Yes this was the boombox used in The Karate Kid

Yes, this is the Karate Kid boombox. The one Johnny Lawrence of Cobra Kai grabbed away from Ali Mills and threw on the ground at Leo Carillo State Beach in Malibu, beginning his epic rivalry with Daniel LaRusso. It's the Sears SR2100 Series, model number 564.21370350, manufactured by Sanyo and featured in Sears' 1983 catalog. It was literally one of the cheapest, most basic radios the producers could find. Retail price was only $59. It's got a massive 2 watts per channel. It's so basic, the cassette deck doesn't even have a Pause button. But hey, why waste a lot of money on a prop that was just going to get trashed?

Note: Sears offered an "upgraded" version with an FM stereo indicator light and a stacked left/right Volume knob that is noticeably taller than the Tone knob. Its model number is 564.21372350. But that model wasn't the one used in the movie.