In many classic scopes, the high voltage transformer that supplies the cathode, blanking, and anode voltages has a tendency to break down. This is mostly because of aging of the potting material used for the high-voltage windings. When this happens, the transformer will run very hot because dielectric loss is too high. It is a thermal runaway.
Currently, the only options when this happens are to find a similar transformer from another scope, or to get a rewound transformer.
A second option is to improve the cooling of the high voltage transformer. This approach has been tested with success on a 556's HV transformer.
The same HV transformer, part number 120-0308, is used in the 543B, 544, 545B, 546 and 547 . Chuck Harris, who frequents the Yahoo Tekscopes forum, has rebuilt several 120-0308 transformers with good results.