I had this last month.
Brand new IM C300 has been in for a week and their onsite electrician decided to change relays in their mains power box with the power turned on
You're on the right track with the dc power supply theory because in MY case they damaged the bridge relay on the dc pcb.
The flow of power goes from the AC board to the DC board THEN back to the AC.
The DC board mostly just looks after the Fuser.
So when the power came back on onsite the surge went from the AC straight to the DC...fried the bridge relay and arced back to the AC taking out the glass fuses (they did their job).
I replaced the AC board originally and the lights in the building dimmed and the fuse on the AC board popped (repairable luckily).
I did this twice to confirm then replaced the DC pcb and all good.
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