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Emphasizing the feature of the HRO tuning dial
Heft 9, September 1939
By Rolf Wigand
(German language, pdf)
The Germans purchased these dials via Portugese sources. HRO was originally an American amateur radio receiver. It was, however, easy to produce and proved to be very popular. Both, the Germans as well as various Allied wireless services utilized these kinds of receivers. Although, the Germans built their KST copy, made by Körting (employing so-called "Steel-valves" of the 11 series). Siemens & Halske redesigned it, which became known as: type R IV.
Please notice also Wigand's 1943 HRO and Hammerlund circuit description
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