so working on super old 2007 PC
monitors. and no PC connected."? it is NOT ANY LED BACK LAMP monitor here.
came new with XP PC's.
the monitos all good to sleep mode if no PC vidoeo connected.
in the usa, even by law, (energy star)
how ever lacking a PC, the screen will show "no signal entering sleep mode"
LED = blue on, amber sleep, LED out, oFF.
NO 1080P mode allowed, (so common now) <<<< big clue here
set the PC to 1024 x 768 (to test ) 60Hz.
using a flash light (torch) tool,.
turn on Monitor, see blue LED
point flash light to screen, see data, now, on sceen in the beam
now you now CCFL backlamps are dead. as most are this old.
Display Modes (Resolution)
640 x 350
720 x 400
640 x 480
800 x 600
832 x 624
1024 x 768
1152 x 870
1152 x 900
1280 x 1024
1440 x 900
proof BELOW. A FREE MANUAL TOO.
page 47, see 4, CCFL jacks and HV supplies here 600VDC
free service manual with full schematics and BOM
in my shop I have VGA signal generator to maKE WORK EASY.