2018-12-31 Zenith Royal 500 Unique Issue

Also to FB group Building Transistor Radios 2018-12-31

I have several Zenith Royal 500 radios with the ‘long range’ eighth transistor added as a RF preamplifier (chassis number begins with 8). See the attached photo. This does not require a third section on the tuning capacitor, as some ‘TRF’ radios have. Zenith added a tuned circuit between this and the next mixer stage. But this tuned circuit was made very broadband so it covered the whole AM band. The broadbanding was done by putting a low value resistor across the capacitor and inductor and that changed the normal peak of the tuned circuit into a flat table with sloping ends. In this schematic it is R1, in parallel with L2.

The Zenith and most other radios of the ’50s and ’60s used carbon composition resistors, which are prone to drifting out of tolerance, typically going higher in value by more than 20 percent. As the value goes up the circuit becomes less functional, until it doesn’t work properly or not at all. The resistor across the tuned circuit might be 2200 ohms, but it drifts higher so it might be 3300 ohms or even higher. The tuned circuit flatness changes to a more rounded peak, giving good reception in the middle of the band but poorer reception at the ends of the band. The resistor can’t be measured in circuit because the coil is a very low resistance in parallel. It must be removed to measure its resistance. If it has been removed, it is very easy to replace it with a carbon film resistor which will not have the drift problem.

An alternate way of checking the performance of the tuned circuit is to check it with a sweep generator, but I don’t know if there are sweep generators broad enough to cover the whole AM broadcast band.

If there are a lot of radio stations in the area and they are strong in the middle but weak at the ends of the band, it might give an indication that this is the problem. But it isn’t certain because other factors could cause a similar problem. In fact, the tech may be trying to align the radio but have the problem with weak performance at the ends of the band and believe it is being caused by poor tracking of the two sections of the tuning capacitor. This may cause the tech to spend a lot of time trying to get the two sections to track, when the problem isn’t there but instead the RF preamplifier’s broadband tuned circuit. Let’s hope this information saves the tech some time in the future.

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