at the end of the day...having replaced the alt and all running fine, i had all the time in the world to review the situation (and it only cost me cdn$143).
i also ordered replacing brushes. and a voltage regulator that looked identical( $17 including shipping!), just to get the small cover for the brushes that broke when removing it.
armed with that, i desoldered the brushes and replaced them with the new, soldered in place and reinstalled, and fitted the small cover. now the old alt is as good as new.
(seems that the wear of the brushes is what kills the alternator. otherwise are built to last forever).
but i learned that this is a job too delicate. that, if in need to replace the brushes is better to replace the voltage regulator (that can be found for about $30).
in my case, the brushes still had about half-life so there was no need to replace them. so it was not the cause of the failure. it was that the surfaces where the brushes touch and make contact were blackened so the contact was erratic. now that i know, the only thing that was needed was to rub them with sandpaper until clean, assemble the voltage regulator and fit the alt in place and go!
and in my case was the small cover that broke that needed the replacement voltage regulator just for it.
so my advice is that,
1-if you have the time and place to remove the alt and check the brushes, do it. this may need only to use sandpaper to clean the surfaces where the brushes touch.
2-or if the brushes are worn out, replace just the voltage regulator.
3-or if you are 1 of the lucky to have an honest mechanic, see with him to do this-if he has not already suggested it.