Test diode (D1) nearest the barrel jack. A diode can be thought of as a switch: “closed” when forward-biased and “open” when reverse-biased.

Diode circuit voltage measurement figures: (a) Forward biased. (b) Reverse biased.

In forward bias condition, around a 0.7V drop is expected, like a closed switch that drops 0.7V across it. In reverse bias, the multimeter should show 'OL' (over limit) or a very high resistance, like an open switch. Mine showed 1.2V when reverse biased, which indicates a bad diode. It was also visibly cracked.