National HRO
  • Power: Requires 6.3VAC @ 3.4A & 240V DC @70mA from an external source.
  • Frequency Range: Uses a micrometer type dial turning 0-500 over 10 rotations. Band is determined by use of removeable, slide-in coil packs.
    • Type J = 50 - 100KHz
    • Type H = 100 - 200KHz
    • Type G = 180 - 430KHz
    • Type F = 480 - 960KHz
    • Type E = 900 - 2050KHz
    • Type JD = 1.7 - 4.0MHz
    • Type JC = 3.5 - 7.3MHz
    • Type JB = 7.0 - 14.4MHz
    • Type JA = 14.0 - 30.0MHz
  • Tubes: 9 total (in HRO-M)
    • 6D6 (x4 - RF & IF amplifiers);
    • 6C6 (x3 - Detector, BFO & Oscillator);
    • 6B7 (AVC, AF Amplifier);
    • 42 (Output Audio).
  • Circuit: single conversion superhet
  • Released: 1935 - 1943
  • Original Cost: US$230

NB: Requires external output transformer of 7,000 ohms impedance and external speaker.

Essentially, the HRO-M was the civilian HRO Senior model.

My Partially Cleaned HRO

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'What Makes the HRO Special?'

A classic military receiver, first built in the early 1930's, the HRO was much favoured by the intelligence community for its high sensitivity and high selectivity - the dial giving an effective scale length of 12 feet (or 3.65 Mickey Mouse Metres). It became a wartime standard and many found their way into private hands as post-war surplus.

The National HRO series of receivers are like no other, at least in appearance! Their distinguishing feature is the use of a removeable coil pack for bandswitching and a large, heavily weighted tuning drive wheel above that gives 10 full turns of indexing. There is no frequency readout althought the front of each coil pack gives a dial/frequency graph for that particular band. However, each chart is calibrated to a particular receiver which does not allow for coilset interchangeability.

But how did it get to look this way? John Hoare, G3PJI, says this:

"As radio communication developed in the thirties, designers were confronted with the conflicting demands of increasing congestion, higher frequencies and increased demand for sensitivity. In high performance receivers, the conventional approach of ganged wavechange switches, plus banks of trimmers, led to large and clumsy 'front ends' with ensuing stray capacitances limiting the receiver's performance. Add to this the need for one or often two stages of tuned RF amplification, and the solutions became difficult and expensive.

In the case of the National HRO the designers 'went back to the drawing board', and returned to the system used by early home-made designs, with plug-in coils. The RF section was turned sideways so that the coils could be built into one unit, plugged in from the front. The four gang tuning capacitor evolved into a gear-driven design driven at a right-angle from the centre, using a worm drive and a micrometer dial with an effective length of twelve feet. This guaranteed accurate logging, although absolute frequency was a matter of interpreting a graph on the coil pack. The resulting receiver was simple and (by the standards of the day) compact, and performance was maintained up to 30MHz with ease."

The HRO, as designed by James Millen, first appeared around 1935, and the price of $300 made
it a luxury item until its adoption for extensive wartime use. Today, unrestored versions of the 'classic' HRO-M and HRO-5 command good prices.

Last updated 11th November 2002