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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
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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
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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
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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
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Default Re: Marconiphone Type 41

Similar to a variometer, more common on crystal sets.
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Old 20th Jun 2018, 9:33 pm   #7
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Default Re: Marconiphone Type 41

I have now started the strip and fix. It is filthy and needs all of the screw contacts tidied up before I move on to coil winding.

The first problem is there are two variable resistors and both are open-circuit as in the conductive surface no longer conducts. One still seemed to have an original paint seal so the carbon has probably just aged and fallen off.

I have a horrible feeling that they were 'made to measure' by applying graphite to the surface and of course I have absolutely no idea what value they were! They both attenuate the signal by applying a resistance across coils so do not carry any DC current. So, if we assume we do not want the resistance to mess up the Q too much when set to its highest level, what should I be aiming for? My guess would be quite high?

I am thinking I may leave them out of circuit and try a few fixed values before I have a go at putting carbon back with a pencil. They are superbly made with spring loaded carbon contacts.
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Old 22nd Jun 2018, 3:15 pm   #8
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Default Re: Marconiphone Type 41

The 'Strength' one has a break in the track giving a no resistance position. I tried an HB pencil and I get a pretty reliable linear variable resistor varying from 2K-1.2M which sounds promising.

The capacitors are simply brass rods with a celluloid(?) wrapper overlaid with turns of tinned copper wire soldered together. They measure about 200-250pF.

Unfortunately, the required diameter for the coil former does not match any plastic downpipe I can find so I may have to make some 'tube' using fibreglass.

Still plenty to do but I am a bit more optimistic that I will be able to get it to work.
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Old 28th Jun 2018, 8:53 pm   #9
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Default Re: Marconiphone Type 41

It's almost cleaned up, just a few finishing touches. I have pencilled in the pots to 1M and hope that will work as they are difficult to remove. A pringles container is exactly the right diameter but they are aluminium lined .

My plan is to dig out a set of 2V valves, PM1HF, PM1HF, PM2DX, PM22A and use my homemade supply with regulated 2V. Wrap 50 turns round some cardboard and put it on the anode holder then use a scope and siggen and find the max/min resonant frequency.

I still would really appreciate input on the coils.

What would be a sensible turns ratio for the secondary of the anode coil that feeds the detector?

The aerial coil relies on aerial capacitance but what is the capacitance of a long wire aerial at 200-500m?

How should the 'proper' coil be wound? Looking at the Marconi V1 it is just a simple coil but the wire is cotton covered so might be litz. The V1 coil is wider so I may need to wind it as 2 layers. Should I separate the layers with something? Would I gain anything by wave winding given the coil already has 1M in parallel and the shorted turn variometer so selectivity was probably not a design consideration?

As we are know reduced to about the number of transmitters available in 1925, maybe selectivity will not be an issue.
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Old 1st Jul 2018, 8:26 pm   #10
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Default Re: Marconiphone Type 41

A forum member is kindly going to send details of a 31 coil.

It must have been stored in a loft outside the cabinet but it is now clean after stripping down every nut & bolt. There is some loss of plating to the spring contacts and surface rust to the transformer and variable resistor casings but as it's mostly plated brass it's not too bad overall. The pic is with the casing off the 'Rejector', a resonant circuit wired in series with the aerial.

I have had a go at the 1M grid leak resistor. It is made of two removable brass end caps and a black tube. The tube material looks similar to other moulded parts and has marks on the outer surface suggesting it was turned or maybe trimmed but an article at Radiomuseum suggests it may be silicon carbide. It was 3M so I soldered a 1.5M internally and it's now 760K so it seems to be variable!
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Old 3rd Jul 2018, 9:07 pm   #11
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Default Re: Marconiphone Type 41

My initial attempt at a rough winding of 50 turns gave a range of about 210-420M measured using a siggen and scope. The coil set supplied with the set would have been 300-500 so I am a bit low on the turns.

To cover the full MW band will take 2 coil sets:
300M-500M is 600KHz-1MHz which is the empty end of the MW band so it probably makes more sense to take some turns off this to make it 200M-.

Not sure how well it will behave at 1.5MHz but I would think the PM1HF should manage. I should probably do some more accurate frequency tests first.
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Old 17th Jul 2018, 12:05 am   #12
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Default Re: Marconiphone Type 41

I have decided to stick with the 50 turns as it covers much of the MW band that still has stations on it. I also managed to find a plastic container of the right diameter to saw up and have constructed a tidier coil with spacing between the layers of around 1.5mm.

This performs much better than my first attempt where layer spacing would have been much less. The test comprises a scope across the coil and light resistive coupling (82K) to a siggen. Not ideal as the probe capacitance will mess it up but good enough for comparison.

I am going to build one further coil, this time with a single layer of finer wire. The theory being that increasing the layer spacing will move the outer layer further from the variometer ring limiting the inductance range assuming that the ring works by reducing the effective area of air it is enclosing.

Once I have built the third coil, I will do some more controlled tests and decide on the best way forward. I will space the secondary out as far as I can and use a finer gauge wire as the resistance should not be critical.

For LW, Litz wire and wave winding will be required to keep the coil compact whilst minimising inter layer capacitance. I am not sure it is required for MW and was not used on the V1 which used cotton insulation so probably a finer gauge than I will be using.

Radio waves are remarkable things, I can 'tune in' a station on the scope even with this crude setup.
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