I repaired one last year. And if memory serves, it was a burnt regulator. Those older units, like many digital devices of that era, used more current than recent devices do. The heat-sinking on the 3-pin regulators was fairly modest, and it is also easy to imagine the thermal compound coupling the regulator to the heatsink drying out over time, allowing one or more of the regulators to overheat and burnt out. I'm not saying that is necessarily what it is, but based on my own experience, that's a good place to start. Check the state of the thermal compound as well.
Earlier this year, I repaired a Diamond Memory Lane pedal for a buddy, and it took several e-mail exchanges with the company's tech support until their tech guy remembered that in one of the earliest runs of the pedal, the distributor they bought their regulators from had sent them a pile of defective regulators, whose heat sink tab was about 1/3 the normal thickness. When they'd power up the pedals to calibrate them, they'd test fine. But because the heatsink was so thin, and the regulator was free-standing without additional heatsinking, it would begin to overheat after about 15 minutes, and drift off-spec, yielding a very annoyingly audible HF whine. Since I imagine they'd set up and calibrate the pedals in less time than that, it took them a while (and probably some customer feedback) to identify the source of the problem.
Three-pin regulators are wonderful things, but they all demand certain thermal conditions to show their best.