EEVblog #524 - Vignetting on a Cathode Ray Tube

[EEVblog]

Dave explains what causes CRT vignetting problems and how to fix it.
A follow up to the HP 35660A Dynmaic Signal Anayser video here:

Whiteboard explanation of how a Cathode Ray Tube works, and how it can cause interference to sensitive measurements and the shielding required in a precision instrument like the HP DSA.

[RupertGo]

Yay! Fixed!

Two things, though - I've always pronounced it ving-yett rather than vig-nyett, because it's really a French word and that's how it's said in la belle France. Could certainly be wrong here too.

However, there won't be a focus coil inside the tube. It'll be electrostatic focussing, so there'll be some sort of plate system to tighten up the beam. CRTs can have any mix of electrostatic and electromagnetic beam control after it leaves the gun, and in the early days of long-neck CRTs there were some that used coils to focus the beam and plates to deflect it. This is quite a short modern tube, though, so it's got coils for deflection but plates for focussing. I don't know of any CRTs that had internal coils - but again, open to re-education.

Bonus fun: when I was much younger, I used to take old portable TVs, slide the deflection yoke off the neck and put a salvaged yoke back on, which I fed from a stereo audio amp. You end up with a really cheap Lissajous display that produces lovely patterns in time to any music you play into it. Quite dangerous if you're not confident of what volts live where, but I managed to get invited to various parties on the strength of being able to put on a freaky sound-to-light show. In the end, I even had a system with a cheap video camera and a junked projector that could throw the images around an entire room, which meant I met far more interesting girls than I ever managed with my ham radio gear.

Result.

[c4757p]

Two things, though - I've always pronounced it ving-yett rather than vig-nyett, because it's really a French word and that's how it's said in la belle France. Could certainly be wrong here too.

I just let Dave have his Aussie-isms - they're as just bad as we Americans are!

[N2IXK]

Thanks for the update video, Dave!

There isn't a focus "coil" inside the tube, but a focusing anode. This an electrostatic focus tube, not an magnetic focus type. Magnetic focus CRTs pretty much went away back in the 1950s, because it was much easier to supply a simple DC focus voltage (at essentially no current) than the substantial DC current needed to energize an electromagnetic coil. Not to mention the savings in copper and steel over a wound focus coil.

[Hydrawerk]

Those CRTs are gonna be like vacuum tubes nowadays... Our children will see them only at a museum, while we grew up in a world full of CRTs in TV sets or oscilloscopes and other signal analyzers...

[Bored@Work]

Those CRTs are gonna be like vacuum tubes nowadays... Our children will see them only at a museum, while we grew up in a world full of CRTs in TV sets or oscilloscopes and other signal analyzers...

These days try to explain rotary phones to kids.

[c4757p]

Two things, though - I've always pronounced it ving-yett rather than vig-nyett, because it's really a French word and that's how it's said in la belle France. Could certainly be wrong here too.

I just let Dave have his Aussie-isms - they're as just bad as we Americans are!

I'd suggest vin-yett. I haven't heard the "g" pronounced at all.

Ah, OK then, "Dave-isms". I've got some pretty cringeworthy Chris-isms, so we're still even

[N2IXK]

Those CRTs are gonna be like vacuum tubes nowadays... Our children will see them only at a museum, while we grew up in a world full of CRTs in TV sets or oscilloscopes and other signal analyzers...

Some kid eventually has to wonder why that big streaming video site is called You*TUBE*, right?

[c4757p]

Well, it's online, and the Internet is a series of tubes... so...

[free_electron]

glad to see it's fixed.

i think what happened is that magnet assembly got jolted loose with the yoke and probably a bit of plastic broke off.
when you re-set the yoke the first time that prevented it from going forward. pulling the yoke back and then re-set it solved that.

[IanB]

The traditional solution for CRT flicker was to run an external sync from the CRT to the camera. Do modern cameras have an external sync input?

[SeanB]

The traditional solution for CRT flicker was to run an external sync from the CRT to the camera. Do modern cameras have an external sync input?

Not at the Prosumer end. You have to go up to the pro market to start to get them with a genlock input these days. Mostly done because they are so small that there is no room for a connector likely to be in most cases unused. You only find spare space on large broadcast studio units which definitely will all be locked to a master clock.

[carbon dude oxide]

Those CRTs are gonna be like vacuum tubes nowadays... Our children will see them only at a museum, while we grew up in a world full of CRTs in TV sets or oscilloscopes and other signal analyzers...

Im quite young and i enjoy vacuum tubes ive finished building a clock using nixie and i have a freind my age who works in a high grade audio shop so i get to see all the vaccuum amps my girlfriend however does not understand why i like them XD

[Fezder]

Nice video! yet another tip, if i ever encounter this problem....none of my broken in-service scopes have this problem :S but they all have screen problem however haha...

[G7PSK]

My father was/is a professional photographer and he always leaves the g silent. The word is late middle english, and is a small ornamental design filling a space in a book or carving typically based on foliage. The word is derived from the French for vine (vigne). How it came to be used to describe a photograph that fades into the background I do not know.   

[EEVblog]

Thumbs up Dave for trying to set the camera to minimise flickering. Did you also set manual exposure?

I usually set manual exposure for most shots. I've set it as a dedicated button on the back of my camera. You'll often see me push the button during a video. For example, if there is a white envelope on mailbag, and then the contents are pulled out and it's under exposed, I'll then hit the exposure button to lock it back to the new contents. If I know a shot will benefit from autoexposure then I'll just leave it on that. The majority of shots are best shot with manual exposure though. For example, a PCB teardown and my (pale white) hand comes into the shot. On auto exposure you'd have the camera compensate for that and the board darkens and lighens every time I move my hand into the shot, and that can be really annoying to watch.

[JoeMuc2013]

Good one, Dave! I knew you wouldn't leave it half-done.
Anyway, why would a manufacturer ever allow for this to happen at all? It does look like a 100% solid device otherwise. So wtf... they saved a few pennies at the wrong place.

[SeanB]

The monitor is a bought in unit, made by a good manufacturer, Matsushita ( National Panasonic) and is about one of the best you could get for reliability and life. No compromises on parts, no overrunning and no bad capacitors. Was designed for a 10 year life on 24/7/365 in a rack. i have seen many that have had a burn on them such that you could read the menu when off, but they still were working.

[tecman]

One other note.  On the back of the deflection coils you can see a black ring.  It is most likely two black rings, which are weak permanent magnets.  You can rotate them to deflect the entire raster; in other words to center the image.  If you notice that the raster was not quite centered in the CTR, by adjusting the rings you could better center the image.  The other magnets that had broken loose are for geometric correction.  To reach the corners of the screen rectangle, it takes some correction since the distance from the gun to the corner is longer than to the center of the screen.  These magnets perturb the sweep's magnetic field to correct this error and linearize the display geometry.

I have a DSA as well.  When I was looking the price on the used HPs was just way beyond my range.  I eventually found an Advantest R9211C.  The menus are not as logical as on an HP, but performance is as good.  The Advantest units were sold by Tektronix in the 90's.  Smaller CRT but more math functions available compared to HP and a built in thermal printer make it a descent unit.  Best of all is that it is 13"x7"x17", quite a bit smaller than the HP and only 35 lbs.  I can actually carry it other locations without a hoist.

paul

[N2IXK]

Nice video! yet another tip, if i ever encounter this problem....none of my broken in-service scopes have this problem :S but they all have screen problem however haha...

Oscilloscopes don't use magnetic deflection coils. In order to achieve higher bandwidth, they incorporate 2 sets of electrostatic deflection plates internal to the tube.

The only neck-mounted coil you might see on a scope CRT is for trace rotation.

[Fezder]

Nice video! yet another tip, if i ever encounter this problem....none of my broken in-service scopes have this problem :S but they all have screen problem however haha...

Oscilloscopes don't use magnetic deflection coils. In order to achieve higher bandwidth, they incorporate 2 sets of electrostatic deflection plates internal to the tube.

The only neck-mounted coil you might see on a scope CRT is for trace rotation.

ahh, i see. didn't know that either thanks for info!

[FrankBuss]

Nice that you fixed it.

Dave, you mentioned something about to be careful about charged capacitors. How did you discharge them or is it sufficient to turn it off and wait an hour, or a day and then poking around with a neon-lamp tester screw driver at suspicious parts to see if there is any charge left? I plan to repair my Vectrex, modifying it for buzz reduction and I don't want to get electrocuted.

[mrflibble]

How did you discharge them or is it sufficient to turn it off and wait an hour, or a day and then poking around with a neon-lamp tester screw driver at suspicious parts to see if there is any charge left?

If you do that, remember to use the right one.

[steve30]

Nice video. I like the look of that green CRT. Looks nice and glowy .

[FrankBuss]

If you do that, remember to use the right one.

Ok, maybe not a good idea to use it, I imagine the sausage in the video would be my finger

Found the service manual: http://www.vectrex.nl/docs/vecman.pdf The voltage at the CRT is 5.8kV. But I don't plan to power it up when the case is open, so I hope that I can still do it when I know how to discharge it safely.