Greetings, Aman,
The problems i tend to see varies by the vendor/manufacturer of the board, and the cost. Typically, the higher the cost of the board, the better the type of test the board is put through before being released.
On the older equipment, assuming no user error (blown fuses, fried resistors...or my favorite..."the voltage leaked out of the capacitor as smoke" ) i tend to see problems due to the frequent handling of equipment...BNC or N connectors bent, often where they connect to the board, encoders that have worn out, dim displays, loosened jumpers... In a couple of cases, we have even worn out the memory simply from the number of times the unit has been calibrated (you can only read/write to it so many times after all).
Most often...i see failures with the power supplies. They tend to be OEM'd by the equipment manufacturers rather than produced in-house, and i don't know if the quality isn't up to snuff, or if small changes made in the manufacture of the power supplies that "shouldn't" make any difference indeed do.
Then...on the newer equipment, i see failures that i suspect have to do with either bad solder, slightly misaligned components, or hairline fractures of the boards, that got worsened during shipping. It seems, though, that doing self-repair or component-level repair is going away quickly...so with these failures, especially while the unit is under warranty, we don't spend a lot of time troubleshooting. Even if a site has the ability to do surface mount soldering (i'm fortunate to have great eyesight and a very steady hand), these new chips...even if they can be purchased, require special hoods or mountings to solder them to the board without damaging them in the process.
So...any more...we seem to troubleshoot down to the module, then check a couple obvious things (specifically fuses and connections) and consider ourselves lucky if we can get ahold of a new module.
It has forced us to look at the repair strategy for a company. In some cases, we have selected more expensive products because they gave a longer warranty, had a better repair strategy, were easier to work with, or were more reliable. And...with some instruments...the extended warranty option turned out to be worth its weight in gold. All depends upon how much the equipment gets abused.
:help: ~blush~ Sorry...i can tend to ramble on a bit...