The epoxy issue with tektronix transformers is well
has been explored extensively. Many people have tried to
field expedient techniques to solve the problem, and all
failed in the long term. Basically, tektronix used an
epoxy that degrades over time, and takes on moisture. The
and degradation increases its already lossy nature, and is
proverbial "straw that breaks the camel's back" in an
touchy HV design.
The HV design is "touchy" because it is a Hartley
is being heavily loaded. It has about 5 watts of filament
in a 547 design, slightly less in the 647 with its 3
5 tube rectifiers. The oscillator needs sufficient
run, and the ferrite core is of an ancient variety first
Allen-Bradley back in the late 1940's. It is starting to
very lossy at 50-60KHz where the oscillator runs, and its
temperature is too close to room temperature for safe
over the temperature range the specified for the 647
Ok, what happens: Well simply, as the loss in the epoxy
the current drawn by the oscillator rises. The rise in
causes the I2R losses in the transformer primary to
further heating the core. As the core gets hot, it gets
to the Curie point, and as such its internal losses rise,
the whole process quickly runs away.
This Hartley oscillator design first appeared in the 513D
as far as I can tell. There it had the entire HV section
in oil, and ran at about 400Hz... it used a somewhat
core to handle the low frequency... a very conservative
The next time I saw this design was in, well every 500
scope made. By the time the 535 came along, it had reached
"modern" configuration, with a beeswax impregnated core,
cooling, and 5, 5642 rectifiers acting as multipliers for
various anode voltages. Its frequency was about 60KHz.
Even though the core was right on the hairy edge of its
curve, because it was in free air, and was mounted right
the fan, everything worked nicely. Tektronix used the
virtually untouched until the early 1960's when the 647
designed... and coincidently, the 545B and 547.
The 647 was an attempt to make the first ruggedized all
state 50MHz scope. For it, they did two things, first
an enclosed plastic box to hold the entire HV section, and
second, changed the transformer to an epoxy varnished
Historical evidence embodied in the circuitry, and the
embodiment of the circuit show that tektronix immediately
the HV section was too hot and too lossy for reliable use.
I am guessing they had spent their wad, and were
and being 20 some years since the original circuit was
by then nobody in tek really understood the original HV
design...so they prepared several expedient patches to the
One patch was to open a hole in the bottom of the plastic
to allow fitting a bar of aluminum, or beryllium oxide
to heat sink the transformer core to the chassis. This
in production 647's, but not 545B/547's. Another was to
replace two 5642 tubes in the 647 supply, with silicon
And another was to add a fuse to the HV oscillator supply
the 647, and add a shunt diode to the screen grid of the
oscillator int the 545B/547 to limit the maximum drive to
safe 100V (120?).
Later, after production, fixes were to replace all the
with silicon, and to change the potting to a black
I suspect that that transformer is triply potted, first in
epoxy varnish, next in silicone, and finally in the black
epoxy... Other scopes, like the 565 used epoxy varnish,
in silicone RTV, in an aluminum can... I'm surprised they
toss some silica gel bags into the HV compartment as part
Nothing worked! The root of the problem is the old formula
core, and they never changed it! It even shows up in the
series scopes and beyond.
Anyway, after trudging through all of the field expedient
attempts with my own 545B and 547, I started to study
HV transformers, and produced a remanufactured transformer
that family that reuses the old core. The winding is
impregnated, and all other parts are new manufacture. A
poking and prodding will reveal that the group is
545B's and 547's that contain my remanufactured
Since I am now the proud owner of a 647A scope with the
transformer disease (Thanks Chris!) I am now working on a
for the 647/647A transformer. I expect to have it
this winter... if I can keep the copper thieves from
return visit to my machine shop...
Like the 545B/547 transformers I am currently making, I
to use a beeswax impregnated winding, and there will be
room for the 5642 filament windings. It will look a little
different, in that it won't be a plastic potted winding,
rather will be naked, like in the older scopes.
I still have some work to do on my winding machine, as
transformer is a little more challenging, and there will
some testing to be sure that the beeswax can handle the
environmental needs of normal use. We will probably have
lose some of the upper end of the temperature range,
it is above the melting point of beeswax. If necessary, I
primed for the effort to use modern epoxies.. but I hope
won't be necessary for this scope.
> I bought a basket case of a 647 mostly to get the
11B2 for my 647A. After
> repairing the 11B2 with help from this group, I
decided to see if I could do
> anything with the 647. I found a wiring error,
several bad transistors, an NPN
> transistor in place of a PNP, and an open solder
connection on one of the heater
> pins on the CRT base.
> At that point it was working, but it had the common
HV transformer problem. After
> about 45 minutes, the 2A fuse feeding the HV supply
would blow. I monitored the
> current to the HV circuit, and found that it took 45
minutes for it to go from
> around 400mA to 1A. When it hit 1A it would get to 2A
within a few seconds.
> Keeping the transformer cool with a fan allowed it to
run all day, but it's not
> very practical. I tried replacing the HV rectifier
tubes with solid state diodes.
> This reduced the current demand by about 50 mA, but
didn't help with run time.
> Interestingly, the transformer didn't appear to be
overheating (the core only got
> to about 50C, and the outside of the windings were
cooler than the core.
> I noticed that when the transformer was hot, the
epoxy potting was quite soft and
> rubbery. I decided to try to remove as much of the
potting as possible. Using a
> hot air gun, I was able to get most of the epoxy off
the windings. I had to be
> very careful not to damage the windings (I used a
plastic tool to chip away at the
> epoxy). I reheated it every time the epoxy became
hard to work with. Be careful
> not to break the 3 floating secondary wires since the
epoxy was supporting them.
> Next, I gave it a 4 hour bath in 120C bee's wax to
try to seal things, and bake
> out any moisture. I did see a few bubbles, but they
may have just been air. If I
> did it again, I would probably skip that step, and
use a more modern HV sealant.
> I wanted to use something that was known to work as a
baseline. Finally, I
> injected some silicone rubber in places that looked
at risk for arcing.
> After installing the naked transformer I noticed an
immediate improvement. Cold
> supply current was down more than 50mA, proving that
removing the epoxy has
> reduced the losses in the transformer. I plotted the
current over time and it
> leveled off at 332mA. Compared to my 647A with a good
original transformer, it
> follows an almost identical curve. The 647A is higher
overall because it still
> has tube rectifiers. I should have plotted the
current before, but based on my
> observations, it would have shown a roughly
exponential rise in current.