Magnavox 42" LCD 42MF521D/37 Dead - no lights

[mrcheap]

Hi everybody. I have a Magnavox 42MF521D/37 that will not power up. There are no lights at all. I was hoping to find capacitors that were visibly bad, but no such luck.

I have tried some of the testing mentioned in this thread:

http://www.badcaps.net/forum/showthread.php?t=11962

Specifically, I tested the voltages recommended by PlainBill in Post #4. The input rectifier measures 165V and the PFC output cap also measures 165V. I also ran across a thread that suggested testing the diodes, so I have done that as well. The diode at position 6062 passed current in both directions while soldered to the board, so I removed it and retested it. Out of the board it tested OK.

I'm not sure where to go from here.

[tom66]

Have you got any output voltages from the power supply, at all?

[mrcheap]

I get 24V on the red/black cables headed toward the inverters. I'm not sure what terminals to test on the cable headed to the main board.

[Dgtech]

I fixed one of these in the past. Here were my notes.

Replaced 4 capacitors - 3 X 1500 1 X 2200

You'll have to get the voltages of these caps from your board. Other than that, I'd start there and see what you got after those are changed out. Just because caps dont show visably bad does not mean that they are good. This will only send you to frustration. Recap first and go from there. After all, it's only 4 caps.

[kevinm34232]

I had one of these too.. symptoms pointed towards the PS board but it turned out to be a short on the main board 12vs line.

There should be a fuse on the main board towards the left side if I remember, check for voltage there..

[mrcheap]

Thanks for the suggestion of new caps, dgtech. I'll try to source some locally but will order them if needed.

Kevinm34232 - I think I found the fuse you are referring to on the main board (thanks to an old thread where dgtech had posted a picture of it). I get no voltage on either side of it.

I attached a picture where I tested the voltage. It is in the upper left corner.

[mrcheap]

New capacitors have been ordered. I couldn't locate them locally.

[budm]

By any chance that you also have the chassis number of this TV so I can look up the service manual for it?
Also, that voltage on the large DC filter cap should go up to around 380V when you hit the power switch, that will indicate if the PFC voltage booster is working or not or if it is not getting the PS_ON from the main board.

[kevinm34232]

Are you getting the 16v to the main board? If I remember you can unplug that harness from the main board to check voltages. My 16v was being pulled down by the main board.

[mrcheap]

budm - I'm not sure about the chassis number. Attached are some pics of the tags I see (one from the back of the plastic housing, one is from inside). Let me know if there is somewhere else I should check.

Also, when I press the power button the voltage on the big cap goes up to around 400V then drops back to 165V after several seconds.

[mrcheap]

kevinm34232 -

Yes, I'm getting 16V to the main board. Pins 1 and 2 read 16V, pin 3 reads 24V. I checked for voltage at the fuse as you suggested but am not getting any (if I'm checking in the right place).

In post #6 I posted a picture with labels on it where I tested the voltage. I'm not 100% sure I tested the right component.

[kevinm34232]

Someone just posted this thread:
http://badcaps.net/forum/showthread.php?t=25165

Uses similar main board. I just remembered my IC7003 was burnt too, I replaced it and it burnt out again, there was a short somewhere further down the main board. Check voltages there let me find that pin layout..

OK check that IC pin 8 see if you have the 16v there, and then check pin 1.
Tom was helping me on this one before, maybe it's the same problem.

[mrcheap]

I get 16V on pin 8 but nothing on pin 1.

[kevinm34232]

You may want to refer back to my original thread then:
http://badcaps.net/forum/showthread....=42MF521D%2F37

I did replace IC7003 and it burnt up again in fact you could even see the burn by pin 8. Check for short to ground on pin 1..

I didn't get any further than this but someone may be able to help you isolate the short..

[mrcheap]

I get 30 ohms from pin 1 to ground. Should I interpret that as a short?

[kevinm34232]

I think after the output of the 12VS at IC7003 it goes to the fuse, so I would think you could remove the fuse and see if you get the 12V on pin1. But that IC is probably fried now anyways, you'd have to replace it first.

I can't seem to find the service manual I had for it Ill keep looking..

[mrcheap]

I connected an external 12V power source to pin 1 and am now able to get the power LED to come on. There are also a couple of little LEDs on the mainboard that lit up. I also get 12V at that fuse now.

TV still won't power up, hopefully because the rest of IC7003 doesn't work. I'm going to replace it and hope for the best. I'll let you know how it goes.

Thanks a bunch for all of the help on this.

[kevinm34232]

That's some good progress on this one.. I hope it works. Make sure you come back with your results. Did you find the part #?

Also what did you use for an external 12v??

[mrcheap]

I got the part number from the diagram you posted - L5973D013TR. Ordered this:

http://www.digikey.com/product-detai...66-1-ND/806442

I used an old ATX PC power supply for the 12V with a fuse wired inline.