I have a Sharp 66GF-64H TV which, when switched on, has sound but the picture is blue with horizontal zig-zag lines on it. I sensed something was going to go because before it turned completely blue it did it for a few seconds when I turned the TV on, before the picture went normal. I want to know if it's a serious problem (e.g. will cost too much to fix and I'm better off getting a new TV) or if it's something simple that can be fixed!

sounds like the pix tube is shorted.

Yes it could be the tube but it could also be the video driver short on blue, Sometimes you can tap the rubbish out of the tube if it is a tube short, so sorry its one of those faults no one can tell you exactly is it worth looking at, try asking on the telephone what a quote would be to see if you think its worth the risk.

blue cathode in crt shorted to g1 or the blue video amplifier is burned.