Austria had mandatory voting in Presidential Elections from the 1920s to the early 1980s.
Turnout was above 90% in all of these elections (97% after WW2 and down to 92% in the early 1980s), but dropped significantly after mandatory voting was scrapped.
The [theoretical] fine for not voting was 1000 Schilling (inflation-adjusted ca. 100-120€ now or some 150$).
But no non-voter ever had to pay a fine, because all Presidents who were elected decided to put out an amnesty for non-voters as soon as they entered office.
That of course did not mean that
all newly elected Presidents would do this, so people were still heading to the polls to avoid an eventual fee ...