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I agree, I would be looking at the various CRT gun potentials, and comparing them to the schematic or the CRT datasheets. Is the 5ADPx even a valid sub for a 5CPx?
The DuMont 304 was one of the first scopes I ever played around with. They were probably DuMont's best selling scope, and a pretty rudimentary circuit to repair. Check the HV caps for leakage, and the 2X2 tubes. Check the divider resistors in the focus/intensity control circuits, and replace any that are out of tolerance.
Be VERY careful measuring HV potentials in this circuit. The voltages are lower than a typical TV, but the 60Hz supply is capable of delivering a lot more current.
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