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Chuck Harris
It is a pretty well known phenomenon... well discussed
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on this group over the years... often by me. Around about the time the 547/545B scopes got designed, tektronix decided to make a universal HV compartment that was plastic, contained the EHT, the 5642 rectifiers, and the capacitors and resistors. The compartment was sealed to help keep dirt, critters, and carbon dust out. The sealing also prevented good cooling due to air motion. This required the transformer to run at a higher temperature than was allowed in the earlier 500 series scopes. The earlier scopes used beeswax to impregnate the transformer windings and tektronix was afraid that the wax would melt in the new sealed compartments (newsflash: it doesn't to any great degree... but I digress.) Instead of using beeswax to improve the HV breakdown characteristics of the EHT, they used an epoxy varnish to coat the windings. Because epoxy is rigid, and the tiny wires used in the EHT were easily broken, they added something to the epoxy to soften it, and probably also something to act as a fire retardant. The epoxy degraded over time, and greatly increased the losses inside of the EHT, making it thermally runaway. This has been dubbed the transformer epoxy disease. Some improvement has been witnessed from long baking at high temperatures, but it is only temporary, as the epoxy truly has changed. The only real answer is to replace the winding with new. Beeswax works better than the epoxy as when it gets laden with moisture, it gets hot, melts the beeswax, and releases the water vapor. Once the water vapor is gone, the EHT goes back to its usual warm self. The 560 series transformers were epoxy varnished, and then potted in an aluminum can using a clear silicon RTV. They still fail at about the same rate as the open core transformers in the 545B/547. Some have opined that the failure mechanism is due to the fire retardant that was added to the epoxy... if there was a fire retardant added to the epoxy. This opinion comes from a long history of problems that have come from fire retardant additives. -Chuck Harris petertech99h via groups.io wrote:
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