National has built into this receiver every feature the most advanced amateur requires. The two preselector stages give remarkable image frequency suppression, weak signal response and high Signal-to-Noise Ratio. The two high gain I.F. stages employ Litz-wound coils and are tuned with air condensers. The useable sensitivity and selectivity are exceptional. Other circuit details are automatic and manual volume control, a vacuum tube voltmeter calibrated in "S" units for carrier intensities, a phone jack, a Send-Receive switch and a Lamb Single Signal crystal filter. This filter makes selectivity adjustable over a wide range and the circuits are so precisely balanced that heterodyning signals may be completely phased out.
FEATURES
| First R.F. | 58 or 6D6 |
| Second R.F. | 58 or 6D6 |
| First Detector | 57 or 6C6 |
| High Frequency Oscillator | 57 or 6C6 |
| First I.F. | 58 or 6D6 |
| Second I.F. | 58 or 6D6 |
| Diode Detector, AVC, First Audio | 2B7 or 6B7 |
| Second Audio | 2A5 or 42 |
| Beat Frequency Oscillator | 57 or 6C6 |
OUTPUT CIRCUIT: The plate circuit of the output tube is brought to the output tip jacks located at the rear left-hand side. The output impedance of the HRO is 7000 ohms, and a dynamic speaker must, of course, have a suitable built-in coupling transformer of 7000-ohm input impedance. A headphone jack is located on the front panel just below and to the right of the "S"-meter. This jack is wired into the output of the pentode section of the 2B7. When the phones are plugged in, the signal input to the last tube is completely disconnected. If the speaker is to be disconnected, a jumper must be inserted in the tip jacks to connect them together.
ANTENNA: The input circuit of the HRO is arranged for operation with either the doublet type or the single-wire type of antenna. Doublet antenna feeders should be connected directly to the input terminals and the flexible ground connection is not used at all. The input impedance of the receiver varies over the total frequency range but averages about 500 ohms.
S-Meter: The S-meter serves to indicate the strength of a received signal. It is calibrated for 1 to 9 in arbitrary units which correspond, roughly, to the definition of the nine points of the "S" scale of the R-S-T system of amateur signal reports.
COIL RANGES: Four Plug-in coil assemblies are supplied as standard equipment for the HRO receiver, each assembly consisting of three R.F. coils and one oscillator coil, all individually shielded and provided with built-in trimmer condensers. Calibration curves are mounted on the front of each assembly. The four assemblies cover all frequencies between 1.7 and 30 megacycles, the division being as follows:
| 1.7 to 4.0 mc. |
| 3.5 to 7.3 mc. |
| 7.0 to 14.4 mc. |
| 14.0 to 30.0 mc. |
A small flat-head machine screw will be found in the left hand terminal block of each coil, looking at the assembly from the front. With the screws in the left-hand positions, the coil range will be that shown in the left-hand, or general coverage, chart. If it is desired to change the calibration to amateur band-spread, as shown on the right-hand chart, it is only necessary to move the four screws to the right-hand terminal block of each coil.
POWER SUPPLY: The receiver is designed to operate from a National Type No. 5897 AB power unit employing a 280 rectifier tube. This power unit will deliver approximately 230 volts at 65 milliamperes and about 2 1/2 volts at 9 1/2 amperes for the heaters.
SHIPPING WEIGHTS: Receiver 62 pounds, power supply 15 pounds.
Standard HRO Receiver, table model, complete with tubes and four sets of coils covering range 1.7 mc, to 30 mc., but no speaker or power supply - 2 1/2 volt A.C. or 6 volt battery model. List Price, $299.50
Standard HRO Receiver, relay rack model - 2 1/2 volt A.C. or 6 volt battery model. List Price, $320.00
5897 AB Pwer Supply - less tube - for above receiver. List Price, $26.50
MCS Table-model metal cabinet and 8" dynamic speaker (7000 ohms). List Price, $23.50
Source: "Notes on Amateur Radio Transmitter Design" by James Millen, 1937
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