The screen are too wide. A resistor burns on the horizontal zone. I replace it but the screen go wide slowly.

Check and replace damaged component/s at its horizontal scan output section circuit. Check the LOT (Line Output Transformer) too for damage. Replace the horizontal scan output transistor. Check the input voltage to LOT. If the voltage is absent check the feeder resistor; if any. If you wish to get some details; check the site linked here. Pull up older posts.
http://electronicshelponline.blogspot.com/ Type in this key phrase " my service table today " in the search box at the top right of the the home page, to get gathered some pages which has the details of practical experiences of the author of that site.

sounds like vertical retrace lines. The horizontal line fault called so. Check and replace damaged component/s at its horizontal scan output section circuit. Check the LOT (Line Output Transformer) too for damage. Replace the horizontal scan output transistor. Check the input voltage to LOT. If the voltage is absent check the feeder resistor; if any. If you wish to get some details; check the site linked here. Pull up older posts.
http://electronicshelponline.blogspot.com/
Type in this key phrase " my service table today " in the search box at the top right of the display window of the home page, to get gathered some pages which has the details of practical experiences to the author of that site.

Vertical output IC shorted. Replace it. Before replacing it, check all the associated components connected to its terminals for failure; especially for any zener diodes for short. It is most possible case. Refer its circuit digram [schematic diagram] for more details.
Contact any service technician.
If you wish to get some details; check the site linked here. Pull up older posts. Surf the site with patience. http://electronicshelponline.blogspot.com/

Hi !

How old is your iMac ?
SOunds to me like it has been left on too long for too many years..
THese screen eventually burn in and then it's the end...
Sorry.

One among the back-light CCFL lamp has burned out. Replace it.

The picture tube is been over-driven by an open resistor on the video board, a quick fix is to slowly turn down the high voltage screen control on the fly-back transformer ( one is focus "don't touch" and the other is marked screen "turn anti-clockwise" until lines just disappear.

some people just dont know what there talking about here(no horizontal section in a lcd your referring to an older tube set)..there is 2 capacitors c17 and c18 that powers the inverter boards in the power supply 10mf at 450v both need replacing.

you just say horizontal lines / and across a top of your television.I expect your trouble is the vertical line of your television . sound ok? try trouble the vertical section the check the resistor?

h/line is due to vertical section failure. 1 reduce brightness as you will get h/line due to vertical failure. 2 check v/section for loose contact or yoke connection. 3 check supply to vertical section. 4 check associated parts in v.section 5 at last check for vertical ic. resistor R410 CONNECTED TO high voltage transformator T444 is low value resistor. .5ohm to 10 ohm. it burnt again due to short cht in vertical mainly due to ic. hope this will help. as per schematic diagram go to www.serviceinfo.com