See my Comment RE the Magnuson–Moss Warranty Act, above.
The act prevents manufacturers voiding the warranty, But it doesn't require manufacturers to make the
technical information and diagnostic tools available, so you can use your own engineering knowhow, or hire
your own engineer to diagnose and repair.
The manufacturers leverage copyright and the DMCA, along with Potting/sealing components and using special
"Genuine/Counterfeit part detection" logic in software to prevent people from replacing simple components,
such as analog sensors.
These days they also started keeping their schematics secret, and if you share technical
information, the big company will use their lawyers and sue or threat of a suit to chill any dissemination of repair details
You are absolutely correct. When I was a rocket scientist, charged with root-cause failure analysis of components, and bound by NDAs out the wazoo, there were still issues. I was supposed to find a single, specific trace in an IC. The thing was in the third of five layers, and less than 100 micrometers long! An impossible task. It was a 'Program-Secret' component (i.e., above Top Secret, and above my clearance level). Despite the burn rate of "having a rocket ready for the launch-pad" costing $3M–$5M per day as the burn rate, it was critically important, yet delayed over the contractor's jitters. Yet, it took the manufacturer of the IC (BAE, Raytheon, or some spook company) ten days to get me the EE-oriented schematics before I could take apart the physical, real-world component. It was stupid. I needed to know where exactly to look, and for that you need a map.
So, yeah, for the average Joe, You are never going to get the schematics. Even the "nation's best experts" have great difficulty in obtaining schematics.
Go to www.sparkfun.com. But a Raspberry Pi. Buy an NI MyDAQ. You can do it on your own, but modern-day 'finished products' are almost impossible to modify on your own – by design.
Oh, don't ask if i got a bonus for saving $10M's. I did not. I received a color laser-jet printed commendation, in a plastic frame and all, for my saving a rocket launch. Fuck the MIC.