Well out of 33 games only one did not work that I have tested so far. I had to clean the open faced carts of activision and such, but all the atari produced carts with the dust cover worked fine. below is a list of the ones I have played on the r77:
asteroids, basketball, air-sea battle, atlantis, berzerk, barn storming, bowling, boxing, breakout, carnival, frogs n flies, combat, centipede,circus atari, deadly duck, chopper command, commando raid, freeway, casino, cosmic ark, crackpots, defender, demons to diamonds, dig dug, dodge 'em, eggomania, e.t., football, donkey kong, dragonfire, fantastic voyage, dolphin, fishing derby.
the only one that did not work was decathlon, this one worked on my original atari but not on the r77.