überarbeitet am 25.4.2012
In several lists, you can find informations about receivers that have been acquired in small numbers and got assigned an E-6xx designation of the Swiss Army.
One of these sets it the HRO-7T, a successor of the National HRO, which itself has been acquired in small numbers.
The HRO-7 T is a single conversion superhet using plug in coil sets for changing bands like the classic HRO. On the coil sets, there is a printed conversion table for micrometer dial reading and frequency, similar but easier to read then the logging charts on the original HRO dials. You find two frequency reference charts for general coverage use and for bandspread in the amateur bands.
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 frequency conversion chart to determine the frequency of a signal from the reading of the micrometer dial.
Most coils sets come with double frequency charts, one for general coverage use and the second one for bandspread use in the amateur bands.
The setting general coverage or bandspread tuning is changed by machine screws in certain positions on the coil sets.
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 (V1: 6K7)
and a second tuned circuit of the RF amplifier stage (V2: 6K7). When mixed with the oscillator (V7: 6C4) signal in the mixer stage (V3: 6J7),
the intermediate frequency of 456 kHz is generated.
A vibrator power supply 686S could be used to power the receiver from a 6V DC source.
Also the crystal calibrator and the FM product detector were not standard accessories but could be ordered separately
and could be plugged in to an accessory socket on the rear of the set.
For the Swiss Army, Telefunken Zurich made a special wooden rack with the intergated power supply, a switchbox for receiver operation with or without speaker, and a storage rack for the frequency band coils.
© Martin Boesch 28.4.2012