unfortunately the spring balances are no where near as good as weights

my current home ~60's has spring balances, fortunately we seldom if
ever open the windows. In SoCal we can open the doors as needed for
air.

Anyway if I understand your description the lower sash jumps up & the
upper sash goes down.

unlike weights which supply a constant force, spring balance force
varies slightly with sash postion and is usually adjustable.

on my windows the spring froce is transferred to the sash via a small
diameter braided cable with a metal clip on the end of it. The clip
hooks to the bottom corner of the sash. Some systems use a thin metal
tape (not unlike tape measure)


In the case of the upper sash that falls, maybe the cable has broken
or sometimes the clip gets mashed into the window sill & detaches from
the sash. So the sash now moves up & down without the spring cable
attached to it.

If it just goes to the middle, that indicates not enough spring force.
The lower sash that jumps to middle sounds like it has too much force.


The link below shows one type of spring balance, there are a few
different types. But they work on the same principle; spring force
takes the place of the weights

http://www.oldhousejournal.com/magaz..._balance.shtml

If you are relatively handy you can dig into the window assembly (w/o
seeing it I really can't be a whole lot of help). Sometimes they have
a little access panel that can be removed to get at the internals.

Having done it on other homes, I can tell you it isn't real complicated
but it not as much fun or satisfying as the weights.

Be careful, the springs can store a fair amount of energy & apply a
fair amount of force; getting cut or pinched is a definite risk.

cheers
Bob