HV transformers

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 120-308, is used in the 543B, 544, 545B, 546 and 547 . Chuck Harris, who frequents the groups.io (formerly Yahoo) Tekscopes forum, has rebuilt several 120-308 transformers with good results.

Documents

• "HV Transformers for Tektronix 500 Series Scopes Wound on ETD44 Core" by Marco Morelli

Pictures

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  Alternative to replacement: cooling with a heat sink

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  Two different HV transformers for 561B & 568. Brown potting (120-0466-00) is bad, clear potting (120-0466-01) is ‎an improved variant to avoid the thermal runaway problems.