Friday Projects 2026

In January we organized a Hell Meeting for a long-standing network of German Hellschreiber enthusiasts. Since the mid-1970s, this group has been communicating every Sunday afternoon using Hellschreiber technology.

The Hellschreiber, named after inventor Rudolf Hell, is a distinctive telegraphy device designed to transmit and receive text-based messages via telephone lines or radio. Its ingenious but simple concept has allowed it to remain in use for decades.

Our main building has two main exhibition rooms, a library and a depot. We also have five garage boxes with many documents and Dutch radio equipment, mainly from the first half of the 20th century. This includes equipment from Kootwijk and former PTT collections.

We are examining whether the depot in the main building can be turned into a third exhibition room with Kootwijk equipment, including the Telefunken tubes blown by retreating German forces in 1945, as well as the Koomans receiver used near the seaside for connections with Indonesia but also used by the Germans (Forschungsstelle Langeveld), being a secret German listening station and an electronic warfare center of the Luftwaffe in occupied Netherlands during the Second World War..

During WWII, the German station received signals sent between Washington and London at Forschungsstelle Langeveld using Philips CR101 receivers. We have a CR101 from the station that appears complete, with its characteristic green cat-eye or magic-eye visual signal-strength indicator. We also have a similar receiver without type identification.

Our foundation has two Siemens T52 Geheimschreibers with 10 cipher rotors, mainly used for secret communications over fixed land lines. During a loan to a museum in the UK, the hood of one T52 was lost.

We were fortunate to obtain a rusty, dented hood found at the bottom of a lake. The first restoration steps included removing mud, rust and lime, then bending the larger deformations back into shape. It fits and will now be further polished and restored.

The 1.5 kW Transmitter was a heavy long-wave transmitter manufactured in Germany by Lorenz and Telefunken for the German Wehrmacht since 1935. It served as a mobile transmitter, installed in the Kfz 72 trucks, or as a stationary transmitter at airports or weather stations for telegraphy, telephony, and Hellschreiber transmission. The frequency range was between 100 – 600 kHz. Below the transmitter a ventilation unit was installed, and for power supply a U1500 Umformer or T1500 rectifier was used. 

Since April we have been working each Friday on the 1.5 kW Sender. It was not stable during operation, with fluctuations in the cathode voltages. A closer inspection of the Verstärker-teil / amplification unit showed damaged capacitors that were removed and checked. The broken ones were bypassed. Some ubes were tested with the RPG1 and RPG3/4 .

Mid june we had replaced the defective capacitors and performed a first test. The system is alive again, with stable filament voltage, all tubes working, and stable voltages.

Defective capacitors

Testing tubes with RPG1 and RPG4

First successful test with the replaced capacitors on June 19 2026

Two short movies from the 1.5 kW Sender/transmitter work are embedded below.