The TS-520 is the grandfather of the Kenwood hybrid line, and it was the  most advanced transceiver of its day.  30 years later it still holds its own since it has the features most demanded by serious amateurs:  100 watts output for CW with a bit more for SSB; a very stable VFO or selectable fixed frequency crystals; an analog frequency dial accurate to 1 kHz in conjunction with a 25 kHz crystal calibrator; receiver incremental tuning useful in receiving a station slightly off frequency; an effective impulse-noise blanker; and VOX with semi-break-in CW.  While the SSB crystal filter is satisfactory for CW operation, an optional 500 Hz wide crystal filter can be installed for excellent CW selectivity.  Provision is made for installing a  kit to drive Kenwood's external DG-5 digital frequency display.  The combination of solid state and vacuum tube technologies provides an architecture best described as simple but elegant, and it produces the fine audio quality for which the Kenwood hybrids are noted.  It is easy to operate, and tuning the power output stage when changing bands can be accomplished in half a minute or less.  Frequently used functions are readily accessible from the front panel.   It can be powered by 120 or 240 Volt AC mains as well as 13.8 Volts DC, which makes it ideal for mobile, portable and emergency use.
If you have a nice TS-520, hang onto it.  It doesn't have many bells and whistles, but it has everything you need for solid SSB and CW QSOs including DX.  It will  serve you well.