My father-in-law asked me to look at a 2 year-old Sony boombox which he said suddenly "doesn't work". It has a CD player, cassette player, and AM/FM tuner. After playing with the controls I find that the CD player reads CD's and displays appropriate stuff on the LCD, and the tuner also tunes stations and displays the frequencies appropriately. But, in fact, the only audio that gets to the amplifier is that generated by the cassette player, regarless of what source is selected.
The switching between CD/tuner/tape is acomplished using electronic pusbuttons. So you can select CD, and the CD spins, but the audio that is routed to the amplifier is actually the cassette player. The cassette player is controlled using old-technology spring-loaded keys.
So the thing is stuck in "tape" mode, even though the logic behind the electronic controls think that the CD or radio is actually selected. I'm still convinced that there is a mechanical switch or sensor that became stuck somewhere when they last used the cassette player. How to troubleshoot without schematic?