Oscillator Coils

(The conventional oscillator coil is an even simpler device than the average antenna or RF coil. The method of checking the coil itself need hardly be explained. An ohm-meter is usually all that is necessary.

To prove that an oscillator is working, it is only necessary to touch the probe of the Oscillator channel of the Analyst to the stator connection of the oscillator tuning condenser, and at the same time, with the Attenuator set for maximum sensitivity, the dial of the Oscillator channel, is rotated to see if any oscillator voltage is picked up at any frequency within the range of the Analyst.

If a voltage is picked up, its frequency can be measured most accurately if the oscillator probe is placed not on the stator connection itself but merely close to it, so as to disturb the circuit least. When the oscillator probe is so placed, the pickup from the oscillator circuit may be so low that the indicator shadow will not close completely and, in fact, may give only a slight flicker as the tuning dial is rotated through resonance, but enough indication will be obtained to determine the oscillator frequency.

Another method of checking for oscillation, but one that gives no hint of the operating frequency, or a method that may be used for checking oscillators out of the tuning range -of the Analyst, is to check the voltage developed across the grid-leak of the oscillator, if it has one. For this test the prod of the Electronic Voltmeter channel is touched to the grid of the oscillator tube and the voltage measured. The isolating resistor in the prod handle should prevent the measuring circuit from having any influence on the oscillator unless it was in a very unstable condition on the border line between operating and not operating. If this method of testing is used it is always advisable to perform the test in two parts. First, check the voltage across the grid-leak, then short-circuit the oscillator tuning condenser to observe that the voltage drops to zero when the condenser is short-circuited, proving that the voltage measured is actually caused by the oscillation.

The fact that an oscillator works and delivers the proper amount of output voltage does not necessarily mean that the oscillator circuit is free from trouble It may be operating at an incorrect frequency due to some fault in the padding circuit, or in the switching ol pads in multiple-wave receivers. In high-frequency oscillator circuits, the frequency is sometimes incorrect because a by-pass condenser has opened up, increasing the length of conductor in the circuit which can cause quite an appreciable effect when the frequency of the circuit is high enough, because a large part of the circuit inductance is in the leads themselves.

If the receiver is operating properly up to and including the Mixer, the oscillator frequency should be equal to the sum of the intermediate frequency plus the input-signal frequency.

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