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Old 17th May 2018, 8:51 pm   #1
PJL
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Default Marconiphone Type 41

I need to make some plug-in coils for this beast. It dates from 1925 and has an unusual tuning method. The coil sits over 'gyroscope' tuning mechanisms which rotate the inner ring between perpendicular and parallel to the coil. Each gyroscope operates a switch which connects a capacitor in/out of circuit so that the rotation on either side of the perpendicular position cover different frequencies. The 'gyrscope' is not connected to anything so I assume the tuning is by changing inductance from the introduction of the shorted turn which explains why they need the 'rejector'.

The tuning dials are marked Aerial and Anode. The Aerial appears to use the aerial capacitance for resonance and switches a series capacitor in/out. The Anode I am still checking but I think this is more conventional with two capacitors in parallel one being shorted by the switch.

The capacitors are wirewound! The big black box contains a conventional tuned circuit used as the 'rejector' for strong stations. It's very dirty (you can see where I have washed the rejector cover) and the cabinet is a bit scratched about but it has potential!

Valves in place were an eclectic mix of Mullard PM3. Cossor Point One RC, Unmarked, and Mullard AC104, but I will swap these out for 2V PM series. The grid resistor is 3M and should be 1ohm (1920's for 1M!). The interstage transformers are both good. All the square wires look to be in place.

Where to start? The coils are pretty big so I would have thought about 30 turns will be more than enough. I am just drawing out the circuit.
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Old 18th May 2018, 11:23 am   #2
PJL
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Default Re: Marconiphone Type 41

Circuit attached. Any RF guys able to provide an explanation of how the 'gyroscope' works? The black L1/L2 are the aerial coil, L3/L4 the anode coil and the black lines are the switches that are operated over half the turn.

I can only assume the shorted turn reduces the effective area of the coil? If this is the case the inductance should fall as it moves to parallel?

Is there any possibility the coil is not conventionally wound? Does anyone have a Type 31 or 41?
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Old 18th May 2018, 12:10 pm   #3
ms660
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Default Re: Marconiphone Type 41

Possibly similar in theory to these? (scroll down to auto series variometer):

http://www.valve-radio.co.uk/literat...ers-catalogue/

Lawrence.
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Old 18th May 2018, 10:16 pm   #4
PJL
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Default Re: Marconiphone Type 41

I'll strip it down and clean it up before prototyping some coils as there are likely to be issues with poor connections/contacts as it is. Aside from the 'gyroscope', the circuit makes sense although the reaction control of a shunt across the aerial coil is a bit odd.

Prototyping the coils would involve:
Wind the primaries, test the resonant frequency and adjust.
Add the secondary to the Anode coil.
Put some valves in and wind the reaction coil to give just enough to bring it to instability across all frequencies.

If it was the same as the Type 31 the original coil set covered 300-500m so only up to 1MHz. I'll go for this for my first attempt. There were no coils sets for any higher frequencies.

I have a couple of questions:
Should I use litz wire?
What turns ratio should I use for the Anode secondary?
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Old 21st May 2018, 11:13 am   #5
PJL
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Default Re: Marconiphone Type 41

I just saw pics of a V1 and it uses some of the same parts. It has a similar tuning method although in this case the ring is on the outside of the coil and it only rotates 90deg. The wire is cotton covered and there are roughly 60 turns including the reaction coil but the diameter is less than the 41.
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Old 21st May 2018, 11:24 am   #6
D.Finney
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Default Re: Marconiphone Type 41

Similar to a variometer, more common on crystal sets.
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