Marconiphone Model 314

This sizeable table model was probably one of the last TRF receivers and was almost certainly aimed at listeners still reliant on gas rather than AC or DC mains. In this respect it's interesting to draw similarities to broadband. Rural areas in some parts of the country do not have fast broadband because it's uneconomical for Openreach (an arm of BT) to provide modern cabling and interconnection cabinets. In the days before WW2 the same was true of mains electricity and occupants of remote farmhouses or farm workers cottages relied for cooking and lighting on gas supplies which had been provided when local distribution was the norm. Even in the 40s and 50s some urban areas had to put up with antiquated DC mains and manufacturers needed to allow for this in the design of radio and TV receivers.

Perhaps with less well off customers in mind this radio was relatively cheap costing a mere £7:17:6d including a set of batteries; HT and grid bias dry batteries plus a 2 volt lead-acid accumulator. It had ganged tuning of its aerial and RF amplifier coils so could claim "single knob tuning".

 

 Below is the uncluttered chassis with three valves: left to right a PT2 (power pentode) or later KT2, S23 (tetrode RF amplifier)and HL2 (amplifier-detector). The set has some peculiar features; the volume control is a rheostat in the filament lead of the S23 and instead of a normal step up transformer for driving the output valve it uses an auto-transformer. The volume control is oddly located concentrically with the tuning control. Because it's a TRF set there's a need for a reaction control.

The control knobs are pretty weird having more quirky design than practicality.

Lots of space for the batteries and as you can see, there's no frame aerial as the receiver was designed for use in the home and the traditional wire aerial strung down the garden or around a back yard. Note the insulated anode top cap no doubt included to avoid shocking users or the odd serviceman checking for hum by touching what he might have thought was a grid lead.

 

 

 
 Above the instruction leaflet inside the case printed on Marconiphone's favourite green paper. Above right, the rather dusty amplifier coil. The aerial coil is mounted under the chassis to prevent unwanted feedback and instability.

 

 Whilst mains receiver dials are usually made from glass, manufacturers of battery sets usually used plastic which over the years discolours and gets easily scratched. The use of a plastic dial is strange because the set being a table model without a handle, and not a portable, could easily have been fitted with a cheaper glass dial. When I removed the set from its case I discovered the dial was a strange brown colour and the glass (not plastic as I'd first thought) was filthy. Probably both the dial and the glass colouration was due to nicotine staining.

By 1937 when this set was introduced the whole of the medium waveband was filled with stations which were then, as a result of international agreements, neatly spaced out and mostly free from interference.

 

 Removing the receiver from it's case was straightforward (the usual four screws accessed from under the case) except I had to cut the speaker leads. First an under-chassis picture. The wavechange switch is quite substantial and very robust. It has a spring ident mechanism and the selection of appropriate springs is via metal links embedded in the ebonite rotor. This same switch also includes springs for switching on & off the LT and the HT supplies. You'll also note that there are very few components as these were expensive back in the early 1930s.

 

The top view shows nothing unusual although the RF valve in the centre may need some cooking foil wrapped around it because the set may otherwise be a little unstable.

 

 Now for the big problem, and no doubt the reason the set was abandoned. My theory, which is often reinforced by my findings, is that sometime in the life of many old sets something serious happened. Maybe a repair estimate had been sought and refused so up into the loft went the set. It had cost so much money the owner couldn't just chuck it away. Decades later it was retrieved and sold.

What you see below, having been detached from the set, is a combination tuning slow motion assembly with an integrated gain control or more accurately an ex-gain control because at some time the winding has been damaged; probably it just wore out and a wire broke.

 
 The broken wire then got tangled up in the wiper and after trying for ages the owner ended up completely scrambling the winding. Because the gain control is a rheostat fitted into the filament lead of the RF amplifier valve, either the filament went out or became very intermittent when a broken wire happened to come into contact with either part of the chassis or maybe the wiper.

 

 Eventually the wiper itself became unduly strained and no longer contacted the metal work.This close-up shows the wiper now floating free. The tuning mechanism is also defunct because the spindle for the slow-motion drive is completely seized. I've soaked the thing in freeing oil and if all goes well I'll rewind the rheostat (which is 50 ohms), take everything apart, clean it up and reassemble having adjusted the springs to enable the rheostat to work again.

 

 

 The scrambled rheostat winding.

  There's something odd about the filament rheostat. The resistance track is flat and horizontal but the wiper moves in a circular path. The business end of the wiper has a leading edge which is very sharp and thus cuts into the track which surely can't be right. I rewound the track with a length of new resistance wire and bound each end with a copper strip about quarter of an inch wide. This connects the resistance wire to the filament connection. Turning the wiper would have cut the new track so I carefully bent the wiper to present less of a sharp edge. This was partially successful but there was only resistive contact over a limited length of the track so I suspect either the design of the original mechanism was rubbish or a part is missing. With the thing working after a fashion, I reassembled the set. After cleaning and lubricating the slow motion tuning mechanism, at least this is working properly.

Refitting the mechanism however appeared to be impossible without bending the mounting plate, then bending it back again (I'd already noticed this when attempting to remove it). Unless I've missed something this is a serious design fault and must have given the assembly line people a headache. Something we'll probably never know now because anyone that worked in the McMichael factory in 1936 would now be at least 100 years old.

 

 
 Above left you can just about see the repaired rheostat below the wiper (the three screws that are visible at the tuning condenser drive spindle are a pair for securing the slow motion drive and the third, smaller screw for securing the dial pointer which is on a long rod which passes through a hole in the spindle); and above right a piece of the old LT lead. The two leads had been compressed by the cable clamp together with the brown speaker leads. The pressure hadn't affected the latter but had resulted in the red and black rubber fusing together and baring the wires. Outside the clamp the rubber had hardened and become very brittle so needed replacing.

 

 A miraculous transformation once the old dial and glass had been cleaned with soapy water.

 

 And the set once the chassis had been returned to the case

 

 The final step was to power the old receiver and see if it worked. At this stage I'd replaced only the LT leads.

The battery leads had all assumed roughly the same colour so I needed to check them by continuity testing. I identified the two HT leads, two GB leads, and the HT negative and GB positive leads. I used my homebrew power supply (click to see this) and having wired up the leads rotated the wavechange switch. There was a hum from the speaker, clearly from insufficient smoothing within my power supply, and some crackling which cleared after waggling the wavechange switch a few times. I twiddled the reaction control and heard a feedback whistle so I plugged in a wire aerial and twiddled the tuning knob. Radio 4 came in loud and clear. Switching to medium waveband I heard an awful raucous noise and the odd heterodyne when I turned the tuning knob. Of course, I soon realised the noise was from my network cameras but by judicious setting of reaction and tuning I heard a medium wave station at decent strength.

Carefully turning the control knob in the centre of the tuning knob I found I could vary the signal strength, so that refurbished control is working to some extent. I even found in my junk box an octagonal knob to match the one on the other side. Time to call it a day, park the radio, and move on to another....

Click to see the Trader Sheet

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