You are right about how it can often be a cheap part that fails on a pcb, capacitors are often the most common failure. Most boards are sealed with conformal coating which can be removed with a strong solvent like acetone. For the most part manufacturers don't repair them, it's just a replacement. However, if you could do a visual inspection on the board and spot an obvious fault like a swollen capacitor can or a dark burnt looking component like a diode or even a burnt track then there would be a possibility of repair without even needing a circuit diagram. That being said I have no idea of the history on the failure of these boards, it could also be an IC in which case there'd be no chance of repair.