British WWII 4, 4A mine detector
In January 2014, I was contacted by a student, Simon K., of Germany, asking for technical help on a British WWII model 4 ( not the later 4A) mine detector he had. He had made a 60 volt battery with button cells, but the mine detector did not operate. He had the technical skills, equipment and schematic to do electrical tests, but needed advice on how to troubleshoot it.
After looking at the schematic and pictures he sent, and though I don’t fully understand how the circuit works, I requested reading the voltages on all the tubes (valves). From Simon’s measurements, it appeared that two tubes and one capacitor were bad. I suggested he move the functional tube to the output stage, to see if there was any sound then. After he switched the tubes around, the mine detector was functional. However, that was a short lived success. It quit shortly after, so he bought some 1T4 tubes on the German ebay at a low price. With those installed, he was up and running again.
As in so many cases in my experience, apparently it was partially a case of dirty contacts, possibly the tubes, switches or connectors. When working on vintage electrical equipment, clean any connectors you can, and with NO power applied, switch any switches and jiggle any parts quite a number of times to help clean the contacts.
Jan 2021 - A British #4 mine detector in great condition sells for $156, but only one bidder.
pictures by Simon K.
A note on capacitor (condenser) tolerance and voltages - use 10% or better tolerance, and a voltage rating of 200 volts is sufficient. Resistor tolerance of 5% is typical tolerance now.
Some of Simon's battery pictures. He uses AG13 button cells
