Discrete components

Older amplifiers from 1950s to 1980s generally use "discrete" components - individual resistors, capacitors, transistors etc mounted on circuit boards. Often when a component fails it can be replaced with like-for-like new parts.

However, some parts (ie Sony v-fet transistors) are no longer in production and are impossible to source.

IC chips

More modern amplifier designs replace sections of circuit boards with IC chips (ie STK chips) which can sometimes be difficult to find reliable replacements for.

Proprietary components

"Proprietary" components are produced specifically for certain models and cannot be replaced with alternative stock components. Generally, if proprietary components have failed then unfortunately it's virtually impossible to carry out repairs successfully.