MG7720 won't print black.

I have a PIXMA MG7720 which won't print black. I did as much maintenece and tests and cartridge replacing I can think of. It won't priny no mater how I trycanning; copying; direct from computer...Nothing. I can't find any answers on line and local alleged "repair" places suggest I throw it away and buy new. One of the stupidest things I've heard of but seems to be the way now. Thow away society. Please help with a real soultion that isn't "give up".