Mark has written up a terrific document detailing how he added the Digital Display to his non “S” TS-520. There was a kit for the TS-520 back in the day to add the CAR/VFO/HET signal outputs, the TS-520S came with this option built in. Mark details how he built the interface, including the buffer amp necessary for the HET signal. Thanks Mark!
Digital readout for the original TS-520 by Mark Mohrmann, WA2FXM