The TS-820 is an upgrade to the TS-520S.   It incorporated additional features and performance improvements while retaining the successful hybrid combination of solid state and vacuum tube technologies. It covers the most popular amateur HF bands (160, 80, 40, 20, 15 and 10 meters) with 100 watts CW power output and somewhat more SSB peak envelope power.   WWV can be received at 15 MHz for frequency and time synchronization.  The receiver sensitivity is excellent -- 0.5 microvolts for a 10 dB signal plus noise to noise ratio, typical of the entire line of Kenwood hybrids.
The TS-820 was designed to accept installation of the DG-1 digital frequency display as a kit.   In a companion model, the TS-820S, the DG-1 display was factory installed.  So the only -- yes, only -- functional difference between a TS-820 and a TS-820S is the presence of the DG-1 display.  With the DG-1 kit installed, a TS-820 becomes a TS-820S except for the name plate.
The TS-820 models include  frequency shift keying with selectable offset, a phone patch that can be used for recording or connection to a computer sound card, a more accurate analog frequency readout with 25 kHz crystal calibrator, an improved impulse noise blanker, and the DG-1 digital frequency display with 100 Hz resolution.  They employ a very stable VFO and phase locked loop (PLL) heterodyne oscillator to provide a single conversion architecture that reduces spurious responses (birdies) and provides exceptional image rejection.  New to this model was IF shift capability that makes it possible to tune out moderate adjacent channel interference.  XIT, RIT, and VOX with semi-break-in CW are included.  Continuing Kenwood's attention to audio quality, this upgrade's speech processing added variable compression to increase SSB average power incrementally.  Normally powered from 120 or 240 Volt power mains, an optional 13.8 Volt DC power module can be installed.   Like its predecessors, it is easy to tune and operate, and frequently used  functions are accessible from the front panel.