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Vintage Radio

Manufactured in the US in the early 1950s, the Philco 52-545 is a 5-valve broadcast-band superhet. It's a transformerless AC/DC that runs directly off 115V AC and so a care must be exercised when working on it.

By Rodney Champness, VK3UG

In both Australia and New Zealand during the valve radio era, it was standard practice to use a mains transformer to convert the 240V AC mains to other AC voltages, as required. By using a transformer, the lethal mains voltage was isolated from the receiver’s metal chassis and this made it safer for both users and servicemen.

However, there were some exceptions to this convention as there were areas with DC mains and areas that relied on 32V DC house lighting plants. Many AC/DC sets were made for use in country areas and these often had one side of the mains directly connected to the chassis!

This meant that, depending on which way around the mains was wired, the chassis could operate at 240V AC with respect to earth. For the unwary, they could be a real death trap and were always dangerous to work on.

Because of this, AC/DC receivers were always totally fully enclosed in a cabinet (ie, with closed backs) and the controls were often fully insulated from the chassis. That wasn’t always the case though. Many AC/DC sets had metal-shafted controls which were attached to the chassis and if a knob came off, users could get a nasty if not fatal shock from the bare control shaft!

To overcome this problem, some sets did not earth one side of the mains so that the chassis itself could be earthed. In these sets, all items that would normally have been earthed to the chassis were instead connected to a bus bar (often a thick solid-core tinned copper wire). This bus was then earthed as far as RF was concerned using a large high-voltage paper or mica capacitor wired between it and the chassis.

In addition, the antenna coil primary winding was often completely isolated from the mains, with one end going directly to the antenna and the other going directly to an outside earth.

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