The National Radio Company HRO-500 is a completely solid state communications receiver with a frequency range greater than that of any other unit available today. All facilities are incorporated for AM, CW, and SSB reception between five kilohertz and 30 megahertz, with unusually accurate dial calibration, high frequency stability, and ease of operation. The HRO-500 is currently in use by every major government agency in the United States for HF/VLF point-to-point communications, monitoring, and for laboratory instrumentation applications, and has rapidly become accepted as the finest and most versatile commercial receiver available for such use. Complete transistorization provided a very high MTBF, together with minimal heat generation and power requirements, and allows the HRO-500 to be used in portable or field applications barred to existing vacuum tube equipment. Ease of operation, accuracy of dial calibration, short and long-term stability and sensitivity are demonstrably superior to competitive tube equipment, and offer a performance combination unequaled by any other commercial receiver. The HRO-500 is remarkably compact in size and weight and may be either table or rack mounted. In addition, a fully portable version is available with self-contained power, loudspeaker, and waterproof carrying case.


  • Complete coverage of the entire VLF/HF spectrum between five kilohertz and 30 MHz, with the same superb dial calibration, stability, and tuning rate throughout.

  • Total transistorization for maximum reliability and versatility with minimum heat generation and power requirements.

  • MTBF per NAV-SHIPS 93820 of 3820 hours results from conservative circuit design and proper de-rating of all components.

  • Operates (with no need for accessory supplies) from 117/234 V.A.C. or 11-16 V.D.C. sources. Current drain at 12.6 V.D.C. is only 200 mA.

  • Complete SSB, AM, MCW, and CW facilities, including both product and diode detectors; Passband Tuning of filter for true selectable sideband reception; fast attack, slow decay AGC; and four discrete bandwidths from 500 Hz to 8 kHz.

  • Dial calibration accurate to one kilohertz over the entire frequency range, with bandspread of 1/4" per kilohertz and 24 feet per megahertz.

  • Selectable tuning ratio of either 10 kHz or 50 kHz per revolution of main tuning control for choice of rapid band-scanning or fine tuning at will.

  • "Instant-on" operation with no warm-up period required.

  • Superb short and long term stability - nominal stability, including 7% change in A.C.input voltage, better than 100 Hz per day and 50 Hz per degree C.

  • Frequency determination by mean of phase-locked crystal frequency synthesizer assures maximum stability and elimination of band-to-band recalibration.

  • Sensitivity and image rejection (averages 80 dB, minimum 50 dB) superior to the previous standard of comparison - the earlier HRO receivers.

  • Unusually flat audio response - 100 Hz to 5500 Hz within 6 db, with over 2 watts of available audio output.

  • Complete remote control provisions including AF and RF gain, BFO on-off, receiver mute, and power on-off.

  • All necessary oscillator, AGC, I.F. and detector outputs provided for use with accessory equipment.

    FREQUENCY RANGE: The HRO-500 covers the entire VLF and HF spectrums continuously - from five kilohertz to 30 megahertz in 60 500-kHz bands. The 5-kHz to 500-kHz VLF range is incorporated in the HRO-500 receiver with frequency in kilohertz read directly from the main tuning dial. The LF-10 preselector may be used to provide one microvolt sensitivity in the 5-kHz to 500-kHz range with protection against possible overload when desired for critical VLF applications.

    SELECTIVITY: I.F. selectivity in the HRO-500 is obtained by means of a tunable six-pole filter employing ferrite cup cores at 230 kHz. Bandwidths (at 6 db) available are 500 hertz, 2.5 kHz, 5 kHz, and 8 kHz to meet any combination of SSB, CW, or AM receiving conditions. Front-end selectivity and image rejection in the HRO-500 is unusually high. Three tuned circuits are used in the RF stage instead of two, and intermediate frequencies are chosen so the image rejection is a minimum of 50 db throughout the tuning range.

    AGC: The HRO-500 incorporates an AGC system of high relative merit (5 .db change in output for input variation between 10 and 100,000 uv) with attack and release times optimized for SSB/CW/AM reception without pops, clicks, or disablement due to noise pulses.

    S-METER: is accurately calibrated in decibels above 1 microvolt as well as the familiar S1-S9 scale.

    SOLID STATE CIRCUITRY: The use of total solid state circuitry in the design of the HRO-500 results in advantages never before realized in commercial communications receivers: greatly improved reliability over tube-type equipment because of the far longer life of transistors and elimination of performance deterioration from tube "aging"; vastly increased versatility in terms of portable or field operation since the HRO-500 may be operated directly from a 12 V.D.C. source as well as from 115/230 V.A.C. 50/60 hertz sources.

    WEIGHT: 32 pounds.

    SIZE: 7 5/8" H, 16 1/2" W, 12 3/4 D.

    ACCESSORIES: RMK-5 rack mounting kit, LF-10 preselector, HRO-500-TS matching table speaker and WPC-5 waterproof carrying case.

    Read the August 1965 Popular Electronics article reviewing the HRO 500

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