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Is Q4 shorted?
You should be able to tell if there is a shorted Zener by checking the outputs of the power supply for a short to ground. Without a schematic might take some guess work. They typically put a Zener slightly higher rated than the normal voltage on one of he outputs to act as a clamp in case of excess voltage to intentionally blow the fuse and protect the electronics from damage. Was the original fuse blackened? Might have been hit with a power surge.
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