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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 21st Aug 2019, 9:06 pm   #1
kestrelmusic
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Default Marconiphone 196

My next-but-one project is a Marconiphone 296 table-top set from (I think) 1934. It was given to me by a friend who bought it for a modest sum in order to extract the PX4 output valve. It has one of the nicest Art Deco cases I have seen in a long time!

Anyway, I don't propose to spend well over 100 for a replacement valve - if I change the socket for a 5 pin one I can use a ML4. The output will be reduced, but I guess it will work, and a PX4 will still fit if one becomes available.

This set used the speaker field coil, connected between the HT centre tap and the chassis earth to provide grid bias, so I can connect the ML4 cathode to earth.

However, the problem is, I suspect, going to come with the capacitors. Most of them are potted in bitumen in a metal box, which means that if more than one or two caps are duff it will be easier to replace the lot. I can presumably melt out the old bitumen and build up a new block with modern components, but can anyone suggest a suitable filling material? Ideally it wants to be liquid enough to be poured, but set reasonably firmly.
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Old 21st Aug 2019, 9:27 pm   #2
Heatercathodeshort
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Default Re: Marconiphone 196

You don't need to fill it with anything. Just leave it open. The reason this was done by EMI was an attempt to stop bodgers messing about with a good quality radio, fitting incorrect components and handing back to the customer with a much reduced performance. It was extremely difficult to open the box and bodgers learned and just stopped taking them in for so called repair. The replacement metal boxes with all the components fitted was a cheap component from EMI exclusive to their agents and could only be obtained if you were an accredited agent. [Marconi/HMV dealer.]
The exact component layout of the innards of the box was given in the Trader sheet for this extensive series. You can make up a tag board maybe with a Paxolin sheet to replicate the original. Hope this helps, John.
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Old 21st Aug 2019, 9:40 pm   #3
PJL
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Default Re: Marconiphone 196

I restored one of these a while back. It had a lot of surface rust but I did manage to remove most of it and get it going.

I believe this is one where many of the resistors are mounted in a moulded frame mounted on top of the capacitor box. I use the cheapest PCB mount capacitors possible and fit them to veroboard making up the required values using 1uF or 0.5uF.

Potting in bitumen would have been an attempt to protect the paper capacitors from moisture a) Doesn't seem to have worked b) Is a PITA for servicing c) Is not required for modern types
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