HV Go, G-M No Go, And Inside the Euromarine Radiofix Mk5

Late last night, I worked up the Geiger HV generator on breadboard. The first attempt didnt give me more than the supply voltage! Clearly, I'd got something wrong in building it, so I pulled everything out and did it again - this time it worked!

Parts ready to prototype, and the HV divider board

Using my newly built HV resistor chain, I was able to set the HV generator up to show 43V on my DMM, which equates to 430V. This is in the middle of the Mullard ZP1481 tubes 'Geiger Plateau'. I didnt have a 100Ω preset for the voltage setting, so was using a 500Ω with a 100Ω fixed resistor in parallel. This was very temperamental, and hard to adjust! Ive some 100Ω 10-turn presets on order which will make that much easier!

HV generator, and Mullard ZP1481 G-M tube

The 4M7 anode resistor was connected right on the tubes anode pin, and a 2x100k potential divider in the cathode to ground path was provided to allow me to connect the oscilloscope to look for pulses.

Sadly, not one single pulse was detected! After a lot of checking and testing, including with various 'beta-light' sources, I've sadly had to come to the conclusion that the tube is dead. I will of course retain it and test it again once I have another working tube and a working counter circuit. I have Russian tubes on order, but they will take some time to arrive.


With the intention of maybe retuning it for better coverage of the Aero-NDB band, I've also had the Euromarine Radiofix Mk5 open. Internally, the most prominent feature is the ferrite rod antenna. From the wiring I can see that there are two coils wound together on this. The circuit itself looks to be a very simple superhet. There are three IF transformers, which are all Toko 468kHz units, plus an oscillator coil (Red). Five BC171B transistors and a germanium diode detector.

Ferrite rod directional antenna

Simple Superhet

 Being a 468kHz IF, this rules out retuning to cover up to 550kHz, as this would require tuning through the IF! But, it seems there are few beacons in the UK above 440kHz anyway (not sure of the rest of Europe), so if I decide to retune, it would only be shifting around 40kHz, and still clear of the IF.