Magnavox 42" LCD no Power

[jbpest]

Hi I'm new in here so please bear with me. I have a 42" Magnavox LCD model 42mf521d/37. The TV has no lights no anything. I replaced 4 blown caps on the power supply they were 1500uf. When that didn't work I replaced the 47uf and 3 100uf still no power. I have check the fuse and it seems fine. If anybody can see anything more I might try please all help is appreciated.

[smason]

Re: Magnavox 42" LCD no Power

I see some questionable solder joints on the transformer, and a couple others.
Have you measured any voltages?

[jbpest]

Re: Magnavox 42" LCD no Power

No I haven't checked that. I'm not sure just how to go about it. I'm pretty knew at this but I hate to just toss a $1300 dollar tv.

[PlainBill]

Re: Magnavox 42" LCD no Power

No I haven't checked that. I'm not sure just how to go about it. I'm pretty knew at this but I hate to just toss a $1300 dollar tv.

Do you have a DMM? Without one it won't be possible to do much troubleshooting. I've marked a few points to check.

First of all, you will be measuring potentially lethal voltages. Work carefully, the life you save, etc. Set your DMM to measure DC voltage, and set it to the highest range. You will be making measurements on the back of the board.

I've labeled the input rectifier as point A (upper right hand corner of the board). With the power cord plugged in, put the red probe on the point circled in red, the black probe on the point circled in black. What is the voltage between those points?

I've labeled the PFC output cap as point B (lower middle of the board). Again, what is the voltage between the points circled in red and black?

Now unplug the power cord and wait a few minutes. If you got a reading at B, be careful, that voltage will take some time to dissipate.

Referring to the picture of the top of the board, What is the part number of the IC circled in yellow? Set your ohmmeter to the lowest range and measure the resistance across the fuse circled in yellow.

What capacitors (Voltage, capacity, brand and series) did you use for replacements?

P.S. By the end of the month it will be about a $400 TV. Still, your point is valid.

PlainBill

[jbpest]

Re: Magnavox 42" LCD no Power

I set the meter at DC V 500

1. Ok at point A the reading was 0 it never moved.
2. Point B the reading was 0 it still never moved.

I unpluged it and at the fuse the ohmmeter read a little over 1. The fuse is a 250v 5amp ceramic.

The Capacitors are 1500uf 35v GXE 96e2.

Thank you for the help. I sure appreciate it!!!!

[Dgtech]

Re: Magnavox 42" LCD no Power

How about a picture of the display board? Some time ago I found a Magnavox that would not power up. Part of the power circuit was located on the display board.

[jbpest]

Re: Magnavox 42" LCD no Power

Ok here are the pictures of the display board. I know that the power switch plugs into this board.

Also the IC has TEA1507p, N6E9M7, 6E07912, HN6391. I'm not sure which one is the part number.

The ohms from the fuse to the IC is 1k.

Sorry I missed that earlier.

[PlainBill]

Re: Magnavox 42" LCD no Power

I set the meter at DC V 500

1. Ok at point A the reading was 0 it never moved.
2. Point B the reading was 0 it still never moved.

I unpluged it and at the fuse the ohmmeter read a little over 1. The fuse is a 250v 5amp ceramic.

The Capacitors are 1500uf 35v GXE 96e2.

Thank you for the help. I sure appreciate it!!!!

Recheck your resistance check of the fuse. Set your DMM to the lowest resistance range (probably 200 ohms). Short the two leads together. You should read less than 1 ohm. Now check across the fuse. It should read less than 1 ohm, very close to what you got with the leads shorted. Some ohmmeters display a 1 on the extreme left if the reading is over range.

A quick tutorial: The AC comes in through the power plug, then goes through a number of filters and to the bridge rectifier where it is converted to DC. There are a VERY limited number of reasons for a reading of 0 volts across the output of the bridge rectifier (point A). A blown fuse is one of them.

WARNING!!! If the fuse is blown, do not replace it until we determine WHY it blew.

PlainBill

[jbpest]

Re: Magnavox 42" LCD no Power

I set the DMM to the lowest resistance range and the meter reads 1 ohm when shorted and it reads 1ohm across the fuse. I don't think the fuse is the problem. I'm not sure the power is ever getting to the fuse.

[jbpest]

Re: Magnavox 42" LCD no Power

Ok, I just figured out that the DMM had a blown fuse in it. I have changed the fuse and here are the new readings again I'm sorry about that.

1. at point A. the reading is 75DC.
2. at point B. the reading is 75DC.

Agin sorry about that PlainBill didn't realize what was going on until I couldn't get a reading on anything but Ohms.

[Dgtech]

Re: Magnavox 42" LCD no Power

There also is a fuse on the processor board. I marked it on this drawing. Check this one also whenever you get a chance.

[jbpest]

Re: Magnavox 42" LCD no Power

I checked the fuse on the processor board and its fine.

[PlainBill]

Re: Magnavox 42" LCD no Power

Ok, I just figured out that the DMM had a blown fuse in it. I have changed the fuse and here are the new readings again I'm sorry about that.

1. at point A. the reading is 75DC.
2. at point B. the reading is 75DC.

Agin sorry about that PlainBill didn't realize what was going on until I couldn't get a reading on anything but Ohms.

OK, well the readings still don't make sense. The easiest explanation is that you are making another mistake. The other possibility is that we will have a lot of troubleshooting to do.

I've updated the picture. I've labeled a point Gnd in red. Measure the DC voltage from two other points I have also marked in red - DC in and PFC out. DC in should read about 165 volts, PFC out may read over 300 VDC.

If you still get 75VDC for both, post a picture of your DMM with the probes hooked up and the range switch set as they were when you measured the voltages.

If you get something close to what I indicated above, on the left side of the board I have labeled another point as Gnd, this time in black. I have also labeled two other points as DC1 out and DC2 out. Measure the voltage from DC1 out to the black Gnd point, and the voltage from DC2 out to the black Gnd point.

PlainBill

[jbpest]

Re: Magnavox 42" LCD no Power

Ok maybe I'm figuring this out. I was using an old AMM I have now bought a DMM and hopefully I'm on the same page as you guys.

Thank you for the patients.

at point A and B on the DMM the reading is 173

DC in is 173
PFC Out is 173

DC1 is 24
DC2 is 0

DC2 is at location 2074 and had a very bad cap that I replaced. Does this mean I replaced a bad cap with a bad cap?

[PlainBill]

Re: Magnavox 42" LCD no Power

Ok maybe I'm figuring this out. I was using an old AMM I have now bought a DMM and hopefully I'm on the same page as you guys.

Thank you for the patients.

at point A and B on the DMM the reading is 173

DC in is 173
PFC Out is 173

DC1 is 24
DC2 is 0

DC2 is at location 2074 and had a very bad cap that I replaced. Does this mean I replaced a bad cap with a bad cap?

More likely a bad solder joint or open diode.

PlainBill

[jbpest]

Re: Magnavox 42" LCD no Power

I'll check the solder joints to see if that fixes the problems.

[jbpest]

Re: Magnavox 42" LCD no Power

I checked the solder joints they seem to be fine but I reheated and made sure they were fine.

at the 2074 or DC2 I get a reading of 16 on the positive side of the cap to the ground.

[PlainBill]

Re: Magnavox 42" LCD no Power

I checked the solder joints they seem to be fine but I reheated and made sure they were fine.

at the 2074 or DC2 I get a reading of 16 on the positive side of the cap to the ground.

Slightly unusual voltages. Are there any indications on either the power supply or the main board what these voltages should be?

PlainBill

[jbpest]

Re: Magnavox 42" LCD no Power

No not that I can tell.

[Dgtech]

Re: Magnavox 42" LCD no Power

In a strange ironic twist of fate, I just purchased a non-working 42MF531D today. As you can see from the picture below, It's the same power board. Also you can see that I havent changed out these capxon's just yet - need to go to the parts store tomorrow to get these three 1500uF's and one 2200uF 25V caps. Then I'll be right with ya.

I noticed that there are two inverter boards in this set. I havent gotten to pulling them out but you might wanna check to see if there are any fuses or bad caps on them before you get too far. Sometimes with the startup sequence, the unit will have so many checks before actually turning on the set. If one thing is not right, the unit just will not start up. I dont have any time to get into it more tonight so it's just sitting on my couch till tomorrow.