Phillips Magnavox 42MF438B/F7 does not power up

[selldoor]

Re: Phillips Magnavox 42MF438B/F7 does not power up

Meant to say can we have a better picture of the bottom of the power supply.
Next time it is together and switched on I am thinking a bit of random looking for voltages is in order. Meter on vDC black pin on the ground screw just probe the Cold Secondary side of the board with the red probe jumper wires are easiest I suppose.
But you could check each side of various components. Do not touch the heatsinks or stray into the hot side

[budm]

Re: Phillips Magnavox 42MF438B/F7 does not power up

OK, from what I see in the picture, there are PFC/VOLTAGE booster, 12/24V PS, 3.3VSTBY PS, BACKLIGHTS INVERTER.
You have indicated that you had replaced the MOSFET and recitifier, are they in the PFC section, the IC may be damaged when MOSFET failed.
Are you getting about 400VDC on the 450VDC cap when you turn on the TV?

[mike123333]

Re: Phillips Magnavox 42MF438B/F7 does not power up

The mosfet I replaced is Q201 which is the one located in the red block you highlighted in the 12V and 24V power supply.I pulled most of the capacitors in the 12v and 24v section as well as some in the primary, they all checked outgood regarding capacitance.as to the voltage on the cap I did not check but I will after I solder the caps back on.

[budm]

Re: Phillips Magnavox 42MF438B/F7 does not power up

How about the voltage on that 450V main filter cap, the SMPS for the 12/24V may be damaged when the MOSFET shorted out, very typical failure.

[mike123333]

Re: Phillips Magnavox 42MF438B/F7 does not power up

I just tested the voltage at the main filter cap its running 388.3 VDC. What should I look for regarding damage? Ipulled the 2 double diode rectifiers and the largerectifier diodes they all tested good out of circuit. aslo the mosfet on that heat sink that tested good as well.

[budm]

Re: Phillips Magnavox 42MF438B/F7 does not power up

So the Standby signal is working because the PFC dose turn on, this same STBY signal is also used to turn on the 12/24v PS. So there should be an IC close to that MOSFET for the 12/24V PS. Can you see it? We will need good closed up pictures of the bottom side of the board.

[mike123333]

Re: Phillips Magnavox 42MF438B/F7 does not power up

I measured the voltage without the LCD hooked up only the mainboard as I needed to flip the PS over so I can measure across the Caps

[mike123333]

Re: Phillips Magnavox 42MF438B/F7 does not power up

let take a quick photo of the underside and will load it up

[mike123333]

Re: Phillips Magnavox 42MF438B/F7 does not power up

Here are the photos for the ic that is in that vicinity. As I mentioned in an earlier post that the trace was blown apart so I repaired it as seen in thephoto. Maybe the ic is shorted? IC has this ID 1207A PHM7( date stamp)

[mike123333]

Re: Phillips Magnavox 42MF438B/F7 does not power up

I measured the voltage on the mosfet...I am getting 388 VDC on the drain and gate/source is 0 VDC

[budm]

Re: Phillips Magnavox 42MF438B/F7 does not power up

If I were you, I would replace the SMPS IC, you are lucky that the IC Gate drive output did not stay in stuck-high out put mode which will blow up your new MOSFET.
Please see my marking in the attached pictures, look real close at the components in the SMPS IC section with high magnifying glass to see if you can locate any blown up components.
When power MOSFET blows up, it will usually take out the IC and the relate components connected to the Gate pin of the MOSFET.

[mike123333]

Re: Phillips Magnavox 42MF438B/F7 does not power up

Thanks for your help and everyone elses.. I was thinking the same thing of buying the part, i was actually looking it up.. I will double check everything in that area.

[mike123333]

Re: Phillips Magnavox 42MF438B/F7 does not power up

I have the part on order, reviewing the spec sheets on that IC it appears that the IC will permanently latch off in an OVP mode. I wonder if this is what exactly happened to this IC and not allowing the 12v and 24v power supply from starting up.

[mike123333]

Re: Phillips Magnavox 42MF438B/F7 does not power up

I replaced the IC today and fired it up. Still nothing no power up of TV. I probed for voltage and seen that the large cap voltage would spike up then immediately start to decay. I probed the rectifier AC voltage and basically could not get a quick reading, and then it would switch over to MV measurement. It would appear that soemthing is causing the voltage to immediatley drive down..

The relay switch does latch on and latches off after the TV power is removed and about 20 seconds later.

If anyone has any input I would appreciate it.. I was relaly hoping that the IC would be the issue.

I tested the rectifier and Mosfet I replaced but there are no signs of shorts. I am wondering whether the relay switch is indeed making a good contact and staying on, even though it is making the clicking noise.

[mike123333]

Re: Phillips Magnavox 42MF438B/F7 does not power up

After more observations it seems that the relay is double clicking meaning latching on then off.. AFter the power is removed then there is another click after 15-20 seconds.

[budm]

Re: Phillips Magnavox 42MF438B/F7 does not power up

If the voltage on the large cap was 388V last time and it did stay up but you were not getting the 12/24V power supply to run, now the 388V on the large cap will only spike up and then discharge fast, that is not the same reaction as before. I can only think that something is causing the PFC circuit to shutdown but I still expect to see 160Vdc on the cap since the standby 3.3V will need that 160Vdc from the cap to run.

[mike123333]

Re: Phillips Magnavox 42MF438B/F7 does not power up

with the Main board attached I lose the 3.3V .. without the main board I do get the 3.3V..The other thing that bothered me was that the IC chips did not have a dot or cutout to identify pin 1.. I followed the data sheet and basically went by the nomenclature on the chip as to layout.,I tried to take a picture of the installed IC but it is very difficult to get a clear micro shot.

[mike123333]

Re: Phillips Magnavox 42MF438B/F7 does not power up

which areas should I focus on for troubleshooting. Is there a possibility that the main board has issues? or just the PS board.

[budm]

Re: Phillips Magnavox 42MF438B/F7 does not power up

I would not plug it in until we verify the pin out of the IC, according to spec, with the slant side toward you the left pin will be pin one. I cannot tell from the picture to be able to see anything clearly, it is not in focus. I look at the earlier picture, I can see '8' on the bottom right, and I can see the slant on the top edge of the IC.
When you get different symptom after new parts are installed, we will need to go back and recheck the works.

[mike123333]

Re: Phillips Magnavox 42MF438B/F7 does not power up

When I was unable to locate a dot that would indicate pin 1, I basically used the data sheet and used the 1207A as shown in the illustration below as a reference or chip orientation.XXXX is the date stamp. I am not following you on the slant on the top of the IC I checked the originally IC from the above photos. There is a band that goes along one edge of the top side of the IC I am not sure whether that means anything.

8 7 6 5

1207A
XXXX

1 2 3 4