How does S-75 SAM's SNR-75/Fan Song radar works?

[Dilandu]

With the Soviets using almost all short-term conscript manpower and often iffy supply availability they would still have one or two missiles that worked in a battery and the fire controls likely would work. Their problem would be compounded by poor levels of training and a general lack of live fire practice.

Considering that during Vietnam war those field crews managed to restore about half of all damaged (mainly by fragments) missiles into working conditions, I think you seriously underestimate both them and quality of components, honestly.

In terms of reliability, that really depends on the crew and parts availability.

The S-75 missiles were from the beginning designed to accomodate a mobile system, working in field conditions. They were not intended to be stored in bunkers till ordered to launcher. So it's more likely reliability was traded for required ruggedness - the missile must work in far suboptimal conditions, even not as reliable as in perfect ones.