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This was a 10 ohm, 1W carbon resistor. I replaced it with a 5w resistor (only one I had) and measured the voltage over the new one when switching on. This voltage was around 3v after a minute or so, which would mean 0.9W dissipation, which sounds fine.
The only thing left to do was to fix the deflection amplifiers to that they have the same deflection for the same signal:
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