Marvin windows are now designed to fail.

Half of the ballasts on the ten double hung windows at the cabin that were installed in 2004 have failed. For $700 Marvin will replace them but, looking at the parts the ones that failed were polycarbonate. The pulleys inside were zinc. Now they are also polycarbonate. The parts are under designed and will cold fliw and snap. Polycarbonate looses half of it's strength due to humidity and heat in only a few weeks after molding. It looks good on paper but sucks in practice. Saving the parts and will make metal versions. Do it right the first time and don't use plastic for parts under stress/tension. That includes screws.