3. Transformerless or AC/DC Radios

Some radios work on both alternating current (AC) and directed current (DC); this was rather common for the smaller table radios, but for larger radios like the Pye P445U on the right it is more the exception. AC/DC radios do not have a power transformer, but rather rectify the incoming mains power directly; this saves the space for a transformer and, moreover, the considerable cost. In Dutch, AC/DC radios are referred to as `Universeel-toestel'.

As a consequence of the design, the chassis of the radios is connected directly to the mains, and they require special care when servicing them or playing them with the cabinet open or removed. Indeed, safety requirements were less severe a couple of decades ago, and nowadays production of these radios would be prohibited without any doubt. As long as you know what you are doing, but only then, there is no immediate reason to throw (or give, hint hint!) the radios away.

For anything beyond the most elementary repair, work on AC/DC radios requires that the radio is supplied from an isolating transformer. If the transformer can be switched to 110V (like mine) you can also use it for American radios or play a European one without heating the large dropping resistor. Unless you are very sure about what you are doing, don't even look at the inside of AC/DC radios without using an isolating transformer. Only with this unit an AC/DC radio can be connected to measurement tools or even, be touched, safely.

Many of the AC/DC radios are quite simple and have no external connections except for an antenna. Barring the unusual power supply, the circuits used in AC/DC radios are very much like those found in the smaller AC-only radios. All external connections run through capacitors, which should be carefully checked and replaced if their DC resistance deviates appreciably from `infinity'. Before taking the unit into regular use, be careful to check that no metal parts, connected to the chassis or the wiring, can be touched from the outside. The Erres depicted on the right was designed very carefully: even when the knobs are completely gone it is not possible to touch any metal part, but the popular Tesla Talisman have their hot parts touchable in such a case. Be extremely aware when the knobs have been replaced or modifications have been carried through: the new knobs or added controls may introduce electrocution risks. Don't put the set into regular use without the back and bottom parts of the cabinet. It is better not to use these radios in damp areas such as the bathroom (;-) or kitchen.

Some more technical points regarding the AC/DC radios. The heaters of the (usually four or five) tubes are connected in series. In the European radios the common heater current is 100mA (tube type numbers start with U), while in the American radios (like this RCA) the current is usually 150mA. The heater voltages add to usually 110 or 120V. In those radios that can be played on 220V (almost all European radios) a large dropping resistor is used in series with the heaters. This large resistor dissipates about 10-12 Watt and becomes very hot. It sometimes contains asbestos.

When the resistor needs replacement you can put new resistors, but it also becomes increasingly popular to put a series capacitor instead; compute the value with the CapCalc spreadsheet in 10 seconds! (Requires Excel.) I did so in my Philips 208U to reduce the amount of heat produced in the small cabinet.

AC rectification in transformerless radios is half-wave and therefore the hum in these radios is always 50 Hz (60 in America) rather than the 100Hz we are used to in AC only radios.

Gerard Tel.