i have a few old mechanical phone ringers one has a .4 mf 500 v DC paper capacitor bolted to it is this suposed do something for the ringer like a capacitor does for a flash in a camera. or is it for something else.

 

Phones used to get their power from the phone line. It was about 48 volts DC. The ring signal was about 90 volts AC imposed on the DC line. That capacitor was there to block the DC and pass the AC ring signal to the ringer mechanism. Here is a link - http://www.epanorama.net/circuits/telephone_ringer.html

And another - http://affordablephones.net/theringer.htm

 

so this capacitor would go in series with the ringer. am i understanding this corectly. this capacitor is rated 500 vdc how can it run ac too. i thought they capacitors blew up when there polarity is reversed or used on ac.

 

It would if it were electrolytic. Sounds like yours is wax and paper. That is a non polar type - not sensitive to the polarity of applied voltage.