cheap. My rule is when in doubt change them if you have the right replacements, or better.




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post #7 of 28 Old 08-21-11, 09:29 AM Thread Starter
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Re: The Magnavox 42mf337b LCD repair thread

Cap's are allready on order with the diodes. The cap's are like a buck each, I needed 4 to replace all the capXon ones. I think the shipping cost is higher then the price of the parts. lol

The diode is a "Transient Voltage Suppressor" And it looks like I'm not the first person to have one go bad in this model. After some google'ing I found a justanswer post on it.

It makes me wonder if I had one shorted the whole time, and that's what put a extra load on the old MOSFET that blew, and that's what caused the squel sound.
That would make sence, since there are 2 in series with each other, it would only kill half wave, so it could still power up, just have a heavy load.
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post #8 of 28 Old 08-21-11, 09:39 AM
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Re: The Magnavox 42mf337b LCD repair thread

These are zener diodes, which conduct in reverse when a certain voltage is reached. I would check the circuit to be sure that there is not current folowing through them continuously. If that is the case something else is wrong. Troubleshooting this type of power supply requires a scope if you want to really sort out what is going on, but be VERY careful. Power supplies are more dangerous than the high voltage in CRT sets.




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post #9 of 28 Old 08-21-11, 10:37 AM Thread Starter
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Re: The Magnavox 42mf337b LCD repair thread

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These are zener diodes.
Yes, but they are classified as a TVS diode (Transient Voltage Suppressors). My guess is to protect the parts that drive the transformer from the inductive kick back.

http://www.mouser.com/Search/Product...994HXaOQ%3D%3D

The diodes I pulled out are a dead short in both directions.
That and the hole in the side was a good indication some thing was'nt right.
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post #10 of 28 Old 08-21-11, 11:21 AM
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Re: The Magnavox 42mf337b LCD repair thread

Are these bi-directional? A TVS diode is usually a bidirectional zener and is designed to pass more current before shorting. If they are in a series circuit this is more likely to double the zener voltage and is intended to clip peaks in a swithing regulator.

Either way, a TVS or zener is very similar. They are both based on junctions that avalanche at some voltage. TVS are usually used in a fashion similar to MOVs but do not have the power handling capacity. They are faster, however.




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