Before becoming interested in synthesizers, I was interested in guitar effects boxes. The 1/4" jacks are always going bad in those. Usually it is metal fatigue on the piece that contacts the tip of the plug. This piece breaks after hundreds of insertion cycles. Or the metal piece bends enough that it does not contact the tip. The encased jack is usually found mounted on a circuit board and the open jack is mounted onto a panel.
On stomp boxes, the 15 cent 'tact' switch for the foot switch is always breaking or becoming deformed, requiring replacement.
On synths, it is the capacitors in the power supply that wear out after twenty years. And the backlights (for LCD display screens) go dim or dead after ten years. Character LCD displays are straightforward to replace and cost $5-$15. Graphic displays require getting one that is exactly compatable. Modern graphic LCD displays with an internal backlight cost $20-$60. Often the synth or tone module has to be completely disassembled to access a screen replacement.
Occasionally the 120VAC power supply for North America needs to be modified for 220VAC when a used synth gets bought on eBay and shipped to Europe. Or the ceramic resonators that comprise the main clock circuit of the CPU start failing intermittently. This causes the power to be cycled many times before the unit actually turns on and stays on. A crystal oscillator of the same frequency as the resonator fixes this.
Keyboards fail wheneven a drunken dancer knocks over a pitcher of beer onto the keyboard and sogs the membrane switches. And there are always broken cases and circuit boards when someone drops the keyboard on a concrete floor.
Aside from all that, MIDI equipment holds up well as hardware. Where they fail is software obselesence. Editors and sequencers that worked fine on Windows 95 don't work at all on XP. MIDI interfaces that were easy to program in DOS become impossibly difficult to program using obscure APIs in C## or Visual Net.
I've given up trying to program MIDI on the PC. I use inexpensive microcontrollers like the Atmel AVR to make custom MIDI controllers in order to change patches and parameters. And I've given up on General MIDI ROMplers. Heard one: heard them all. Only real synths for me from now on.
