Zenith Royal 3000-1 Fuzzy Sound

The first audio frequency (AF) stage consists of a transistor 121-374 (2N2439) which amplifies the signal from the final IF stage. From this point forward, there is sufficient AF component in the signal for power amplification to drive a loudspeaker.

The power amplifier circuit consists of a driver transistor 121-375 (2N2438) which drives a matched pair of transistors 121-373 (2N2431) in a Class-B push-pull configuration. Since each transistor deals with half of the waveform, a transformer T13 is used to recombine the signal in the primary windings whilst the secondary windings drive the 3.2 Ω loudspeaker producing 500 mW output.

The sound quality of this type of output stage is very reminiscent of valve amplifiers producing that warm glowing sound rich in even harmonics. A 4-inch × 6-inch oval speaker mounted in a metal enclosure produces that unique clear sound that is ear splitting loud.

It's The Fuzz...

The fuzzy low volume sound was the easiest to solve, because in a Class-B push-pull output stage, there are two transistors, each amplifying opposing halves of the waveform. If one transistor were to fail, by going open, then the audio output would be half-waveform from only one transistor, hence the fuzzy low-volume sound. A quick inspection on the circuit diagram revealed a matched pair of transistors 121-373 performing this function. They are easily located on the chassis situated side-by-side.

I pulled both transistors out and the sound went completely, and then tested each one by inserting it back in on its own. Sure enough, one transistor was completely open. Well, that was not so bad I thought to myself smugly, I am only five minutes into the diagnostics and found one fault already. I found an equivalent matched pair of replacement transistors, which came out of an old Ferranti set so I decided to use those instead. However, whilst experimenting, I noticed that a matched pair of transistors 121-47 can also replace the matched pair (121-373) for the power amplifier stage. The new output transistors solved the problem and the sound was ear splitting loud and clear. It occurred to me that maybe these transistors can also fail by going short, in which case I would anticipate a hum due to the overloading in the power supply rail. However, in my case, the transistor had gone open.

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