When I started in ham-radio, some 25 years ago, I bought this nice receiver. It's all with tubes and I changed the mixer tube to a better one. (6CB6 to 6JK6) On 40 meters, it was better listening. For schematics look at: http://www.radioamateur.eu/kenwood.html Later I sold the JR310 and bought a TS520 transceiver line. P.S. All those 6CB6 mixer tubes are also used in other Kenwood equipment. Replace them !!!