Both electronic and mechanical components experience wear and other changes over time.
Capacitors are common wearable electronic parts, specifically wet aluminium electrolytic types. Wear rates vary according to type, quality of construction, thermal loading, age and hours of use. I’ve seen capacitors fail after 5 years and older parts that are still perfect after 50 years, which is extraordinary and common for Elna capacitors for example.
Transistors, diodes and resistors also deteriorate and can fail, sometimes catastrophically. Failure tends to follow certain types, scenarios and the old nemesis – heat. Some resistors are prone to increasing in value or becoming noisy. Likewise, certain small signal transistors and diodes are ‘fragile’ and commonly fail, others fail when pushed close to their voltage or thermal limits.
Testing is the only accurate way to determine deterioration and wear. Parts can be tested, measured and replaced with new ones that meet or exceed OE specs in most cases.
Belts, gears, switches, potentiometers, knobs, meters and connectors are the most common mechanical parts that suffer from deterioration and wear. This includes rotary and linear controls and switches, speaker terminals, RCA connectors and so on.
Many of these components can be replaced and others are serviceable and can be brought back from non-functional to working perfectly, with the correct service techniques. Other parts like connectors can be replaced with new parts.