Marconi RB12 Smokers Cabinet radio 1924.

[Geordie McBoyne]

I think I have the answer to the mystery of those coil assemblies.  We know that the tuning coils are housed in 6 inch diameter vulcanite cases.   In the earlier Marconi V2 receiver the tuning spades are simply slide in and out of the plug-in range units. Such an arrangement is not possible in the RB12 coils assemblies.
Instead of the tuning spades moving in the manner of the V2 I'd suggest to facilitate tuning a flat circular metal disc is moved towards and away from the pancake type coils.  When the disc is close up to coils this will be the minimum inductance setting. The HF end of the waveband.
The PCB was donated to me by Trevor to assist with the development of the variable inductance coil assemblies.

Geordie McBoyne.

[Geordie McBoyne]

Connected the printed coil via a 22pF capacitor to a signal generator. The response of the tuned circuit monitored using an oscilloscope. No external capacitor across the coil. The circuit resonates at 1650Khz. Placing a 90cm aluminium disc close to the coil results in a resonance of 2700Khz.
So we know that the tuning disc concept works. Placing the disc 1cm away from the coil results in a resonance of 1900Khz.
The advertising for the VB4 tells us that the tuning system employs two sets of coil pairs for long waves and short waves. To tune to the shortest waveband a pair of coils are connected in parallel. For the next waveband the same coils are connected in series. It's the for the long wave coils, the HF end of the long waveband the coils are in parallel and series for the LF end of the waveband.
The receiver was designed to tune between 300 to 3000 metres in four stages.
I'm not entirely convinced if this is correct, it could be that the two coil assemblies for long and short waves were in fact single coils with taps for the two tuning stages.

Geordie McBoyne.

[Murphyv310]

Hi David.
A square of thin steel should reduce the resonant frequency, even a ferrite rod should do the same to a lesser degree.

[Ed_Dinning]

Hi David, a result!!
I think Ali is probably the best material, or perhaps brass. Steel or ferrous materials my be too lossy.
Looking at the pictures today, I think some shaping of the tuning mat be possible by shaping the leading edge of the "sliders"

Ed