Shrinking pic on projection tv

[jellero]

hi folks, first post. i have a 52" magnavox projection tv i bought at sears about 7 years ago. a couple years ago it started shrinking/distorting the picture down to the lower right 1/4 of the screen. if i turned it off and on, it would be o.k. for as much as a year. now that doesn't work. i moved it out on the porch and it works every time when temp is below 50 or so and does not work when it is warmer anymore. i guess it is a bad connection but am hoping someone could give some idea where, like the soldered connections on bottom? how long does set need to be unplugged to be safe? thanks. j

[kschang]

You have an overheating problem, which means one of the transisters on the board is about the die. If you want to keep it working, I'd suggest getting one of those non-touching thermometers (it's like a gun and you point it at a spot and you get the temp) and see which section is the heat coming from, and try to point a small fan into the area.

[edDV]

Next you could move to Alaska.

Hopefully it isn't the main projector tube.

[redwudz]

how long does set need to be unplugged to be safe?

If you've never worked around high voltage circuits, not a good place to learn about electronics.

If it uses a CRT tube (Very high voltage) it still needs to be discharged to be safe to work around. And if it's sat long enough to discharge the HV, you won't likely be able to tell which part is failing from heat unless you see obvious burned parts. .

But if it's a low voltage setup, like in a PC, the only place any dangerous voltages are would be inside the power supply. Still, not a good idea to be poking around in a live chassis. You can still melt the end of a screwdriver with low voltage, high current power supplies.

A IR temp reader as mentioned is one method. What many techs use to diagnose components that fail when heating up is a can of component cooler. You spray it on suspect parts and see if it cures the problem temporarily. But it can cause damage by cooling parts too fast, so you have to be careful.

All in all, I would let a trained tech do the repair.

[jellero]

overheating at 60 degrees? it is messes up when i turn it on at 60.
so it will always have current to zap me without discharging? my main concern. hate that. thanks. j

[redwudz]

I'm mostly concerned with high voltages, if they exist in the set. If you open it, just don't put any metal objects in there, like a screwdriver, don't touch any exposed connections with your bare hands. Good rubber gloves are cheap insurance.

Generally if you let it set overnight unplugged, most voltages will dissipate. In a TV set, the picture tube has a high voltage plug on the side of the tube, leading back to the high voltage supply. That is the one to avoid. I don't know about a projection TV, depends on what does the projection. If it's a CRT type, it has high voltage.

It's unlikely that a bad solder joint would cause this. What I would look for would be a discolored component, probably near the high voltage supply. Most parts have lettering or a color code on them. If one appears burned, might be the problem. While you have it open, blow out any dust, etc. That may help with cooling.

Just be careful.