In the early 1960s, the National Radio Company, realizing that the future of communications receivers lay with solid-state designs, started engineering work on a totally new receiver. The company had a long history in the receiver industry, stretching back to the 1930s.
Rather than take an existing design based on vacuum tubes and adapt the circuitry to transistors, National reasoned that the new receiver would have to be an almost complete break with the past. It would have to be state-of-the-art, and it would have to employ the latest electronic design philosophies.
The result of all this work was the marvelous HRO-500 communications receiver. First models were available in October 1964, and were immediately the talk of the Amateur Radio community. Nothing quite like it had ever appeared on the market, and neither of the two other major manufacturers (Hallicrafters and Hammarlund) had anything that could compete with the receiver.
One noticed first the HRO-500’s cosmetic design. The familiar National PW dial was retained, a link back to the earliest National receivers. This version of the dial had an updated, sleek metallic look. The panel layout was very symmetrical, with all operating controls within easy reach. Above the PW dial was a secondary frequency indicator, a drum that rotated to indicate the 500-kilohertz band segment in use. On the left side of this indicator was another, smaller dial, to indicate the tuning of the antenna circuits; on the right side, a meter to indicate relative signal strength.
The real story of the HRO-500, though, was its advanced circuit design. Most commercial receivers up to that time were tuned by relying on a variable frequency oscillator. No matter how well the oscillator was designed, it could never be completely stable with respect to frequency. Variable oscillators were subject to drift from the frequency to which they were tuned, thereby detuning the receiver. Only with crystal control of the oscillator circuit could there be any hope of stability; however, crystal control implied an oscillator tuned to only one particular frequency. Even if an engineer opted for double superheterodyne conversion, a tunable receiver still required at least one variable oscillator.
The designers of the HRO-500 solved this problem by using a frequency synthesizer circuit. The synthesizer, ultimately based on a stable crystal oscillator, produced the necessary frequencies that would normally be generated by the variable oscillator. The crystal oscillator determined the overall stability of the synthesizer, and inaccuracy was virtually eliminated since the variable oscillator in the synthesizer was phase-locked to the crystal oscillator. The HRO-500 was so stable (better than ten hertz in any ten-minute period) that it could even be used as a frequency counter.
This circuit made it possible to tune the HRO-500 in sixty bands of 500 kilohertz width, across its range of .5 to 30 Megahertz with accuracy of one kilohertz across the entire spectrum. Only the military R-390 series of receivers could equal such an engineering feat. The operator could dial up a particular frequency and be assured that the receiver was indeed tuned to that frequency -- and stay there.
Other advanced features were present in the HRO-500. Among them were four selectable bandwidths; a passband filter for accurate tuning of single-sideband signals; flat response across the audio spectrum; a true product detector circuit for single-sideband reception; complete solid-state circuitry to ensure total stability.
Unfortunately, all this did not come without a price. The transistors in the HRO-500 were socketed, instead of being soldered directly to the circuitry. This tended to cause problems when corrosion developed between the transistor lead wires and the sockets themselves. Also, the calibration oscillator circuit had occasional problems starting. The complexity of the HRO-500's circuitry made servicing a tricky and time-consuming affair.
National had hoped that the HRO-500 would revive the company’s failing fortunes. Although it was well received by amateurs and commercial communications operators, it was not the huge success the company needed. As Raymond S. Moore puts it in Communications Receivers, Fourth Edition:
"Perhaps it was a little ahead of its time. The HRO-500 was popular and widely admired ... the HRO-500 immediately became the ultimate receiver. Unfortunately, it did not save National ... the Japanese were waiting in the wings."
Production of the HRO-500 continued through late 1972, when National discontinued the receiver and introduced its successor, the HRO-600. Since that time, a certain mystique has grown up around the '500. Collectors quickly snap up surviving models; it has lost none of its original value, and the HRO-500 is prized today even more than it was during its lifetime.
Moore, Raymond. Communications Receivers, Fourth Edition. La Belle, Florida: RSM Communications, 1997
Osterman, Fred. Shortwave Receivers Past and Present. Reynoldsburg, Ohio: Universal Radio Research, 1998
Weigand, Neil. "HRO-500". Radio Bay. <http://www.io.com/~nielw/nat_list/hro500.htm> (26 December 2002)
personal experience (I own an HRO-500.)