2467B CRT geometry puzzle

[jonpaul]

Hi there: Working on  a clean 2467B, before CAL, I am adjusting CRT.

Have a 1/10 div upward  bending of horizontal lines at the upper right.
In all other respects the CRT display is fine. At lower and middle lines are straight.
I did trace rotation, this is  a bending distortion

Using geometry trimmer, R4350, distortion only gets worse and screen scale changes.
Best adjustment is original position, full CCW.

Normally, this trimmer should be near center?
Anyone have experience in geometry problems?
I Need to fix this before I can do a CAL!

Many thanks

Jon

[macboy]

Maybe try degaussing the CRT with one of those handheld AC powered degauss coils. I know the 2467 has a very special micro channel plate CRT to support its bandwidth so I don't know how safe this is for that type of CRT.
Also note that orientation vs. the earth's magnetic field as well as other stray magnetic fields in your lab can cause geometry issues.

[helius]

If scope CRTs required degaussing they would have built-in coils, it's not expensive to add. Remanent magnetic fields produce color purity errors in color CRTs, they don't cause gross geometry problems.

[tecman]

In the service manual, starting on page 5-7 is the section covering CRT adjustments.  One part is the geometry adjustment to straighten lines.  If you did not try these adjustments go there first.  If you still cannot reach the desired results, start looking at voltages on the CRT's various electrodes.  A power supply improperly adjusted or out of tolerance can result in geometry issues.

The MCP will not cause a geometry problem as it is a 1:1 straight line from front to back.  It;'s purpose is to increase the intensity of fast sweeps and transients by multiplying electrons from the CRT gun to produce a brighter image on the phosphor.

paul

[jonpaul]

Hi:

1. Degauss was a faint hope as the CRT is surrounded by a heavy MuMetal custom designed shield. These units were in labs and never dropped (as far as any visible damage!!) Nonetheless I have ancient bulk tape degaussers and could try one near the faceplate. No isdsue to degauss the 2467/B as the MCP plate is not magnetic.

2. Symptom is barely noticeable, at 0.1-0.2 div, I can live with it.

Perhaps its a CRT manufacturing spec issue and is normal?

Any TEK veterans out there to comment?

3. As GEOM pot changes CRT gain, any change will affect V and H CAL.

4. Fortunately I have another 2467B, and will carefully compare, and report.
and to check the POD lens and GEOM voltages.

Good weekend!

[David Hess]

If scope CRTs required degaussing they would have built-in coils, it's not expensive to add. Remanent magnetic fields produce color purity errors in color CRTs, they don't cause gross geometry problems.

Color CRTs have a metal shadow mask which besides having a huge effect on convergence, is also susceptible to being magnetized.

The metal structures inside the Tektronix CRT might be susceptible to being magnetized.  I thought this might be the problem causing poor geometry in one of my 7834 storage oscilloscopes however removing the CRT and degaussing it and the mu-metal shield had no effect and did not improve the geometry.  I now suspect that the CRT was damaged in shipping.

2. Symptom is barely noticeable, at 0.1-0.2 div, I can live with it.

Perhaps its a CRT manufacturing spec issue and is normal?

Any TEK veterans out there to comment?

That level of distortion at the edge of the CRT is within the specifications.  Some of my Tektronix oscilloscope CRTs have about that much distortion at one corner or side and are not clearly bad or damaged.

[jonpaul]

Hello all: great notes and comments!

I setup all 4: 2X  2467B, 2465B and 2467.

3X  fine, just barely noticeable < dim trace width, and << 1/10 division,  or no nonlinearity.

Upon closer examination, the suspect 2465B, with the geometry issue and geom  trim full CCW:

a. Deviation is ~ a dim tracewidth thickness,  less than 1/10 division. Noticeable at both top and bottom of CRT.
b. Both top and bottom traces are  bending slightly upwards at right but seem straight at left.
c. Most noticeable at 0 and 1st vertical bottom and 7, 8 at top.
d. You really have to look carefully to see it.
e, Not affected by intensity, focus, astigmatism.

A tip for HV board adjustments, The PCB is recessed 1" in back of the metal shield.  If you do not remove the shield on the HV board, it is difficult to center the tool, especially with a plastic slotted alignment tool.  I use a small metal Phillips driver, and a headlight to see thru the holes.

About degaussing, MuMetal shield seems thick and very complete, so accidental magnetization seems unlikely. 
Any stray remnant magnetic field would be uniform, and affect only the corners.

Bonne Journée

Jon

PS: Any 246X aficionados have more than 4?   
I accumulated since 1992, in V. G. - exc. cond.
Enjoy  fixing and calibrating, also have  many original manuals. 
The digital scopes have many benefits but I still prefer these analog scopes.

[jonpaul]

After tweaking best results, 4 ch ON

Chuck your comments? Compared to yours?

Jon

[jonpaul]

TEK spec 2465/7/B

"X-Axis Low-frequency Linearity
0.1 division or less compression or expansion of
a two-division, center-screen signal when
positioned within the graticule area.

Y-Axis Low-frequency Linearity
0.1 division or less compression or expansion of
a two-division, center-screen signal when
positioned anywhere within the graticule area."

This is really compression of the amplifiers, vs screen position, not line curvature or geometry!

two have almost perfect geometry, the is other off by ~ a dim tracewidth <<1/20 div
The 2467B with CCW GEOM trim,~ 1/20 div.
All are well within the 0.1 div TEK linearity spec.

Can any TEK CRT veterans assist?

What we need is a ROM generating staircases run scope  in X-Y mode.

Pity that the TEK CAL has no dot pattern for this purpose!

Cheers,

Jon

[David Hess]

About degaussing, MuMetal shield seems thick and very complete, so accidental magnetization seems unlikely. 
Any stray remnant magnetic field would be uniform, and affect only the corners.

The mu-metal shield is absolutely required to protect the CRT from the switching power supply as I discovered when I reinstalled my 7834 CRT without the shield.  I do not think they intended it to shield low frequency and DC magnetic fields as they sometimes included other steel shields for this.

[0culus]

I remember in a physics lab a long time ago, the instructor demonstrated deflecting an oscilloscope trace with a high powered permanent magnet. Now I'm wondering if that didn't damage something...

[David Hess]

I remember in a physics lab a long time ago, the instructor demonstrated deflecting an oscilloscope trace with a high powered permanent magnet. Now I'm wondering if that didn't damage something...

My father has a photograph where the magnetic field from the experiment made time go backwards on the oscilloscope.  I have a 7B92A timebase which did the same thing until I fixed it; naturally it was marked as being from the physics department.

Another of his photographs shows an explosion ... from inside the CRT when the experiment unintentionally applied high voltage to the oscilloscope inputs.

[0culus]

 

I'd love to see scans of those photographs if you get the chance ever.

[jonpaul]

Hello again

I went thru the CRT adjustment again, then setup to make a better test pattern.

I took a PG508 and set to 10 uS width, ext trigger and used a 500 uS setting on TG501 to trigger. The pulses were then widened with the rise/fall time controls, so they are vertical lines. If you increase vert scale and move the baseline below the CRT edge the result is as shown. It only vertical and not perfect lines but seems to work, the horizontal could be filled by the cursors and other 3 channels.

To Siggi, RE cameras, they all have some pincushion lens distortion especially at the wide angles for scope photography. Backed away ~ 6" I found 3% lens distortion, on a compact Panasonic Lumix LX-7, corrected with Photoshop CS6 Lens Correction filter.

The first shot is the full image of the 10 pulses from PG508, the second is the trace moved down and at increased vertical scale.
The distortion is just slightly noticeable but better after the adjustments. The remaining issue is edge focus, at left corners, adjusting the edge focus trimmer, precise focus possible at upper or lower left,  I had to compromise with both slightly out. Oddly the right corners seem unaffected by edge focus.

From Chuck, the  list of possible causes,

1) physical abnormality inside of the CRT.
2) incorrect bias point for the quadrapole lens.
3) a stray magnetic influence.

I feel that the fast writing rate, wide BW and other requirements caused the CRT designers to make some compromises in the design. If you have visited a prototype glass blowers shop or a CRT line, you would marvel at the complexity and precision of the technique. So perhaps its manufacturing tolerances or compromises, category 1.

Cheers,

Jon

[David Hess]

The 2465 series CRTs all rely on greater scan expansion for increased bandwidth which makes the tolerances of the CRT elements more critical.

[tautech]

An interesting watch:

https://youtu.be/G0Dci5RPe94

[jonpaul]

I discovered a different Geom adj procedure:

The geom test pattern could be more easily generated: Used TM501 trigger output, very narrow pulse with 50 uS PRF or even better the old 2901 marker generator, very narrow pulse on marker amp output.

Horizontal traces are CH 2,3,4 with no signal.

I Used only vertical lines, turned off cursors and other traces,  with  geom trim, you now see a  clear transition from barrel to pincushion distortion uniform over the screen area! Geom Trim is now about centered, with vertical lines no distortion, but horizontal still have very  slight bow at upper 2 div right side only. If you try to trim out the horizontal line bowing, the trim is full CCW and the vertical line test now is showing uniform distortion!

Overall a better display.



Jon

[jonpaul]

Hello again:

1/ Magnetic shield, degauss: I found an ancient Akai bulk degausser, and used a variac. With 40 VAC line, slow approach to the crt face gives the characteristic waving traces, beating at line freq. About 6" with the waving ~ 1 div, and gradual removal, I saw no permanent change in any part of the display. So geometry issue is not magnetization or the CRT Mu shield.

2/ I used a TEK 2901 marker gen, at 50 uS PRF, and with marker amp for 25 V pulses. The vertical is set to show just the middle of the pulse to avoid the fall time slope.

3/ Result: 2467: Perfect
2465B: Perfect
2467B#2 similar to 2467B#1 in the photos, but not a bad:

vertical lines almost perfect, same horizontal: fine in lower half, upper 2 or 3 div have just perceptible upward bend on right side of display.

The SNs of the two 2467B:

B0508xx, B0502xx so very close.

NOTE: DO NOT adjust any CRT trimpots unless necessary, since they can change the deflection factors and thus require a complete CAL 01, CAL 02, Hor and Vert!

Enjoy!

Jon

[jonpaul]

I cranked up 7104/7A29/7B15. These were originally made for AES nuclear testing and 1000s used at LLNL, Los Alamos, LBL, etc.

It was  CALed by a silent key, Walter, a super TEK/HP/Ham/Broadcast Engineer. 

Setup squarewave test,  transitions on graticule grid.

Perfect, NO observable distortion over the entire CRT faceplate! !

As the 7104 is 1 GHz BW and a huge bench scope,  the CRTs must be completely different designs, although both use the MCP.

Perhaps the CRT of 2467B was a compromise?

bonne journée

Jon

[jonpaul]

I just got my trusty 7904 and tried the same test with squarewave.

Same result as the 7104 , precisely rectilinear, no visible distortion.

Cheers,

Jon