Needs a Motherboard for a Sony KP-51WS500 rear screen projection television

It sounds like the focus pack needs to be replaced (p/n 122392535) and re-adjusted. The re-adjusting could be the tricky part if you haven't done these before.

The Blue CRT has shorted and shutting the set down. You can verify this by simply sliding the Blue CRT drive board off the neck without unplugging any of its wires, Just let it hang suspended and then try powering up the set. If the set stays on you will know the Blue CRT needs replacement.

yes this could be the start of bad picture tubes . but you can adjust g2 controls and get rid of the line

u have bad blue picture tube , $300 - $500

Sounds like from your description the Blue CRT ( picture tube ) is shorted. I believe they are no linger available. If in doubt, I would call a service company to verify this.

sony has a problem with that model the pix tubes short not going to be worth fixing

Tv repairman. the green picture tube has an intermittent short in it. This is an expensive fix.

your tv needs to be looked at by a professional tech..it may need reconverged..

Try this link leave them an email with your model number and which tube you need they should be able to locate one for you.
http://servicemanuals.vstore.ca/

tubes ok may need adjustment.