Not printing New printheads, new ink cartriges... says recving fax but won't print. Rings for incoming... wont print. Can't print anything? Printed fine one minute, then just stopped.

Check to see if the fax machine is in 'answer' mode. Different fax machines have different features, though.

Check to see how many rings it is set to answer incoming faxes.

Test it by sending from a known good fax machine to your fax machine.

If a human calls the fax machine, nothing will be printed.

please manually clean the noozle openings with a soft cloth

Always try replacing the ink cartridges first.

It sounds like the print heads in the 7780 are bad. Don't confuse the ink cartridges with print heads. The cartridges fit inside the print heads. You will need to change the heads. Here is a link to HP's how-to.
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00841890&tmp_track_link=ot_faqs/top_issues/en_us/c00841890/loc:1&lc=en&dlc=en&cc=us&lang=en&product=2512010

Could have the same problem with the Dell.

Ok. Remove both cartriges. Put them aside. Remove the new printhead from its bag and all the protection things. To remove the old one, press the button at the top left of the printhead. Put the new printhead in, secure it by pressing the sticker with the finger, and replace the cartriges and print the alignment page.

The Thermal printhead is most likely burned out on one side and will need replaced. It could also be the printhead connector has worked loose and is only making partial contact. Remove the connector and reinstall. If that doesn't solve the problem then it would be cheaper to purchase another fax.

Cartriges have a shelf life. It's printing in blue either because it has dried up or is out. Different cartriges have different timelines but if you haven't been using the printer often, it could have dried up or is simply out of ink. I would suggest first to try cleaning the head of the cartrige where the ink comes out with alcohol and a Q-tip and then if that doesn't work, buying a new black cartrige.

are copies light too? if not then it is sending fax, but most times it is inferior ink or drum causing the problem sometimes damp paper (try printing on reverse side as well)

Move the ink cartridge carriage (the printhead) to the middle of the printer like you're going to change cartridges.  Take out all the ink cartridges.  There should be two black springs on either side of the printhead.  Remove these and the printhead should pop out.  Clean the nozzels (the little parts that squirt out ink) and reassemble.

If this doesn't work, then you'll have to replace the printhead.

First with a clean soft cloth and a very small amt of alcohol clean off the underside of the printer carridge (the holder for the print heads) also where the print heads cleaning station is clean out any large masses of congealed link. Then remove the ink cartridge in question and with a pencil eraser gently rub the contacts and both the cartridge and the carridge replace the cartridge. Using the software tools for the printer clean and align the printheads ( if there is an option to do so). If this dosn't fix the problem try a different ink cartridge.

No, its not possible. The problem is likely the ink has dried in the purge unit or one of the small tubes going to the waste pad. The new printhead is the fix for the ERROR 41. The problem is there is sometimes other things that go bad, purge unit being just one example. One of the biggest problems with inkjets is that consumers leave their printers alone for long periods of time and the ink dries and clogs the tiny hoses inside. Even the ink cartdriges are just ink tanks, I have replaced those many times and have seen bad ones that just won't flow, replaced the inks and it works fine. You also have to put in the id# for the printhead after you replace it. Did you do that or get the instructions?