The Radionette Kurer is a step nearer to working but I've run into a problem. Apart from an intermittent volume pot the audio stage is operating, but the receiver is otherwise dead on all four wavebands.
The IF and detector stages are also working, because a modulated 465kHz test signal can be heard OK from the speaker. My next thought was that perhaps the Local Oscillator wasn't running, and this was where I began to struggle a bit.
All of the receivers I have worked on have either had separate oscillator and mixer valves, or have had these two functions combined into a valve with two distinct sections, typically a triode-hexode such as the ECH81. This one uses a single DK96 heptode valve as a self oscillating mixer. Here was the problem; I'd never seen this configuration before and I couldn't understand how it worked.
The answer came from Wikipedia in an article entitled Pentagrid Converter. This explained how the first grid is used for the oscillator, with the second grid acting as an anode. The incoming RF signal from the antenna tuning circuits is applied to grid 4. The remaining grids 3 and 5 are connected together to act as screen grids. Grid 2 allows some of the oscillating electron stream to leak through and couple with the mixing section of the valve.
The Wikipedia article can be found here.
The only circuit diagram I have for this set is a very small picture pasted inside the rear cover. I don't have any component layouts or test voltages either, so given my unfamiliarity with this type of circuit this one it's going to be a bit of a challenge!
I need to spend a few days away from the workshop so regrettably this job will have to be put away for a while, but before I do I will check if the LO is actually working. The easiest way to do this will probably be to use another radio. The LO will be running 455kHz above the receive frequency, and should be receivable on another radio in close proximity.
Keep listening!
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