Magnavox 42MF438B/27 - either shuts off completely or loses picture but keeps sound

[Joshster]

Magnavox 42MF438B/27, Chassis LC8.1U LA

The TV will come on and I can watch for anywhere from 5-30 minutes before either powering off completely, or losing the picture only (sound will still play). I would then unplug it and it would do the same thing after restarting. Usually, after the second time unplugging it and restarting, it would stay on. Leaving it unplugged overnight seemed to allow it to stay on longer.

I replaced all 22 cylinder shaped capacitors (although none showed obvious signs of failure) on the power supply board, but the problem remains. It actually seems to shut off or lose picture faster than before. When the picture disappeared but I still had sound, I was able to see the menu items when I held a flashlight up to the screen.

Just to be clear: The TV does both randomly and continually - either shuts off completely OR loses picture but keeps sound.

I am not at all experienced in electronics but willing to learn and try to fix this TV if possible and not too expensive.

Any guidance is much appreciated.

[Joshster]

Re: Magnavox 42MF438B/27 - either shuts off completely or loses picture but keeps sou

Forgot to add pics to original post.

[Dgtech]

Re: Magnavox 42MF438B/27 - either shuts off completely or loses picture but keeps sou

Inverter boards??

Checked any regulators on the main??

[budm]

Re: Magnavox 42MF438B/27 - either shuts off completely or loses picture but keeps sou

Since this is power supply/inverter combo board, there should be a pin labeled with INV_ON, or BL_ON on one of the, see if it changes state when the backlights go out.

[Joshster]

Re: Magnavox 42MF438B/27 - either shuts off completely or loses picture but keeps sou

Ok thank you for the replies. I will try to test the inverter boards today. Is this something I can do with a basic $50 multimeter? Like I said, I'm very inexperienced with electronics...don't want to break anything or get fried. Thanks.

[Joshster]

Re: Magnavox 42MF438B/27 - either shuts off completely or loses picture but keeps sou

I found the pin labeled INV_on and measured with my multimeter. It measured the same whether the screen was lit or not (3.244 V). On the SSB, I noticed two green LED lights. They both stay on even after the backlights go out. Not sure if that is relevant, just thought I would mention it.

I took measurements of all the pins just in case (power was on but the backlights went out):
CN6
3V3 ST = 3.313 V
standby = 15.6 mV
GND = -1.7 mV
GND = -1.7 mV
GND = -1.7 mV
12 V ssb = 12.04 V
12 V ssb = 12.04 V
12 V ssb = 12.04 V
+12V aud = 13.4 V
GND_aud = 1.0 mV
-12V aud = -13.2 V

CN7
12V ssb = 12.04 V
12V ssb = 12.04 V
GND = -1.2 mV
GND = -1.2 mV
INV_on = 3.244 V
DIM = 1.802 V
boost = 1.488 V
GND = -1.4 mV

Awaiting further instruction, thank you.

[budm]

Re: Magnavox 42MF438B/27 - either shuts off completely or loses picture but keeps sou

The INV_ON/OFF should change state when the TV is toggle on and off, it sounds like the main board is the problem.

[Joshster]

Re: Magnavox 42MF438B/27 - either shuts off completely or loses picture but keeps sou

budm,

The INV_on measures 3.244 as long as the power light is on. The voltage doesn't change regardless of the screen's backlights being on or off, as long as the TV is getting power. When I press the power button to turn the TV completely off, it drops to 0 V.

Is the main board something I can fix or will it have to be replaced?

Thank you!

[Joshster]

Re: Magnavox 42MF438B/27 - either shuts off completely or loses picture but keeps sou

Suggestions, anyone? Kids really miss their TV...

[Joshster]

Re: Magnavox 42MF438B/27 - either shuts off completely or loses picture but keeps sou

FIXED! I went back and touched up the solders on a handful of the replaced caps. TV has been good for 2 weeks now.

[Joshster]

Re: Magnavox 42MF438B/27 - either shuts off completely or loses picture but keeps sou

UPDATE
OK, so after nearly a year working well, this TV has developed a problem similar to the original. It doesn't power off completely, but the picture goes black while the sound stays on. This is usually accompanied by a burning smell and the back of the TV feels hot. Thinking it may be low quality capacitors used in the first repair, I pulled the back off the TV and noticed no obviously bad caps. However, the Power Supply Board has an obviously burned spot on the large component just beneath the CN2 and CN3 connectors.

I have no idea what this piece is. Can this be fixed, and if so, can I just simply replace this piece?

[martino]

Re: Magnavox 42MF438B/27 - either shuts off completely or loses picture but keeps sou

looks to me that solder joint is really bad....Might just have to resolder that joint,but make sure you use lots of solder,I would almost take a thicker gauge wire and lay it on that trace,so you can solder that wire to that joint but also a bit away from the burned spot...

[Joshster]

Re: Magnavox 42MF438B/27 - either shuts off completely or loses picture but keeps sou

Thanks Martino. So you think the solder joint was loose and that's what caused it to burn? No need to replace whatever that piece is?

[budm]

Re: Magnavox 42MF438B/27 - either shuts off completely or loses picture but keeps sou

"The INV_ON/OFF should change state when the TV is toggle on and off, it sounds like the main board is the problem."
"The INV_on measures 3.244 as long as the power light is on. The voltage doesn't change regardless of the screen's backlights being on or off, as long as the TV is getting power. When I press the power button to turn the TV completely off, it drops to 0 V." That is normal and glad you found the problem, that is one reason it is important we we get the correct info when the TV is ON and when the TV is OFF.
BTW, that bad joint on the primary side of the inverter transformer had lot spark due to about 400V switching DC is applied and enough to cause black marking on the board, I would remove the old solder and clean that section real well then use good flux and solder on the joints, might as well do the same on the other pins as well, and inspect the whole board.

[Joshster]

Re: Magnavox 42MF438B/27 - either shuts off completely or loses picture but keeps sou

Thanks all. Followed your advice and seems to be working fine again.