Re: Quick question 564B

Chuck Harris <cfharris@...>

It is a pretty well known phenomenon... well discussed
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

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 wrote:

Please tell me more about the 564B HV transformer epoxy disease!

Pete On Sunday, January 3, 2021, 8:07:20 a.m. EST, Chuck Harris <> wrote:

It is a cute little scope, but very low bandwidth,
something like 10MHz.  Its HV transformer suffers
from the epoxy disease that affects 547 scopes.

They are quiet, not having a fan, and the plugins
contain all the deflection circuitry (as I recall).

I used to like them quite a lot back in the day, for
audio work.

The storage functions are finicky.

-Chuck Harris

Stephen wrote:
Hi all, and Happy New Year 2021,

Is a 564B a good grab, is it desirable at all?
It’s not considered a 500 series, right?
56x family I suppose...

Thanks 😊

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