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79:04 error indicates the firmware needs updated. You need version A.04.02 (link below).
Link to firmware:
https://driverscollection.com/_4475275365015ebc7c64e8cfb88/Download-HP-Designjet-500-42-in-Firmware-v.A.04.02-free
Firmware utility tool is included.
Download the files to a folder on the desktop.
Connect printer to PC via USB.
Run the firmware upgrade utility program.
Push the firmware to the printer.
Display will show an unplug image when transfered.
Unplug\Plug in, boot printer.
Display will ask for language.
Completed.
This video may help.
