EKCO AD36 Vintage Radio Receiver

EKCO Radios Includes:
EKCO round radio series     EKCO AD36     EKCO AD65     EKCO A22     EKCO AD75    

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The EKCO AD36 was one of the last TRF or tuned radio frequency radios to be made by EKCO. It was introduced into their range as a low cost radio because the cost of radios represented a major expense for families.

It was one of their classic round radios which had Art Deco style and manufactured using a Bakelite case.

In view of their styling, these vintage radios, along with others in the series from EKCO are still sought after today as classic antique radios.

AD36 specification

The AD36 had a reasonably good specifications considering that it was a tuned radio frequency receiver. Its attraction over other superhet radios was a significant cost reduction.

Although the AD36 used the same number of valves as the equivalent superhet radios, the circuit was much more straightforward using fewer wound components and this reduced the cost, making it more affordable for many households.

Operation was a little more difficult than that of a superhet, but in general it was easy to use. The performance was also not up to the same standard as that of the superhet radios.

AD36 front panel

The EKCO AD36 front panel is dominated by everything being circular or round. Not only is the case round by so are all the other elements that are visible on the front.

As expected with a radio from EKCO, the case is made from Bakelite and it is rounded at the edges to give a smooth contour.

The loudspeaker grill takes the centre of the whole of the front of the radio. The loudspeaker grill is circular, although at the bottom there is a circular piece that is taken out to enable the semicircular tuning dial to take a central position at the bottom of the front area of the case.

The dial is calibrated not only in the wavelength in metres, but also stations are marked on the dial for easy reference and selection.

There are three controls to the bottom of the front of the case. The main control at the centre is for tuning. This is a standard tuning control for selecting the stations, and there is a link to enable several turns of the tuning knob to be required for a complete scan of the band in question. This makes tuning much easier.

At the bottom left is the volume control, which is a capacitor on the input tuned circuit and works by balancing out the input to prevent overload from local stations and also reducing the volume.

To the right of the tuning knob there is the reaction control. This controls that level of regeneration in the leaky grid detector. Increasing the level of reaction increases the sensitivity and selectivity, but ultimately the circuit will burst into oscillation.

The band switch is rather unusually located on the lower left hand side of the radio as it is only possible to have three controls on the front of the radio to maintain the looks of the design.

Valve line-up

A total of four valves were used within the AD36 vintage radio - three for use within the actual radio parts of the circuit, and one for the power input rectifier. Interestingly, this is the same number of valves that were used for some of the superhet models of the EKCO round radios.

EKCO AD36 circuit description

The EKCO AD36 is a classic vintage radio, and the circuit is a relatively straightforward tuned radio frequency design using four valves.

However there are some warnings that need to be kept in mind when using or servicing this vintage radio as soem of the techniques used are no longer standard and would certainly not conform to modern safety standards.

When working on the radio, the fact that the chassis is connected directly to the mains must be kept in mind at all times and it presents a significant electric shock hazard.

Fortunately all the controls are isolated from mains by the fact that they are made from insulating material, but the spindles themselves are connected to the circuit and they also present a shock hazard when the knobs are removed.

However it is essential to make sure that the mains neutral line is the one tat is connected to the chassis. Whilst this does not make the radio totally safe, it reduces the risk of shocks.

There are several different areas to the overall AD36 receiver circuit diagram.

• Aerial input:   The aerial enters the receiver and immediately the signal pass through an isolating "condenser' or capacitor. It is worth remembering these capacitors may deteriorate with age and could become leaky, so it is worth ensuring that no mains reaches the antenna from the circuitry which is directly connected to the mains.

  The signals are then differentially coupled to the main input inductor arrangement via a differential variable capacitor. There are switched taps on the coils to provide the required impedance match. Also the coil arrangement is switched to provide coverage of the long and medium wave bands.

  One capacitor in this arrangement is used to provide a "zero balance" and provides a means of obtaining a low minimum input from a powerful local transmitter which might otherwise swamp the receiver.

• RF amplifier and V1:   The first valve of the receiver, V1 is a Mullard SP13. This is a metallised RF pentode that operates on the signal frequency.

  This circuit is then coupled to the next stage using an inductor in the anode to act as a load and a capacitor to provide the link to the next stage.

• Detector:   The second valve, V2, in the AD36 is a Mullard metallised HL13 triode. This stage acts as a leaky grid detector and incorporates reaction to increase the selectivity and gain.

  This is coupled to V3 by using a resistor in the anode of V2 as the load and a coupling capacitor.

• Audio output:   The final valve, V3, in the receiver lineup is a Brimar 1D5. This stage is an output pentode with a transformer for the loudspeaker in the anode circuit.

  Tone correction, as it is termed is provided by placing a capacitor between the anode and cathode of the circuit.

• Mains power input:   The radio operates on both AC and DC. Incoming power passes into the rectified diode which acts as a half wave rectifier. The diode remains in circuit for DC, where it causes a small loss of voltage which does not affect the operation of the circuit.

  Smoothing for the circuit is provided by the loudspeaker field coil and two electrolytic capacitors, one either side of the coil.

• Valve heaters:   The valve heaters along with the scale lamp which is shunted by a resistor to enable the right current to flow and also the ballast resistor are all connected in series across the mains input. There is also a filter at the input which suppresses any incoming mans borne interference. Even in those days EMC was an issue!

The EKCO AD36 was a low cost version of their round radio range. As the cost of any radio would have been a major expense to any household, having a low cost radio with slightly lower cost would have appealed to many people.

The AD36 is well sought after today as a valuable vintage radio, and they are soon bought when they appear on the antique radio markets.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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