Repairing Valve Equipment like radios, amplifiers and TVs was quite profitable a few decades ago. Partly because the equipment was so expensive but mostly because the circuitry was very simple and easy to diagnose.

With the advent of transistors, the equipment got smaller and cheaper, making repair work barely worth the expense - why pay to have a radio to be repaired when you can buy a new one for half the price and twice the quality. As time went on valves were replaced more and more by transistors and ultimately by ICs (Integrated Circuits).

Valves were still being used in high voltage + high power applications, but as transistors got more and more powerful valves got replaced there too.

Repairing Valve Equipment (radios, amplifies and TVs) was a lot easier than with modern trasistor electronics. The article explains why.