Repair: Broken 'Counterweight' Cords
A few windows in our new home have snapped cords so the sashes don't stay up. Thought it might be an easy fix, but these are not your old fashioned sash cord with a counterweight in the frame type of windows apparently. The white track that the sashes ride in is a vinyl piece over wood frame. It is held in with a couple of screws on one side, but mostly pressure fit. I was able to remove the upper portion on one side, and the header but found no clues as to where to go next. I removed the trim around the window frame, but I cannot see any counterweight system nor is there space for one. I am beginning to think that the metal plates at the top of the frame are housing some sort of friction/spring-based resistance mechanism, but I have no idea how to access that without destroying the window frame. They seem to be held in by the jambs on the outside edge, and a single staple on the inside.
Any tips on how to access the mechanism, or how to go about this repair safely, and without having to completely disassemble and rebuild the window? Are these even serviceable?