The component in there is only present in the mk2a, and helps to protect a 74LVX04 (IC33); on a mk2, pin 9 of one of these (the one nearest the unused internal LCD connector) connected directly to the power jack switch.

If AC/DC sense dies on a MK2, and the fault is *not* with the plug itself, then you need to swap out IC33 which is a bit harder than a SOT23 but not too bad. The biggest issue is getting a TSSOP 74LVX04, in fact.

Hugo