1980's Andersok Windo

Hey there!

It sounds like you may have an Andersen sash window balance box. I recommend watching the video below. It will go over the details of the replacement. 

To access the balance boxes, you would first lower the top sash and remove the two screws on the left side jamb liner. Raise both sashes and remove the bottom screws. Now carefully pull on the bottom of the vinyl jamb liner out away from the window and pull down the top from the sashes to remove it. Bring the bottom sash down, swing it out, pull out the bottom white nylon pin attached to the cord in the sash, and carefully bring it up to the top. Do the same on the other side. Bring down the top sash and repeat the cord removal from the sash. Now at the top of the window, you will see screws holding the vinyl header. Remove the screws and carefully pull down the header to snap it out from the frame. Now you can see the balance boxes.

In that case, it must be held in by pressure in some way. Lower the top sash and see if you can pull down the top vinyl header cover to access the balance boxes to read the numbers. You will need to remove sashes when changing the balance boxes.