I ask to the ZX81 user is never happens for distraction, to insert the power jack, on the ear or the mic connector.
And sadly power up. On ear and mic connector is any type of DC block protection?
No, the mistake seem to be … safe for the ULA ship.
C10 & C12 can isolate the ULA, but in the french display mod, the transistor TR3 can be toasted by the 9v voltage on the Mic socket.
And the computer start properly.
But, in the real world, the Mic and the Ear jack was connect at any time, and the power socket was the only one to be feed !
No, the main problem is to invert the power supply polarity… it's a component firework… RAM, ULA, the regulator…
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Looking at the schematic for the French version, I can’t see how transistor TR3 would be damaged.