HMV Model 486

 Another old radio in my collection. This was stored away and has only just seen the lght of day. It was made in 1936 but looks almost 10 years younger. It has a very unusual design, both the case which is very stylish and expensive to make compared with a plain box shape, and its power source which is AC/DC mains. In the 1930s not everyone had AC mains (in fact this extended into the 1950s) so some manufacturers supplied sets that would work on a variety of uncommon voltages including DC. This means that the set has no isolating mains transformer and therefore has a "live" chassis.

 
 The rear view showing deterioration of the valve metallising and light corrosion of aluminium due to damp. The valves are unusual, being designed for current operation rather than from a specific voltage. From left to right, an X31 frequency changer, W31 IF amp, D41 detector/AVC, behind this is an N31 output valve and not visible to its right the rectifier a U30. On the right, a balloon shaped Marconi type 30 baretter which regulates the heater current for the series-connected valve heaters. The U30 rectifier is unusual, having a B7 base and an insulated heater because the cathode will be at the HT voltage for the set. The U30 will behave as a rectifier for AC and offer some cleaning up of the DC supply.

 

 The cabinet shape must have been tricky for the cabinet maker. Note the slide-wavechange switch.

 

 The dial showing short, medium and long wave stations is simple and really clear. The calibration is back-to-front compared with most sets.

 

 The label is glued to the inside of the cabinet and must have been designed for more than one model. Below this is a icture showing the model number and serial number. The number on the chassis is different to that on the label. Note the strange-looking mains connector. It was very important to get the polarity the right way round otherwise the U80 wouldn't pass HT current when DC mains was used. In the case of reversal of AC mains connections, the U80 would work OK but the chassis would be live.

 

 

 The circuit diagram of the HMV 486. Aswith any AC/DC set having dial lamps in series with valve heaters, a failure of one would cause the set to fail.

 

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