Almost from day one, battery eliminators were used to reduce the cost of running battery-powered radios. That’s because portable radios were so convenient that they were often used in the home as well as outdoors, but the cost of running a set on batteries alone was prohibitive.

The Monarch BLK battery eliminator front panel.

To overcome this problem, the radio manufacturers developed mains-powered battery "eliminators". The Monarch BLK battery eliminator of 1947 is one such device and was a big improvement on the designs used in the 1920s. Monarch equipment was made by Eclipse Radio and family resemblances to Peter Pan and Astor can be seen.

The Monarch’s nominal output voltages suits sets using 1.4V valves with 90V high tension (HT). Physically, it is about twice the size of two 45V batteries and it has just one control – an on-off switch. The unit, in its original form, had a 4-core lead and an octal plug on the end of the lead so that it could plug into the sets that it was designed to power. I decided that I would use it as a general-purpose eliminator instead of one designed specifically for particular sets hence the unterminated wiring on the unit shown in the photos.

To dismantle it, the control knob is first removed and the front panel sprung to remove it. After that, it’s simply a matter of undoing three screws so that the works can be removed from the case.

As can be seen from the circuit diagram (Fig.1), the unit is quite simple and so the restoration is also simple. There are two secondary windings on the transformer – a high-voltage wind-ing to supply nominally 90V for the HT supply and a centre-tapped 6V (3V + 3V) winding to power the filaments and indicator lamp.

The high-voltage winding employs a half-wave dry metal "Westalite" rectifier. The HT is filtered in the normal way using two electrolytic capacitors and a 1kΩ resistor and this is quite effective. The actual HT voltage developed depends on the current drain. With no load, it is around 115V. The designed maximum current drain is 20mA.