Re: 549 transformer question

Chuck Harris <cfharris@...>

Hi Roger,

We know everything about the cores, as the full Allen-Bradley
specifications are all over the net. It is A-B's "W5" material,
and the core shape is an E core.

A-B number for the ungapped half is: E1960S011A
A-B number for the gapped half is: E1960S021A.

The gap is 0.015" and on the center leg.

Le = 2.98" (effective length)
Ae = 0.271 sqin (effective area)
Ve = 0.812 cuin (effective volume)
Aw = 0.287 sqin (window area)

Saturation Flux Density 4900 Gauss @ 10 Oersteds @ 16KHz
Initial Permeability 2800 ui @ 16KHz
Core Loss (Pcl), (uW/cm3 -Hz) @ 1350 Gauss 2.5uW/cm3-Hz

TDK bought A-B, and liquidated them. TDK made everything that
A-B did, except the old fashioned cores, which they did not want
to make. A-B was bought to eliminate a competitor.

And being a gapped core, with 0.015"" gap, the core
material characteristics are relatively unimportant.

The problem is the core's shape. The core is of the E style,
but it has an octagonal center leg, and two rectangular outside

The dimensions are:

overall thickness = 0.52"
overall length = 1.960"
max winding diameter = 1.408"
outer legs = 0.52" x 0.267"
center leg = 0.52" x 0.488" (octagonal)

Initially, tektronix bought ungapped halves, and used 0.008" tape
to apply the gap to all three legs equally. Then they started buying
one gapped, and one ungapped half core, with 0.015" gap. And finally,
they got smart, and bought the cores gapped to 0.075", and assembled
them for 0.015" gap, simplifying inventory.

They used this same core for all of their tube scopes after the earliest
oil dunked scopes that operated at 1KHz (or should I say 1Kc?).

In modern magnetics, you have essentially four choices:

1) toroid
2) pot (cup) core
3) sectioned pot (cup) core
4) E core.

Toroid is out for the obvious reasons. Cup core is out because
the access to the winding area is limited, and the inner area
doesn't allow you to keep your winding away from the ferrite.
Sectioned pot cores have better winding access, but the legs are
still too close to the outer wrap of the winding for HV.

E core would be the best choice, except that square windings are
a really bad thing for HV, as the corner bends launch corona and
stress the wire insulation. So, you would have to use a round
former for the winding, and waste a lot of your winding space.

I have searched all the major companies, and none have anything
even close to suitable. Usually the dimensions are done to
maximize efficiencies for low voltage switchers, and A-B was
making HV cores.

Few of the owners of 500 series scopes own them as daily drivers.
They are more of a collector's item. It is desirable to have them
work, but it is also desirable to keep them as original as possible.

The old ferrite is best.

-Chuck Harris

Roger M wrote:

Hi Chuck,
Yes, I agree, the best solution is a rewind of the transformer.
Yet, in the continuum of poor, fair, good, better, and best, there
may be a method that is acceptable to the extent that its "good

This topic interests me but I have to admit I'm hampered by not
having even one failed transformer in my possession. In my life
I've owned and used a prototype 546 (for some 25+ years),
a rack mount 547 (some 15 years) and now a couple of recently
acquired bench model 547's. None of which have ever exhibited
the HV disease.

Some weeks ago I purchased some Tek parts needing a new home.
Among which are some apparently rewound 120-0308-00 transformers
at least one of which I'd swap for one in otherwise good condition
that has the thermal runaway issue.

On the topic of the unobtainable ferrite cores I should ask:
was Tek's vendor Stackpole? Do we know the mix type of the ferrite?
Not wanting to risk damage in disassembling a transformers I have
on hand, I assume the core is gapped at one or both of the center
legs. Have you noted what the gap dimension is?

Looking at what is currently (and readily) available, there's
the Ferroxcube core E47/20/16 in 3C94 material (for example).
Its too thick for direct fit into Tek's plastic HV enclosure,
but that's what a clamp jig and a Home Depot (Harbor Freight?)
diamond blade tile saw are for.

I plan to order some and "dink around" as time permits...


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