iMac desktop won’t turn on but keep doing 1 long beep
Every time I turn on my iMac desktop there always 1 beep every 5 seconds but the screen remains black
Current Pro Desktops
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That is an indicator that the machine does not recognize valid RAM installed.
If this is coming from a machine you have been using it may indicate that the RAM is failing or corrupted.
About Mac startup tones. - Apple Support
How old is this Mac? Which model and year is it?
What is the installed macOS version?
Has it been serviced or hardware updated recently?
Can you access the physical RAM modules and determine if they are properly installed?