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:
Later I sold the JR310 and bought a TS520 transceiver line.
All those 6CB6 mixer tubes are also used in other Kenwood equipment. Replace them !!!