US Sources

They arguably started in 1790 and have been continuing ever since on occasion. The most recent was in 1979.

Sources :

• “Has the U.S. Government Ever “Defaulted”?” Congressional Research Service, counts 3 times.
• Alex J Hill(Former US treasury employee) counts 4 times here
• this paper covers the 1790 debacle a little bit, along with other countries defaults, which you might find interesting.