Three-point tracking in AM broadcast receivers To achieve three-point tracking (alignment at low, mid, and high ends of the band) in a superheterodyne receiver, which component is specifically adjusted or added in the oscillator section along with RF trimmers?

Introduction / Context:
Tracking ensures that the RF tuned circuit and the local oscillator maintain the correct frequency relationship across the tuning band. Three-point tracking refines this alignment at the low, mid, and high ends of the band to minimize dial error and maintain sensitivity/selectivity.

Given Data / Assumptions:

• Single-conversion AM superheterodyne with a gang (ganged) tuning capacitor.
• RF and oscillator sections each tuned; small alignment capacitors are available.
• The goal is precise oscillator-to-RF tracking at multiple points.

Concept / Approach:

While the gang capacitor tunes both RF and LO simultaneously, fine correction is needed because coil stray capacitances and mechanical tolerances cause frequency-dependent errors. A padder capacitor (usually a series capacitor in the oscillator tank) is adjusted to set the low-end tracking, while trimmer capacitors are used to tweak the high-end alignment. Together, these provide three-point tracking across the band.

Step-by-Step Solution:

Verification / Alternative check:

Alignment procedures in service literature specify using a padder for the low end and trimmers for the high end, achieving three-point tracking with the ganged tuning system.

Why Other Options Are Wrong:

“Gang condenser” is the main tuning element but is not the specific adjustment for low-end tracking. “Variable selectivity network” affects IF bandwidth, not tracking. “Any of the above” is incorrect because selectivity networks and AGC capacitors do not correct tracking.

Common Pitfalls:

Attempting to fix tracking errors by misaligning IF; proper method is padder (low end) plus trimmers (high end) with the gang capacitor.

Final Answer:

Padder capacitor