The 3BP1 Anderson monitor

[Harry Dalek]

Harry, Looks better. May be that the largest possible size is not the best setting. At a certain moment you are going to see the lines separating. Then you are too far. As well, when you are scanning too close to the edges of the tubes screen, the scanning becomes non linear. the rectangle of the picture then gets the shape of a pillow or a barrel. This is also called "pillow distortion" and "barrel distortion".
So it is not unwise that the corners of the picture are free of the edges of the picture tube.

On the Grant Dixon test picture you might use a ruler to make the circle a true circle..... Grant calculated it to be a circle.

You will see that this picture has a vertical grey scale at the left and at the right. This grey scale is two lines at both sides. If your frame sync is delayed you will see a wider grey scale at the left side (two lines) and a narrower (one line) at the right side.

Good luck..... It is amazing

HI Klaas
i tried this size i think this is about tops before the bending would happen as you and Steve mentioned in hes post also ...I will try your ruler a compass for making circles also do the trick careful not touching the screen with the pointy bit ! the eyes pretty good noticing how off it is but yes a correct measurements do need a tool. Heres a few gos i had this afternoon for fun trying the maximum size i think is usable for the 3BP1.

[Steve Anderson]

There's not that much information 'out there' about the history of CRTs, you'll more than likely find datasheets and the like, but where and for what, when and by whom were they used? For what it's worth here's what I've been able to find on the 3BP1...dig through the text to find the pertinent bits...

Steve A.

[Klaas Robers]

Harry, I grabbed the Grant Dixon pattern off the screen (Prt SC) and pasted it into a Paint canvas. Then I could measure the height (142 pixels) and width (142 pixels). So the circle is a true circle. Well done.
In the same picture I could see that your monitor displays both lines of grey scale, as well at the left, as at the right. Also well done.

However I see also the individual NBTV lines, with black spaces in between. I would make the picture size somewhat smaller, such that the lines just touch. Another trick is to add a high frequency wobble frequency, say 100 kHz, to the horizontal deflection voltage. then the lines become wider but you keep the same definition in the vertical direction. This is called "spot wobble".

You see, there are always improvements possible once it is running....

[AncientBrit]

Harry,

Not sure if this will work but it's worth a try.
If you are not surrounding the tube with any form of mumetal or steel screening try bringing a small magnet near the outside of the tube towards the front.

You might be able to effect some form of partial correction for the barrel distortion.
Swap the magnet poles to get correction in the correct sense.

Cheers,

Graham

[Harry Dalek]

There's not that much information 'out there' about the history of CRTs, you'll more than likely find datasheets and the like, but where and for what, when and by whom were they used? For what it's worth here's what I've been able to find on the 3BP1...dig through the text to find the pertinent bits...

Steve A.

Its older than i thought reading the pages you uploaded Steve,i was thinking mine might be from the 50's ....would be interesting to know its history apart from it was made in the USA way back when .
Mine had twin that got lost some where during the move to Melbourne in 2001 i am pretty sure i had this CRT since the 80s so a good 30 years ..

[Steve Anderson]

Unless you 'pull' a CRT from a completed instrument there's a good chance that it's unused, especially if you also have the original packing. Those with burn marks in the phosphor, particularly a straight line across the centre from oscilloscope use, are probably nearing end-of-life. They're useful as a 'test CRT'. As a test engineer for Decca Radar in the late 70s we fitted a test CRT when the equipment was first switched on. Once all the faults had been found then soak tested for a few days the 'real CRT' was fitted, the one the customer would be staring at for hours.

When a customer required a new CRT fitted to replace the old tired one after a few years the old CRT was returned to us and became those test CRTs. It didn't matter during the test process that the scan was a bit dim and the centre of the tube was really dim. As long as we could see something, that was fine. If the odd test CRT got fried due to some fault in the build, it didn't matter, but it was quite rare.

Now you might understand my caution when building something based around a CRT and you have only one example. These days getting a replacement is getting increasingly harder and/or expensive.

Amongst my small collection there are only two varieties I only have one example of, a single 3FP7 and a single DH3-91 (aka 1CP1). Otherwise I have at least two of each.

Steve A.

[Harry Dalek]

Harry, I grabbed the Grant Dixon pattern off the screen (Prt SC) and pasted it into a Paint canvas. Then I could measure the height (142 pixels) and width (142 pixels). So the circle is a true circle. Well done.
In the same picture I could see that your monitor displays both lines of grey scale, as well at the left, as at the right. Also well done.

However I see also the individual NBTV lines, with black spaces in between. I would make the picture size somewhat smaller, such that the lines just touch. Another trick is to add a high frequency wobble frequency, say 100 kHz, to the horizontal deflection voltage. then the lines become wider but you keep the same definition in the vertical direction. This is called "spot wobble".

You see, there are always improvements possible once it is running....

Thanks Klaas
I must have a good eye for circles that was only done via looking and adjusting .
If i defocus it a touch the line gap tends to go ,i must admit don't mind it that much seeing the lines .
I did an experiment this afternoon with MR Catweazle from 1970 taking pictures and adjusting the focus for different results ..Taking pictures of NBTV is a bit tricky as what the camera sees is not what the eye sees ..i see even with lines a pretty good picture where as the camera can be hash it sees faults where i don't ,i suppose since it can take a snap in time it will see a scan where i see a few lazy brain lazy eyes !

Yes Steve i understand these tubes are rare these days i think even the old colour picture tube i see every day on the side of the road as just rubbish now no one wants will become pretty rare in the new few years , Do you plan one using all your CRT 's sort of best existence to be used as intended .

Good Idea Graham as you say the effort worth the result ,its one of those things you will never know unless you try if works for sure i say !

[Klaas Robers]

Harry, these are exactly the type of experiments that you should do in this phase.

In fact I like the blurried picture best, however it becomes also unsharp. That is the problem with defocussing: the spot defocusses in both directions, horizontaly and vertically, while you would like to defocus only in the horizontal direction. Then you loose the sight on the individual lines, but you keep the definition along the lines.

This is what spot wobble does, it deforms the spot into a small dash. But it is not too easy to implement. It should be possible for you to adjust the dash width as well.

[Steve Anderson]

As a stop-gap measure you could always adjust the 'astig' control in conjunction with the focus to get a 'sort of' elliptical spot. It's not ideal for sure but it may give you a clue as to the improvement that could be obtained.

The only thing is - I can't recall if I added an astig control to this project! I'll need to look back through the files, but I've got a feeling I didn't...trying to keep things simple.

However, to add 'spot wobble' in this case is quite easy. You simply need to inject a sawtooth or triangular waveform into the horizontal deflection amplifier. A 555 could generate a reasonable triangle waveform and inject this at reduced amplitude into the deflection amplifier at the 'position' pot circuit...opposite to the ramp input, it is a differential amplifier, and at 100kHz plus a few harmonics, the amps should be up to it.

Harry, if interested I'll look into it.

Steve A.

Well, dang my bat! I did add an astig control, VR902! Harry, twiddle it in conjunction with the focus pot, see what you get. Don't expect miracles though!

[Harry Dalek]

As a stop-gap measure you could always adjust the 'astig' control in conjunction with the focus to get a 'sort of' elliptical spot. It's not ideal for sure but it may give you a clue as to the improvement that could be obtained.
The only thing is - I can't recall if I added an astig control to this project! I'll need to look back through the files, but I've got a feeling I didn't...trying to keep things simple.

OH yes top right hand control its sure there as i recall with no raster the dot circle can be changed to an oval shape .

However, to add 'spot wobble' in this case is quite easy. You simply need to inject a sawtooth or triangular waveform into the horizontal deflection amplifier. A 555 could generate a reasonable triangle waveform and inject this at reduced amplitude into the deflection amplifier at the 'position' pot circuit...opposite to the ramp input, it is a differential amplifier, and at 100kHz plus a few harmonics, the amps should be up to it.

Harry, if interested I'll look into it.

It depends if you are keen try or want it added to the Anderson monitor ,i think klaas is wondering this might be an improvement ,i think i could add another little board if need be ..I am fine to add that little circuit .

Steve A.

Well, dang my bat! I did add an astig control, VR902! Harry, twiddle it in conjunction with the focus pot, see what you get.

I think i did try a bit with the focus ,but i should try as in the gif pictures i did but one control setting at a time and see ........as again i wish the camera would take what i see it does at times but have to take many pictures till the silly thing catches up by chance and takes that one shot .

[AncientBrit]

Harry,

Did you see my post re magnets, about 8 posts ago,

Cheers,

Graham

[Harry Dalek]

Harry,

Did you see my post re magnets, about 8 posts ago,

Cheers,

Graham

Hi Graham i replied in the Catweazle posting i think i would keep the display around that size to stop the bending but i do have small magnets handy to try your experiment ,i recall seeing magnets used on the edge of old deflection yokes wonder what it was used for may the same sort of idea .
EDIT
Not a great video as i was trying to film so results in the film not really as good as what i saw same time doing this and over exaggerated results ,but what i found with one magnet top of the raster depending on the pole used and getting the right position it bent the top of the raster lines to either a arc or a parabola shape so i suppose distance and such of each line moves up a touch from end to middle and back down again bottom of raster was not effected so you would need one there as well ...as far as how well it would work on a large raster with the screen bending still need to check this ....not really great for NBTV in any case as you don't want to large a picture ..might be worth it for larger raster line system on these sort of tubes.
I did take 2 pictures using one side of the magnet so the bowl looking curve top of raster.

[Harry Dalek]

Also today had a look at the focus and astig control the focus is more helpful ,what i found is when i defocus i have to adjust the brightness control a touch brighter . Not quite a full defocus is a nice compromise for results .
Either way i am not really to bothered it gives pretty good results as is .

[Steve Anderson]

That's it! If you're satisfied, then fine. Others may wish to continue to chase perfection, but if you're happy, leave it be.

You could go on and on with this into the realms of diminishing returns. I would sit back and feel rather chuffed with myself in having got this far. How many others have a CRT kicking around in a drawer for the past 30 or so years and have done nothing with it? I have to admit to being a bit guilty on this crime here, but so far it's not quite the same duration...but it so easily could become so.

Steve A.

[Harry Dalek]

That's it! If you're satisfied, then fine. Others may wish to continue to chase perfection, but if you're happy, leave it be.

You could go on and on with this into the realms of diminishing returns. I would sit back and feel rather chuffed with myself in having got this far. How many others have a CRT kicking around in a drawer for the past 30 or so years and have done nothing with it? I have to admit to being a bit guilty on this crime here, but so far it's not quite the same duration...but it so easily could become so.

Steve A.

Yes i am happy with it Steve does what i wanted or really hoped it would do ,as i said before i don't mind the lines look it is 32 lines can't be to fussy ~! and monitor can be adjusted to remove the look if i want it to with the focus control.
Any future constructor going over this design i would say its well worth the work making one !