The diode may function as a reverse polarity protector for the power jack. If the circuit expects a positive power tip, for instance, and you accidentally connect a negative power tip, the diode blocks the power connection. If this is the case, you ought to expect about 4.3 VDC on the diode's cathode when it's connected to a 5 VDC adapter. You need to disconnect one leg of the diode to make any meaningful measurements. Otherwise, the remainder of the circuit interferes.