What's making the picture on my TV green/purple?

About a year or so after buying my TV (Sanyo) the picture in the top left hand corner turned light purple. It was still watchable so I did nothing about it. In the last year the picture on occasions disappeared and the screen became varying shades of vertical purple stripes before shutting off. This started happening more frequently until eventually it turned off altogether. Just before this the freeview set top box stopped working (I get sound but no picture - it was a v cheap model so I assumed it had broken although I had only had it around 12 months). I moved a portable TV (Goodmans) in the place of the old TV and within a week half the screen is green. I am assuming this is something to do with a tube (I know nothing about TVs and am not technically minded at all) and may be caused by some form of magnetic force. Apart from the set box and DVD player I have nothing else in or around that area? What can I do to prevent this happening to a new TV?