Zenith Royal 3000-1 Schematic

Looking at the schematic / circuit diagram, this is a straightforward heterodyne design with a positive chassis ground. For some it will take seconds to understand it, whilst others will take the rest of their natural lives. The frequency modulation (FM) front end is separate from the amplitude modulation (AM) band circuitry, however both share the same intermediate frequency (IF) stages and the final audio amplifier circuitry.

The first, second and third stages of IF comprise of 121-352 transistors (2N2089). Each stage amplifies the signal within a specific frequency band nailed by the slug-tuned coils. These tuned stages will amplify signals vectoring at 455 kHz, and reject anything else. Both AM and FM bands share these stages, so providing you get sound from at least one band, then these transistors will be fine and you will need to focus on transistors critical to the missing bands.

For the AM reception, a local AM oscillator circuit consisting of transistor 121-350 (2N2089) produces a sine wave of frequency specific for each band. The AM mixer circuit consisting of the 121-351 (2N2092) transistor mixes the radio frequency (RF) with the local oscillator signal. The resultant of this mixing process produces signals of higher and lower frequencies, and the IF stages are tuned to amplify the lower ones.

FM Band

The FM band sounds a little different compared to modern receivers, because it has a ratio detector at the IF stage. This results in a lack of hiss that would normally accompany the FM band in modern receivers. Many valve hi-fi purists would regard this with some surprise; however, this type of detection produces just as linear a response as the Foster-Seeley discriminator, which was the standard with valve technology back then. Mathematically, we can show that both types of detection are prone to distortion, however the designers have cleverly implemented the IF amplifier stages with a "Gaussian" response to eliminate any possibility of this occurring. My first thought was that these people really understand radio engineering; I suppose that is why they were one of the most successful radio manufacturers in the world.

RCA Socket Circuit

This Royal 3000 radio has a RCA socket on the chassis at the back marked "Tuner", which provides an audio output signal to feed a hi-fi amplifier. When using this facility, the volume control is required to be at the minimum position to prevent the speaker behaving as a pickup microphone.

By tapping the signal from the collector junction of transistor 121-374, which is the output of the first audio stage, it has the correct amplitude to feed an external hi-fi amplifier. At this point, capacitor C57 provides inter-stage decoupling; therefore, it is most suitable as an output to other amplifier equipment. However, this same point is also the input to the driver stage, because the audio signal passes through the potentiometer, and its wiper feeds the base junction of the driver transistor 121-375. Therefore, this arrangement will also amplify signals fed into the RCA socket.

In the previous model, the Royal 1000, they provided a two-position switch to choose the function of this socket. In the "Phono" position, it behaved as input to amplify signals from a high impedance cartridge of a record player. In the "Radio" position, it behaved as an output providing an audio signal to feed an external hi-fi amplifier.

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