Yes, TUNER is a Scrabble word!

Letter values in Scrabble US: T 1, U 1, N 1, E 1, R 1.

Is it a valid word?

TUNER is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

someone who tunes pianos
an electronic receiver that detects and demodulates and amplifies transmitted signals

Also: piano tuner, radio receiver, receiving set, radio set.