Screen on iMac with external monitor gets black shortly after resuming from sleep

rodion15

Posts: 165   +2
I have an iMac 27inch mid 2010 connected to an external monitor in mirror display mode. The problem is that seconds after resuming from sleep, the iMac screen turns off, while the external monitor remains on.
I've tried resetting the PRAM and SMC and testing with all possible settings in display preferences to no avail.
 
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