The D70 has a AE-L, AE-F Lock button on the body of the camera right next to the viewfinder.
Go to the menu of your D70 Go to the tab with the pencil and choose menu item 15.
You might have to switch the menus to advanced mode and make sure the item 15 is set to EA Lock.
Hard to explain, since I have a german manual.
But howerver. Take your D70 Manual. Look at the pictures of the camera. On the backside to the right of the viewfinder is the button AE-L, AF-L with the corresponding pagenumber of the manual.
Nikon D70s, Sigma 8 mm, 4 to 12 shots, Agnos MrotatorTShort, PTGui, PS CS3, and lots of other software :-)