you have what is called frame collapse ,this is caused by the frame out put chip and a capacitor know as a flyback capacitor , you may be lucky and find it justs need soldering , if befor it went you had the picture not filling the screen at top when it first comes on and white diagonal lines at top then goes ok as it warms up the chip and capacitor need changing this is not a big job and if done on site should cost no more than £40-50.
dave, September 2009
