How do you replace a print head for MP780 and what is the best way to do it so the printer works well after the print head has been replaced

Not normally but the black print head nozzles maybe blocked.
Inkjet printers need to be used frequently to prevent ink drying out and blocking the print head nozzles.
If the print head cleaning maintenance routine does not clear the blockages, then print head needs replacing, but it may be expensive.
It may be more cost effective to buy a later model and better printer than replace the print head.

Inkjet printers need to be used frequently to prevent ink drying out and blocking the print head nozzles.
If the print head cleaning maintenance routine does not clear the blockages, then print head needs replacing, but it may be expensive.
It may be more cost effective to buy a later model and better printer than replace the print head.

If that is a new ink installed in the printer then it's time to replace either PRINT HEAD or PURGE UNIT.

the print head is defective and print head control board, need replacement. normaly very expensive spare parts, above printer price.

Try with head cleaning, replacement of other inks, or with new print head

The print head is clogged.Remove it from printer and wash it with hot water.Best put it at under, a stream of hot water, where the nozzles are.Then wait to dry nice before put in the printer back.If this not help replace the print head.It's part number is QY6-0049.

that definitly sounds like it needs to be replace, have you tried using the printhead cleaning utility?

Yup I had the same problem and there is only 1 valid solution. Change the print HEAD. its a removeable item that is easily replaced. all you need to do is remove the ink cartridges (All) and flip up the lever where the cartridges are . This will allow for the removal/replacement of the head assembly. You can alays try cleaning this head assy with (Ammonia D) just in case you are one of the lucky one just to have a defective connection. Note: Worked for me and everyone else I ever saw with this error. (I bought my head on ebay)

PIXMA MP780 "WRONG CARTRIDGE" MESSAGE?
Here is what I did and it now works just fine.I turned off the printer and unplugged the power cord, and unplugged it from my computer. I then removed all the ink cartriges and removed the print Head I got some rubbing alcohol put it on some toilet paper and wiped down the back and bottom of the print head.Plugged everything back in and It now works better then ever.

go here

http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&tabact=SupportDetailTabAct&fcategoryid=238&modelid=10444

type in :
no black text.

Follow steps to fix your problem