I found a usage example for nchenga nchenga on Twitter:
“tv ya nchenga nchenga” = blurry tv image, usage example twitter.com/Dzonx_tha21st/”¦ ”¦ c.f chiperoni.ch/wordpress/abou”¦ ”¦
— âœ___CollectThisTweet (@nchenga) May 31, 2013
“Nchenga nchenga” is an expression I heard on a Blantyre radio talk show. A lady was complaining about bad TV reception. She said, all I can see is “nchenga nchenga”.