How does an oscillator work? We learned that the oscillator is tuned by the variable condenser, and is added to the radio signal in the converter tube. Now let’s learn a little about how that signal is actually created.

Looking at a close-up of our schematic, we see that the oscillator circuit is comprised of, the converter tube cathode, grids one and two, the oscillator transformer, one section of the variable condenser, a resistor, and two capacitors.

The transformer primary, sends a positive voltage to grid two. This attracts electrons from the negatively charged cathode, and current begins to flow to grid two.

The magnetic field from the primary, is picked up by the secondary, sending a positive voltage to grid one. This causes even more current to flow.

The resistor only allows so much current from cathode to grid one, though, and as it reaches its limit, it reduces the voltage on grid one, causing less current to flow to grid two, and less current to flow through the resistor.

Very quickly, though, grid two and the transformer start current flow to increase again, and the process repeats, hundreds or thousands of times a second. This creates our oscillator frequency, which alternates at the same rate.

Also in the circuit is capacitor C12 used as a bypass, and C13 which keeps direct current off grid one.

C2 and 11 are the oscillator section of our variable condenser. Their capacitance, along with the coil’s inductance, rated in henries, determine the oscillator frequency.

As with the antenna coil, the oscillator coil in our radio is actually three coils. One primary, a secondary for the AM band, and a secondary for the shortwave band. The same switch that selected the secondary in the antenna coil, also selects the secondary in the oscillator coil.

Antique radio coils usually work fine, but sometimes, the tiny wires can become damaged. Coils are easily tested with a multimeter... And should show a measurable resistance. If it tests open or as short the coil will require repair.

Fortunately, the coils in our radio were all working correctly. It’s possible to fix or rewind a broken coil, but it can be a delicate and time-consuming operation

#radiorepair #radio #electronicsrepair #restoration

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