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Tuning the 10.7MHz Intermediate Frequency in Vacuum Tube FM Radios
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posted by Vincent Zhang
Tuning the 10.7MHz Intermediate Frequency in Vacuum Tube FM Radios
A crucial step in tuning vacuum tube FM radios is adjusting the 10.7MHz intermediate frequency (IF). Typically, this is done using a sweep generator. However, in the absence of a sweep generator, the following method allows for meticulous adjustment of the IF curve, achieving results closely approximating those obtained with a sweep generator.There is an example below for you information and learning by analogy.
Procedure:
- Connect the Instruments as Shown in the Diagram:
- Follow the provided diagram to correctly connect all instruments.
- Disable the Local Oscillator:
- Disconnect the circuit at point × to stop the local oscillator from oscillating.
- Maximize Voltage at Point A:
- Connect the probe to point A.
- Adjust the primary core of T3 to maximize the voltage reading on the multimeter.
- Then, adjust the secondary core of T3 to maximize the amplitude displayed on the oscilloscope.
- Maximize Voltage at Point B:
- Connect the probe to point B.
- Adjust both cores of T2 to maximize the voltage reading on the multimeter.
- Maximize Voltage at Point C:
- Connect the probe to point C.
- Adjust both cores of T1 to maximize the voltage reading on the multimeter.
- Optimize Signal at Point C:
- With the probe still connected to point C, re-adjust T3 and T2.
- First, adjust the primary core of T3 to maximize the multimeter reading.
- Then, adjust the secondary core of T3 to maximize the oscilloscope display.
- Finally, adjust both cores of T2 to maximize the multimeter reading.
- Fine-Tune for Optimal Audio Reception:
- Tune to a radio station with good audio quality.
- Make fine adjustments to the secondary core of T3 to achieve the best possible audio quality.
- The adjustment process is now complete.
Additional Information:
- Adding Components for "Magic Eye" Output:
- If a 220K resistor and a 0.01µF capacitor are added to the circuit, point M will become the output for a "magic eye" indicator.
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