We were jammming last night and I just realised that Trouble On My Mind has only the I and IV chord:

Once I had an old banjo,
Head was strung with twine.
Only tune I could play was
Trouble On My Mind, boys,
Trouble On My Mind.

Trouble, trouble,
Trouble On My Mind,
Only tune I could play was
Trouble On My Mind, boys
Trouble On My Mind.

I went down to Lynchberg town
To buy me a bottle of wine.
They tied me to the whipping post
And give me ninety-nine, boys,
Give me ninety-nine

I went back to Lynchberg Town
To buy me a bottle of gin.
They tied me to the whipping post
And give me hell again, boys,
Give me hell again.

Raining, hailing,
Falling from the skies.
My true love's gone back on me,
Surely I will die, boys,
Surely I will die.

Once I had an old banjo,
Head was strung with twine.
Only tune I could play was
Trouble On My Mind, boys,
Trouble On My Mind.

I learned it from a Michael Cooney recording that I borrowed from a friend about twenty years ago and have since returned. I think it was called Singer Of Old Songs.