Marconiphone T21A

The Marconiphone T21A radio receiver was issued in July, 1949. It cost £17 9s 6d, plus purchase tax to buy at that time. It is an A.C Mains powered device, but the difference is that its mains transformer is that of an Auto-Transformer type - a single winding used to drop the mains voltage in order to supply the heater chain for the valves. Hence the chassis is not isolated, so there is a risk of it becoming "live", (depending on how the mains plug is wired up). It is best to make sure that the neutral wire of the mains lead is connected to the receiver chassis ! The radio waveband coverages are Short :- 16.5 to 52 mtrs Medium :- 192 to 570 mtrs Long :- 900 to 2,000 mtrs There are also facilities for three preset stations. The valves employed in this receiver are:- X147 - Frequency Changer W147 - I.f Amplifier DH147 - Double Diode Detector & Audio Amplifier N147 - Output Valve U147 - Rectifier When I first obtained this radio, the cabinet was in a bad state - the veneering had peeled away due to damp storage conditions. I eventually obtained a replacement cabinet with its veneer in a much better state. With regards to getting the radio repaired, I noticed that the wax capacitors required replacement. I replaced them with polypropylene (1,000 volts d.c wkg.) types. The main concern, however, was that the U147 Rectifier valve anodes were connected together by a special wirewound resistor (centre-tapped). This resistor was to contol voltage surges to the Rectifier tube, and its value was 240 ohms total, but each section of the "tap" was 120 ohms. Naturally, the device was completely open circuit, and finding an equivalent resistor was simply "out of the question". To get around this situation, I bought two 120 ohm resistors (about 7watt -cement types) from Maplin Electronics in Leeds, and wired them in circuit to the same valve pins as the original had been. It was a successful repair, as the radio then came back to life. The reason for using a centre tapped resistor on the anodes of the U147 Rectifier, is that its two sections are then wired in parallel, in order to use it as a half-wave rectifier. I had to replace the two scale lamp bulbs (6.5 Volts @ 0.3 Amps m.e.s). When working on the chassis out of its cabinet, some care needs to be taken that the dial lamps do not make contact with the chassis, or some damage can occur to the Auto - Transformer. The lampholders have to be removed from their location places behind the dial plate, since the dial and its plate are not removed from the cabinet when the chassis is removed for servicing (although there is a servicing hatch under the cabinet). I had to re-string the dial cord, as this was found to be broken. This set has a real nice looking cabinet !