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external monitor with iMac

For several years I used an old 19” Acer monitor as a second display, attached via its VGA port to my iMac (2011 model). After I bought a iMac 2019 model, I again used the same monitor attached via its VGA port to my iMac 2019 with a lightning to VGA adapter cable. 


This worked well for about 2 years, but abruptly the monitor screen began to flash off then back on, not in a regular pattern but on average several times a minute. When it did so, the iMac screen was affected in a similar though not identical way, keystrokes begin to lag, and sometimes the whirling beachball starts whirling very fast.


I could not find out what had gone wrong, so I disconnected the monitor and bought a new Acer LCD monitor—model K202HQL . Today I attached it via its VGA port to my iMac 2019 with a lightning to VGA adapter cable. To my dismay the exact same problem happened right away. This tells me I am doing something wrong, but what? The iMac continues to run OS 10.15 (currently 10.15.7). When the Acer is disconnected, the iMac performs well.


Any ideas on what I can do to fix this?



iMac 21.5″, macOS 10.15

Posted on Apr 18, 2022 12:36 PM

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Question marked as Top-ranking reply

Posted on Apr 18, 2022 1:26 PM

+1 VGA is analog. Your best bet is to use a USB-C to HDMI, DisplayPort or DVI adapter, depending on what the Acer has for digital inputs.


Also have a look at > Connect an external display to your iMac - Apple Support


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5 replies

Apr 19, 2022 1:13 PM in response to den.thed

good answer. the cable was good, and because the adapter was only 14 months old i figured it could not be that--but obviously it was. i got a usb c to dvi adapter (the new monitor has vga and dvi inputs) and it is working fine now.


also i learned a lesson--dont assume anything, test it


thank you. i hope i have been able to change my evaluation from helpful to this solved it

external monitor with iMac

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