National entered the semiconductor age in 1964 with the HRO-500. Apart from the name and the use of the same epicyclic tuning dial any similarity with its predecesor ends here.

The HRO-500 is a double superhet receiver with a first IF of 2.75 to 3.25MHz and a second IF of 230kHz. It tunes from 5kHz to 30MHz in 60 500kHz wide bands. The first local oscillator is synthesised providing the band selection. The second local oscillator is a variable frequency oscillator for the 2.75 to 3.25MHz interpolation receiver. The dial is direct kHz readout. No longer is a conversion chart required.Variable passband tuning and an IF notch filter are also included.

The set uses 37 transistors all mounted in plug-in sockets with not a printed circuit board in sight. This results in extremely complex under chassis wiring which must have been costly to assemble. However, the same excellent mechanical design as its predecessor is apparent.

I don’t think I’ve seen a receiver to beat it ergonomically but in terms of performance it was a poor second to competitors like the 1957 Racal RA17.

The HRO-500 remained in production until 1972. National followed it with its HRO-600. In a similar way to its predecessor it uses a sythesised first local oscillator to provide band selection followed by an interpolation receiver. It tunes from 16kHz to 30MHz in 30 1MHz wide bands. Its double superhet with a first IF of 54 to 55 MHz and a second IF of 5MHz. The second local oscillator is provided as a front panel plug in assembly. A precision variable frequency oscillator, fixed frequency control unit or 10Hz step synthesiser option are available.