Hp office jet l7680, paper jam, however no visible paper remaining, jam indicator still flashing.

Probably a switch is knocked off its mount

Yes; there is a great (official HP !) video tutorial on how to take it apart properly here:

There does not appear to be a Model C270 in the hp photosmart printer models. Do you mean it displayed a "C270 error code" in what Model No. Printer? The error code C270 indicates a paper jam/misfeed error.You possibly have paper remnants still inside Machine. You could remove drawers from machine, and look inside for remnants of paper. Maybe try turning machine off and on using red switch at knee level. If there is truly nothing blocking a jam sensor, then problem is likely a bad sensor. 99% of the time HP does not make replacement parts available to out-of-warranty printers. That's why I had to dump my C5550 HP Photosmart All-in-one printer; my office machine repair business could not get the replacement sensor! Remember: "Brother model" offer free toll-free phone tech service for the LIFE OF THE PRINTER! That's why I bought a Brother this time!

Hello,

May i know the printer model # / name ( full ) to assist you better. ?

indicate paper jam light but not paper jam in it , some times it it works what shall i do

There are possible 2 places for the paper jam, either the Feeder has jammed or from the input tray.
For Feeder, you just have to open the feeder from the top and see if there are any pieces sticking in.
Else, you can open the print head cover and look for pieces of paper jammed in it.
Another place you may see is by removing the rear of the printer, it is called a duplexor which can be taken off easily. You may find some bits of pieces in rear of the printer.
To avoid getting another jam, try to keep the paper in a cool and dry place. If the paper absorbs any humidity, it starts crumpling which results in paper jam. You can also try fanning the papers before putting in a bunch and use a recommended paper for the printer.
Hope this helps.
Cheers...

Please follow all steps here:

http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01182176&cc=us&lc=en&dlc=en&product=3434455

hp officejet v40 indicates a paper jam and it is cleared nothing

Here's a video that should help...

http://h30460.www3.hp.com/?fr_story=2304f3aa563aa6dbf7a24b8a764d74f1b43314d2&rf=bm&fr_operatingMode=OneSpecificStory

the paper path may have some jam. It may have some solid particle or paper that's causing disturbance.
If not the paper moving mechanism is having problem.
Better get your printer serviced.....