All screens on earth do that, if the PC desktop is dead, all do , it goes to ITS very own sleep mode. and black screen in 15 seconds. after PC dies, or is PC already dead, not connected or cable to PC is bad. or both ends connected wrong.. #2 cause is CCFL backlamp tubes dead. 2007 year made has those CCFL and most are dead now. or even only 5 years old they die. to prove that , CCFL deal. dead when monitor is cold, and no Power power it on (its button) only, no PC needed and push the menu button under 15 seconds does the OSD menu pop up, if yes monitor is not dead. or do this test any time that" 20 times dead "thing happens, do the OSD test first. CCFL deathh symptoms. dead tubes, all or 1/2 the screen dim tubes intermittent, and random dead fails only warm or cold, only. thermal. yellow tinged tubes, white = yellowish. bad luck and any CCFL can fail, for lots of reasons. we use a flash light pointed at the screen to see , OMG there is data there. the menu button does work , only CCFL are dead. now the acid test gee test the monitor on a 2nd known good PC. what can be more easy and best test. if that simple test works, then the PC#1 is dead, we then fix it next. the secret is he OSD is inside the monitors brain, no PC needed. danny tell your story and do the tests first see data? ????? PC good, or OSD is up for 15 seconds,?
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https://www.manualslib.com/manual/145335/Samsung-Syncmaster-920nw.html?page=11#manual