Topics

7904 excessive shadow/flare

Ed Breya
 

Haha - this tidbit of terminology just popped in from my unconscious long-term storage. It was called "MSE" for "meshless scan expansion." Googling the the full phrase quickly got a lot of hits, including this one:

https://patents.google.com/patent/US4137479A/en

Ed

Ed Breya
 

I noticed this thread a while back, but didn't look into it - apparently just the usual issues of the well known halo etc artifacts especially notable in the 7904. I finally read through it all, and was fascinated with the discussion and theories about the cause. So, I guess I'll put my one-cent worth into the mix.

First, I think pretty much all the conjectures are valid, and contribute to the observed effects to varying degrees, depending on operating conditions.

Second, it was always my impression that most of the grief in the faster 7K and other models was due to the mesh expansion dome. I still believe that is the main culprit, including effects from diffraction and secondary emission in the mesh. I vaguely recall discussions inside Tek way back when, that the new quadrupole lens was primarily developed to get the desired expansion without using a mesh. I think this was likely used in the latest (and last) high performance analog scopes like the 2465 and 7104. I believe the older high bandwidth 7K scopes all use the mesh.

I've never junked out a 7K CRT except for a 7844 one, and the gun assembly is still likely kicking around somewhere - I always save stuff like that. As I recall, it had a big mesh, especially to accommodate two guns. If I can find it, I'll report on it.

Here are a couple of other things to consider.

The electron beam diverges due to mutual repulsion of the electrons, and the beam current produces a magnetic field that will interact with "loose" electrons that may be be around, further complicating the situation.

Since the total acceleration voltage available is about 24 kV (3 from cathode, 21 from PDA), X-rays up to 24 keV may be produced inside. The leaded glass face and other parts keep most from getting out, but internally, the X-rays can light up phosphors or cause electron emission from whatever they hit. I'd guess that a decent amount of the halo is from X-rays that happen to be emitted in the phosphor plane. This would of course be aggravated by needing higher PDA voltage. The rays in other directions will tend to be absorbed in the materials, and contribute to the visual effects only by adding extra electrons to the situation.

The simple concept of CRT operation can get very un-simple, the more you think about what's going on. Interesting stuff.

Ed

Albert Otten
 

Hi John,

If there is effect of the beam current on the bouncing back electrons then the effect would increase when intensity (so beam current and the B field) is cranked up. I didn't notice this.
I also played with Trace Rotation. Trace rotation did nothing special; the primary spot and image stay in line. Probably the field of the trace rotation coil is to weak in the region where bounced back electrons fly.

[I can't attribute to the discussion until after the weekend.]
Albert

On Thu, Jul 4, 2019 at 04:42 PM, John Griessen wrote:


There is going to be some magnetic field action. We have a strong current of
e- going straight down the tube perpendicular to the phosphor. When an e- does
a bounce, it is going in a partly outward direction, but the
rest of the e- beam is going straight, and making a current surrounding
magnetic field that is unaffected by the
odd bouncing electrons.

The strong main B field is circles around the impact spot since it is caused
by the e- beam, and the direction
will be counterclockwise looking at the phosphor surface from inside the tube.

If an electron is bouncing back off perpendicular, its outward velocity will
interact with the B field and it will feel a force. the force will be
perpendicular to velocity X B, (thumb of right hand rule), so while bouncing
outward and upward, (v X B) points down and out, and as that makes it go more
outward the effect is more downward, but getting weaker with distance. The
weakening with distance could mean that as e- falls it is just
mostly newton mechanics trajectory, and it is curve more only close in.

Chuck Harris
 

Hi Adrian,

A discharge implies a short circuit, and perhaps a plasma... nothing
of that sort happens. A continuous beam current occurs, as there is a
continuous stream of electrons in the beam, and that is the very
definition of current.

The electron beam electrons fly through the vacuum, attracted by the
Post Deflection Accelerator (PDA), and the positively charged
aluminized phosphor screen (at anode voltage). The field has to be
very uniform in its composition, or the beam will have odd distortions
that will render its use inaccurate.

My hypothesis assumes that the field the electrons bounce off the screen
into, is uniform, and that like gravity, it drags the reflected electrons
back to the screen, where if their quantity is sufficient, they will
cause a visible green glow.

I would expect the reflected electrons take a parabolic path in their
route back to the screen, where they become part of the shared electron
shell of the aluminum layer, and are conducted away through the anode
cap of the CRT.

-Chuck Harris



Adrian wrote:

So this is all way above my pay grade and I'm quite likely to have missed the point
entirely - but is it that the electric field dragging the errant electrons back to
the face of the tube is not actually uniform in the area where the aluminium layer
has been discharged by the main beam, thus more of them hit outside that annular area?

On 7/4/2019 3:42 PM, John Griessen wrote:
But why does this lead to a fuzzy ring (at higher brightness the arc is a closed
ring, with really dark interior)?


John Griessen
 

On 7/4/19 10:31 AM, Adrian wrote:
is it that the electric field dragging the errant electrons back to the face of the tube is not actually uniform in the area where the aluminium layer has been discharged by the main beam, thus more of them hit outside that annular area?
That aluminum layer is the return path for the electrons hitting it, so the field bunches up there.
There are so many ifs it's just wild assed guessing, or SWAG, (scientific wild assed guessing).

--
John

Adrian
 

So this is all way above my pay grade and I'm quite likely to have missed the point entirely - but is it that the electric field dragging the errant electrons back to the face of the tube is not actually uniform in the area where the aluminium layer has been discharged by the main beam, thus more of them hit outside that annular area?

On 7/4/2019 3:42 PM, John Griessen wrote:
But why does this lead to a fuzzy ring (at higher brightness the arc is a closed ring, with really dark interior)?

Chuck Harris
 

If the 453 has a mesh, the even larger screen'd 454
would have to have one too.

I think that the schematic may be a bit inaccurate, as the
geometry anode is usually after the horizontal plates, but
the schematic shows it to be before the horizontal plates.

I have a feeling that tektronix was so busy with starwars
in those years towards the end of the cold war that
documentation took a distant back seat.

-Chuck Harris

Albert Otten wrote:

In the Concept Series CRT book the 422 and 453 are listed as having a a mesh-grid construction but not so the 454. But the 453 and 454 schematics both show a grounded grid where the mesh-grid should be.
Following the hint in the book I displayed a fully defocused spot on my 454 CRT. I could recognize the mesh grid image, by the lines in vertical direction but less pronounced or even absent in horizontal direction. Nothing like that appears in my 7704A.
It's a pity that the 1985 version of the CRT book (CRTs Getting down to basics) contains the old information w.r.t. mesh grids and is not updated with newer scope data (except for storage scopes).

Albert

On Thu, Jul 4, 2019 at 03:13 PM, Chuck Harris wrote:


I think I was wrong when I said that neither the 465 nor
the 475 exhibit the "7904 phenomenon". They do, they just
have to be at very high intensity before the phosphor gets
enough electrons to light up.

I don't think the "7904 phenomenon" is related to the dome
mesh lens, as I am pretty sure the 7904 doesn't have one...
but it would be nice to know for sure.

-Chuck Harris


John Griessen
 

On 7/4/19 8:29 AM, Albert Otten wrote:
I understand that there will be some random scatter around the theoretical "landing" point. But why does this lead to a fuzzy ring (at higher brightness the arc is a closed ring, with really dark interior)?
Here's a theory: Reflected electrons traveling within a narrow angle from perpendicular to the phosphor/aluminum sputtered surface, are bouncing back and some land and are seen. Some go off course and are not seen anymore because they got as far back as the deflection plates, and then they went sideways... Others reflect with varying speeds and a fairly sideways trajectory, and most land outside the ring. (observed dark ring).

So all that above motion is mechanics and we expect a Gaussian distribution,
but see a cutoff distribution instead.
Something is missing.

There is going to be some magnetic field action. We have a strong current of e- going straight down the tube perpendicular to the phosphor. When an e- does a bounce, it is going in a partly outward direction, but the
rest of the e- beam is going straight, and making a current surrounding magnetic field that is unaffected by the
odd bouncing electrons.

The strong main B field is circles around the impact spot since it is caused by the e- beam, and the direction
will be counterclockwise looking at the phosphor surface from inside the tube.

If an electron is bouncing back off perpendicular, its outward velocity will interact with the B field and it will feel a force. the force will be perpendicular to velocity X B, (thumb of right hand rule), so while bouncing outward and upward, (v X B) points down and out, and as that makes it go more outward the effect is more downward, but getting weaker with distance. The weakening with distance could mean that as e- falls it is just
mostly newton mechanics trajectory, and it is curve more only close in.

Without getting quantitative it's hard to see why the dark ring exists. Fun to think about though.

If we knew that the speeds and angles bouncing off were in a tight range and the force was enough to cause a curved trajectory that would explain a fuzzy ring outside a dark circle.

If.

Chuck Harris
 

Hi Albert,

On my 7854 and 7904, the reflection is a near perfect circle,
with a dim bright dot in the center. It looks like a perfect
copy of the high intensity spot with its halo.

There is a slight difference between the two scopes. The 7854's
ghost is perfect and uniformly illuminated, the 7904's ghost is
more of a bullseye pattern, with the outer ring slightly brighter
at the edge furthest away from screen dead center.

Also, it is pretty clear that the center point of the screen,
the image, and the ghost image, always line up on the same imaginary
radial line extending from the center point of the screen.

And, it is pretty clear that the distance the ghost is away from
the true image depends on how far the true image is from dead
center on the screen. At dead center both the true image, and the
ghost image overlay.

A lot of this would make more sense if the electron beam's cross
section looked exactly like what we see when we turn the intensity
up too high: a bright center dot, surrounded by a circular halo.

In other words, the electron beam is like one of those fuzzy pipe
cleaners: the wire in the center is the beam we all know and love,
and the fuzz is a lower density cloud of electrons that come along
for the ride...

If that is so, then the ghost could be an exact reflection of that
beam cross section, and should look the same.

-Chuck Harris

Albert Otten wrote:

Hi Chuck,

I understand that there will be some random scatter around the theoretical "landing" point. But why does this lead to a fuzzy ring (at higher brightness the arc is a closed ring, with really dark interior)? I would expect a lighted region around the center of the ghost, like what is visible around the primary spot.

Albert

Albert Otten
 

In the Concept Series CRT book the 422 and 453 are listed as having a a mesh-grid construction but not so the 454. But the 453 and 454 schematics both show a grounded grid where the mesh-grid should be.
Following the hint in the book I displayed a fully defocused spot on my 454 CRT. I could recognize the mesh grid image, by the lines in vertical direction but less pronounced or even absent in horizontal direction. Nothing like that appears in my 7704A.
It's a pity that the 1985 version of the CRT book (CRTs Getting down to basics) contains the old information w.r.t. mesh grids and is not updated with newer scope data (except for storage scopes).

Albert

On Thu, Jul 4, 2019 at 03:13 PM, Chuck Harris wrote:


I think I was wrong when I said that neither the 465 nor
the 475 exhibit the "7904 phenomenon". They do, they just
have to be at very high intensity before the phosphor gets
enough electrons to light up.

I don't think the "7904 phenomenon" is related to the dome
mesh lens, as I am pretty sure the 7904 doesn't have one...
but it would be nice to know for sure.

-Chuck Harris

Albert Otten
 

Hi Chuck,

I understand that there will be some random scatter around the theoretical "landing" point. But why does this lead to a fuzzy ring (at higher brightness the arc is a closed ring, with really dark interior)? I would expect a lighted region around the center of the ghost, like what is visible around the primary spot.

Albert

On Thu, Jul 4, 2019 at 02:45 PM, Chuck Harris wrote:


Hi Albert,

If you take a BB gun, and shoot a brick sitting in the
middle of a sandbox, at about the same spot, 10 degrees
off of orthogonal, the BB's will bounce up into the air,
and fall back down onto the sand, where they will stick.

The locations where they fall to should form an arc like you
are seeing on the CRT screen.

The arc comes about because not all BB's will hit the same
spot on the brick, and the slight irregularities in the
brick's surface will cause slightly different trajectories.

The surface of the phosphor in a CRT is very rough, like
a very flat, non reflecting paint. And the aluminum layer
matches that surface to an atomic level. And electrons are
very, very, small.

To the electrons in the CRT's beam, that phosphor surface
looks like bucket filled with railroad cars would to a BB.


-Chuck Harris

Albert Otten wrote:
On Wed, Jul 3, 2019 at 10:24 PM, Chuck Harris wrote:


Most telling would be to use a stationary spot, and a
straight edge to see if the radial construction line I
imagined truly predicts the ghost image's spot location.
Chuck,

How to explain that the ghost image of a stationary spot is not a spot but a
(fuzzy) full ring of almost 1 div diameter? That's the situation in my 7704A
and also more or less visible in the video by Max:
https://www.dropbox.com/s/tfy91d8tq1b6ql9/7904_shadow_flare%20003.AVI?dl=0

Could this be related to the cristal structure of the aluminium layer?
[Apart from that ghost image, in my 7704A there is a disk-like lighted area
around the spot with gradually decreasing brightness at larger radii.]

Albert

Chuck Harris
 

I think I was wrong when I said that neither the 465 nor
the 475 exhibit the "7904 phenomenon". They do, they just
have to be at very high intensity before the phosphor gets
enough electrons to light up.

I don't think the "7904 phenomenon" is related to the dome
mesh lens, as I am pretty sure the 7904 doesn't have one...
but it would be nice to know for sure.

-Chuck Harris

Raymond Domp Frank wrote:

On Mon, Jul 1, 2019 at 03:32 PM, Chuck Harris wrote:


The 465, and 475, are known to me to have dome mesh lenses,
...I have seen them broken... but neither exhibit the
phenomenon observed on the 7904. The dome mesh lens shows
itself, in these scopes, by a fuzzy, less sharp beam when
compared to a conventional CRT, that has no lens.
I guess I misunderstood the above and placed a few images showing (a.o.) that precise "7904 -behaviour" on a 465, wondering what phenomenon was meant if not that.

I assumed that since you replied to this thread with
a picture of the same phenomenon we were discussing, only
this time on a 465 CRT, that you knew what we were
talking about.
I thought and think I was and am but when you wrote

neither exhibit the phenomenon observed on the 7904
I got confused since I remembered that exact same behaviour from the 465, albeit only at very high intensity. I guess I should forget about that phrase then.

I agree with you that it's hard to gauge intent over this medium, so we have to be careful. Apology accepted, of course.

Raymond



Chuck Harris
 

Hi Albert,

If you take a BB gun, and shoot a brick sitting in the
middle of a sandbox, at about the same spot, 10 degrees
off of orthogonal, the BB's will bounce up into the air,
and fall back down onto the sand, where they will stick.

The locations where they fall to should form an arc like you
are seeing on the CRT screen.

The arc comes about because not all BB's will hit the same
spot on the brick, and the slight irregularities in the
brick's surface will cause slightly different trajectories.

The surface of the phosphor in a CRT is very rough, like
a very flat, non reflecting paint. And the aluminum layer
matches that surface to an atomic level. And electrons are
very, very, small.

To the electrons in the CRT's beam, that phosphor surface
looks like bucket filled with railroad cars would to a BB.


-Chuck Harris

Albert Otten wrote:

On Wed, Jul 3, 2019 at 10:24 PM, Chuck Harris wrote:


Most telling would be to use a stationary spot, and a
straight edge to see if the radial construction line I
imagined truly predicts the ghost image's spot location.
Chuck,

How to explain that the ghost image of a stationary spot is not a spot but a (fuzzy) full ring of almost 1 div diameter? That's the situation in my 7704A and also more or less visible in the video by Max:
https://www.dropbox.com/s/tfy91d8tq1b6ql9/7904_shadow_flare%20003.AVI?dl=0

Could this be related to the cristal structure of the aluminium layer?
[Apart from that ghost image, in my 7704A there is a disk-like lighted area around the spot with gradually decreasing brightness at larger radii.]

Albert

 

Hi John,

About the only one I know of still using the capabilities Tek developed is Bernie Vancil whose company, e beam, inc, is located in Beaverton. Bernie worked in Tek Labs. He is currently making propulsion systems for deep space applications (think ion engines).
His lab is an almost endless collection of unusual equipment for high vacuum, high voltage, physics, scientific glass blowing, quadrupole lenses, deflection systems, and on and on. It is a real treat to visit e beam. I only wish I could work there for a year for what I would learn about the glory days of Tek.

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of John Ferguson via Groups.Io
Sent: Wednesday, July 03, 2019 7:32 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7904 excessive shadow/flare

Thank you all for a really fascinating (and enlightening) discussion.

It appears that Tektronix developed astonishing expertise in the display of data on CRT's.

Has this expertise continued to serve in any industry today or has it become more solutions in search of a problem?
--
Dennis Tillman W7PF
TekScopes Moderator

Albert Otten
 

On Wed, Jul 3, 2019 at 10:24 PM, Chuck Harris wrote:


Most telling would be to use a stationary spot, and a
straight edge to see if the radial construction line I
imagined truly predicts the ghost image's spot location.
Chuck,

How to explain that the ghost image of a stationary spot is not a spot but a (fuzzy) full ring of almost 1 div diameter? That's the situation in my 7704A and also more or less visible in the video by Max:
https://www.dropbox.com/s/tfy91d8tq1b6ql9/7904_shadow_flare%20003.AVI?dl=0

Could this be related to the cristal structure of the aluminium layer?
[Apart from that ghost image, in my 7704A there is a disk-like lighted area around the spot with gradually decreasing brightness at larger radii.]

Albert

 

On Mon, Jul 1, 2019 at 03:32 PM, Chuck Harris wrote:


The 465, and 475, are known to me to have dome mesh lenses,
...I have seen them broken... but neither exhibit the
phenomenon observed on the 7904. The dome mesh lens shows
itself, in these scopes, by a fuzzy, less sharp beam when
compared to a conventional CRT, that has no lens.
I guess I misunderstood the above and placed a few images showing (a.o.) that precise "7904 -behaviour" on a 465, wondering what phenomenon was meant if not that.

I assumed that since you replied to this thread with
a picture of the same phenomenon we were discussing, only
this time on a 465 CRT, that you knew what we were
talking about.
I thought and think I was and am but when you wrote

neither exhibit the phenomenon observed on the 7904
I got confused since I remembered that exact same behaviour from the 465, albeit only at very high intensity. I guess I should forget about that phrase then.

I agree with you that it's hard to gauge intent over this medium, so we have to be careful. Apology accepted, of course.

Raymond

Chuck Harris
 

The 7904 phenomenon is what we have been discussing.

When a slow, but bright trace sweeps across the screen,
you see both the dot of the normal bright trace, and a
dimmer, fuzzy, ghost spot. The ghost spot appears to be
located on a radial line beginning at the center of the
screen, and passing through the centers of the bright
beam, and the fuzzy "ghost" spot.

It appears to the operator as a fuzzy trace that is
magnified version of the original trace.

For simplicity, I called this the 7904 phenomenon.

I have observed it to exist on many high performance
CRTs, but it is particularly evident (some say annoyingly so)
on the 7904 CRT.

I posit that the effect is caused by the electron beam
striking the CRT screen, and being reflected back into
the CRT at the angle of incidence.

The PDA anode voltage attracts the bounced electrons back
to the screen, where they once again strike, lighting up
the phosphor, and appear in the form of a fuzzy ghost image

The ghost image is always there, but it is most evident
at higher intensities, and at slower than 5ms/div sweeps.

I apologize for taking a "tone" with you.

I assumed that since you replied to this thread with
a picture of the same phenomenon we were discussing, only
this time on a 465 CRT, that you knew what we were
talking about.

I never presumed you weren't knowledgeable, only that you
were intentionally being difficult. It is hard to gauge
intent over this medium.

If the ghost images are indeed caused by electrons bouncing
off of the CRT screen, and the bounced electrons are indeed
brought back to the screen by the PDA's attraction, the
distance the ghost images are from the trace should be
greater with a higher energy electron beam.

This appears to be so, based on your 465 pictures.

Most telling would be to use a stationary spot, and a
straight edge to see if the radial construction line I
imagined truly predicts the ghost image's spot location.

If you try this, be very careful, stationary beams burn
CRTs... even well protected ones, like in the 7904..

-Chuck Harris

Raymond Domp Frank wrote:
...

As said, I'd appreciate you tell me what that specific "phenomenon observed on the 7904" is. I guess you already told us and I missed it but maybe I'll learn from it and please leave out the implied "I'm knowledgeable, you aren't".

Raymond

 

On Mon, Jul 1, 2019 at 03:32 PM, Chuck Harris wrote:


The 465, and 475, are known to me to have dome mesh lenses,
...I have seen them broken... but neither exhibit the
phenomenon observed on the 7904. The dome mesh lens shows
itself, in these scopes, by a fuzzy, less sharp beam when
compared to a conventional CRT, that has no lens.
What "phenomenon observed on the 7904" were you referring to?

Indeed, now show that it doesn't happen with an overly bright
non mesh lens system. I have seen the same stuff on my very old,
and probably now no longer working 513D, which has neither a lens,
nor an aluminized CRT phosphor. It did have a very high anode
voltage...12KV.

I have also seen it happen on a 545, and 547. The principle
difference, from the 7904, is how bright the beam has to be
before I can see the bounce image, and how far from the original
image, the bounce image appears.
Again: What specific phenomenon on the 7904 were you referring to? Maybe the brightness or the high PDA voltage?
Is it not just a gradual difference, as opposed to a specific "7904 phenomenon"?


Trust me...*** I already know you don't***... but high energy electron
beams bounce off of aluminum, gold, phosphor, rock, just about
anything. Because of the PDA, the electrons are dragged back to
the phosphor where they present as a magnified, fuzzy, glowing
image of the original trace.
I added the above emphasis with asterisks. I have no need to trust you on something I know. A less patronizing and arrogant
teacher is a better teacher, trust me... Did I not praise you (and Dennis) enough in my earlier mail?
As said, I'd appreciate you tell me what that specific "phenomenon observed on the 7904" is. I guess you already told us and I missed it but maybe I'll learn from it and please leave out the implied "I'm knowledgeable, you aren't".

Raymond

John Ferguson
 

Thank you all for a really  fascinating (and enlightening) discussion.

It appears that Tektronix developed astonishing expertise in the display of data on CRT's.

Has this expertise continued to serve in any industry today or has it become more solutions in search of a problem?

Chuck Harris
 

Indeed, now show that it doesn't happen with an overly bright
non mesh lens system. I have seen the same stuff on my very old,
and probably now no longer working 513D, which has neither a lens,
nor an aluminized CRT phosphor. It did have a very high anode
voltage...12KV.

I have also seen it happen on a 545, and 547. The principle
difference, from the 7904, is how bright the beam has to be
before I can see the bounce image, and how far from the original
image, the bounce image appears.

The high voltage of the anode accelerates all electrons coming
from the gun to nearly equal speeds. Electron speed is the same
as electron energy. Intensity controls beam current, which is
the electron density of the beam, not the energy of the electrons
in the beam... that is controlled by the anode voltage, and the
length of the beam path.

Anode voltage controls how far the electrons bounce when they
hit the screen. How far they bounce controls how far away they
land from the original spot, for a given angle of beam deflection.

Trust me... I already know you don't... but high energy electron
beams bounce off of aluminum, gold, phosphor, rock, just about
anything. Because of the PDA, the electrons are dragged back to
the phosphor where they present as a magnified, fuzzy, glowing
image of the original trace.

The principal aberration you see with a mesh lens is an indistinct
fuzzy looking spot. This is mostly due to the lens magnifying the
already indistinct fuzzy looking electron beam. The spot is just
a picture of the cathode surface after collimation and focusing...

-Chuck Harris

Raymond Domp Frank wrote:

On Mon, Jul 1, 2019 at 03:32 PM, Chuck Harris wrote:


The 465, and 475, are known to me to have dome mesh lenses,
...I have seen them broken... but neither exhibit the
phenomenon observed on the 7904. The dome mesh lens shows
itself, in these scopes, by a fuzzy, less sharp beam when
compared to a conventional CRT, that has no lens.
I have added 3 images from a 465 showing not just the halo but also some artifacts that are a bit like those on the 7904's CRT, here:
https://groups.io/g/TekScopes/album?id=90746
They are in the main photo album of this topic. Sorry Unclebanjoman, no intention to hijack your album.
The intensity on the 465 must be turned up quite a lot (much more than on the 7904) and it's a bit difficult to photograph but I guess it's visible.
465 Artefacts-1: Slow-moving spot, fluffy stuff, halo etc. *and* "spot moving at much higher speed than main spot, with center at second graticule from left, one division down from the middle. Quite visible in live picture. It looks and moves just like on the 7904 but its brightness is not enough to show up in the sinewave pictures that follow.
465 Artefact-2: A second image, barely visible in picture, as lighter area ("copy" of trace) just left of darker band between first and second graticule from left. It runs through .4 position right of "10", at about -45 degree angle from horizontal.
465 Artefact-3: Shows slightly compressed copy of main trace, mainly visible between first and second graticule horizontally and from seventh to the right. It is smaller in both X and Y, as opposed to the 7904's.

Raymond