E-606 (old) / E-607 / E-610
überarbeitet am 25.4.2012
After the sudden outbreak of World War II and the mobilisation of the Swiss Army, it turned out that the Swiss Army had no high quality all wave receivers to intercept foreign shortwave traffic and to monitor foreign news broadcasts of the international broadcasting stations.
For the needs of American flight communications, James Millen, an engineer of the National Corp. had developed an all wave receiver which completely changed the design of all commercial shortwave receivers - the earliest HRO appeared in 1934 and the set has been produced in different variations and with several improvements until the End of WWII.
The HRO was one of the earliest receivers featuring not only separate RF and AF gain controls, an automatic gain control (AVC) but also a signal strength meter to determine the signal level of a station and a crystal filter with a control to set the passband or rejection curve.
When you look at the outside of the radio, you can clearly recognize it's roots from commercially used receiving equipment. The metal cabinet comes with a hinged lid to access the chassis for changing valves from the top. An important accessory is the mains power supply, usually the "doghouse" shaped National 697 power supply delivering heaters and B+ / plate voltages. A wooden chest is used to store the coil sets not in use, the set could drive headphones or an external speaker.
The frontpanel of this very early sample of commercial receiving equipment is not that clearly structured as found in later receivers: most controls in the earliest sets came without lettering, but the operator simply knew which knob is for which control:
The main tuning knob with the "micrometer" dial, on which the numbers of the logging dial can be read, is found in the middle of the front panel. On the coil set just below the tuning knob, there is a calibration chart which helps you determining the frequency of a signal. Some coils sets come with double frequency charts, one for general coverage use and the second one for bandspread use in the amateur bands. In the right column of controls, the top button, in early sets a bar type knob with an arrow pointer, lets you regulate the passband width of the crystal filter, the rotary control below turns the crystal filter on and off and acts als rejection control like a phasing filter. The next switch will turn on B+, the plate voltage: let the B+ switched off when the receiver is in standby, and B+ must be turned off when changing coil sets!
At the left, you find the signal strength meter, in early HROs, it's activated by a pushbutton, in later variants by a toggle switch. The next control is the AF gain control, the volume control. The switch underneath will turn off the automatic gain control (AVC) when in it's right position, the bottom control activates the BFO for the reception of telegraphy signals.
Technically, the HRO is a single conversion superheterodyne with two RF preamplifier stages. The signal coming from the antenna will pass a first tuned circuit of the RF preamplifier (6D6, in the late version HRO-5 6K7) and a second tuned circuit of the RF amplifier stage (6D6, later 6K7). When mixed with the oscillator (6C6, later 6J7) signal in the mixer stage (6C6, later 6J7), the intermediate frequency of 456 kHz is generated. This concept using tuned RF circuits could be realised by National because of their excellent quadruple tuning condensor and the plug in coil sets.
In a list of receivers which were used in the Swiss Army, there are several types of HRO receivers. I suppose, that in the earliest time, maybe even single receivers got their own E-xxx designation. As far as i know, the HRO has never been acquired in large numbers, maybe, we can add more informations about the use of the HRO receivers in Switzerland later.
© Martin Boesch 25.4.2012