Most legacy paper capacitors in electronic equipment have been replaced by which modern construction type due to improved reliability and stability?

Introduction:
Older paper-dielectric capacitors suffered from moisture absorption, leakage, and value drift. Advances in materials science led to plastic film dielectrics that offer far better performance and reliability in most general-purpose applications.

Given Data / Assumptions:

• Comparison is among widely used capacitor constructions in low-to-medium voltage electronics.
• Focus on stability, leakage, and manufacturability.

Concept / Approach:
Plastic films (e.g., polyester, polypropylene) provide low dielectric absorption, good insulation resistance, and stable capacitance over temperature and time. Metallized-film variants self-heal after minor dielectric breakdowns, further improving reliability over paper types.

Step-by-Step Solution:
Identify limitations of paper: moisture ingress raises leakage current and changes capacitance.Compare with plastic films: lower moisture permeability and better dielectric strength.Assess practical outcomes: longer life, smaller size for same rating, and tighter tolerances.

Verification / Alternative check:
Modern catalogs show broad availability of polyester and polypropylene film capacitors replacing paper in coupling, timing, and filtering roles, confirming the industry shift.

Why Other Options Are Wrong:

• Electrolytic: High capacitance per volume but higher leakage; not a drop-in for paper in many signal paths.
• Oxide casing on paper foil: Retains paper's weaknesses.
• Waxed paper reissue: Historical, not an upgrade.
• Mica-silver: Excellent stability but cost/size limit universal replacement.

Common Pitfalls:

• Equating high capacitance (electrolytics) with overall superiority in all roles.
• Confusing dielectric constant with reliability metrics like leakage and self-healing.

Final Answer:
Plastic film (polyester, polypropylene, etc.)