Ion trap magnet - setting up in correct position

[RT 1006]

The restoration of my KB KF40 console set is almost finished. This lengthy restoration is the subject of an earlier thread which I will update once all work is complete.

The final thing I'd like to do is check the position of the ion trap magnet. The magnet was removed when the tube was removed during the restoration. I marked the exact position of the trap prior to removal and refitted it back in the same place.

As can be seen from the pictures the magnet is sitting at a jaunty angle. There is no saying this is the correct position so I'm wondering if someone could help me out with the correct setting up procedure with regards to the location of the trap? Thanks in advance, Kev

[Freya]

It is a simple case of moving it to obtain the brightest raster, avoiding any edge shadows in the process.
Most likely its in the best position as it is anyway, even at the angle.

[Peter.N.]

Yep, used to be a more or less daily job in the '50s as the fibre strap used to brake and the trap would be found upside down due to the weight of the magnet.

Peter

[jmedley]

Looking at a plain raster, rotate the magnet and also check forward and rearward movement of the magnet to achieve the most even and highest all over screen brightness

[Mad Mal]

We used to have an old Marconiphone two channel VHF set when I was a kid that required this doing every so often; usually more so as the CRT aged. With the change to "narrow neck" CRT's the Ion trap gradually reduced in size and when UHF came along,it was replaced by a flat copper loop in a paper sleeve(working I gathered because of induction). Some Phillips Branded CRT's in the late 60's had a weird split element type of these sleeves. That Marconi was passed around the fammily and ended up being sold on to someone who ended up in the Highlands of Scotland. After that I have no more info. it would be interesting to know how long it soldiered on.

[HamishBoxer]

Wonder what the year was when the set went up the Highlands?

[Nuvistor]

Originally Posted by Mad Mal

We used to have an old Marconiphone two channel VHF set when I was a kid that required this doing every so often; usually more so as the CRT aged. With the change to "narrow neck" CRT's the Ion trap gradually reduced in size and when UHF came along,it was replaced by a flat copper loop in a paper sleeve(working I gathered because of induction).

Not quite.
The ion trap was used in CRT's that had the gun offset and did not point at the screen. The ion trap bent the beam back to the screen, that's why it's adjusted for max brightness without corners shadow or consistent overall screen brightness. Theses CRT's did not have the aluminium coating on the back of the phosphor. The ions went off at tangent to the electrons and stopped the ions burning the screen
The later narrow neck CRT's like some later wider neck ones had the aluminised screen so the bent electron gun was not required.
The copper loop in the paper inserted under the scan coils was the horizontal linearity control, care in adjustment was required, usually a specific distance in to the coils or damage could occur.

Frank

[Peter.N.]

I had a friend in Lairg from the '70s who was involved in the spread of TV througout Sutherland, he covered practically the whole of it, about 6000 sq miles setting up passive relay systems on mountains and he commissioned the first relay station on the north coast, he lives in Australia now.

Getting back on subject, I don't ever recall seeing an ion trap on a narrow neck tube, that must have escaped me, I thought they all had straight guns by then.

Peter

[Heatercathodeshort]

There was only one size of ion trap last used on the 90 degree AW43-80 of 1957 vintage. [Mullard and Mazda traps were not interchangeable due to the differing method of bending the gun.] Mal might be getting confused with the little magnetic ring that was often fitted to correct for corner cutting and even focus across the entire tube.
It was vitally important to adjust an ion trap for maximum brightness, not for the removal of corner cutting, picture positioning or focus. Inexperienced engineers used to get puzzled when adjusting ion traps with a high emission tube. With the trap 180 degrees out of position a dim out of focus illumination would appear on the screen. [Ha ha!] They soon learned.
I have seen tubes with severe astigmatism caused by the tiny hole in the grid becoming slightly oval due to electron bombardment with incorrect positioning. Of course this is over a very long period and is of little worry with our vintage receivers.
The line linearity loop mentioned by Frank was a nasty idea and often resulted in the scan coils overheating resulting in their plastic moulding falling to bits..
What a life! John.

[Peter.N.]

Never heard of that happening John, but of course we always used to set ours correctly.

Peter

[Restoration73]

This is the manufacturers recommendation :

[Mad Mal]

Originally Posted by Heatercathodeshort

There was only one size of ion trap last used on the 90 degree AW43-80 of 1957 vintage. [Mullard and Mazda traps were not interchangeable due to the differing method of bending the gun.] Mal might be getting confused with the little magnetic ring that was often fitted to correct for corner cutting and even focus across the entire tube.
It was vitally important to adjust an ion trap for maximum brightness, not for the removal of corner cutting, picture positioning or focus. Inexperienced engineers used to get puzzled when adjusting ion traps with a high emission tube. With the trap 180 degrees out of position a dim out of focus illumination would appear on the screen. [Ha ha!] They soon learned.
I have seen tubes with severe astigmatism caused by the tiny hole in the grid becoming slightly oval due to electron bombardment with incorrect positioning. Of course this is over a very long period and is of little worry with our vintage receivers.
The line linearity loop mentioned by Frank was a nasty idea and often resulted in the scan coils overheating resulting in their plastic moulding falling to bits..
What a life! John.

My memory not what it was (down to my illness I suppose) You and Heatercathodeshort (had a number of line op valves do that with spectacular results) are right. That linearity coil did cause overheating, but as the guns became straighter the need for the ion trap disappeared with the advent of Precision In Line CRT's that linearity sleeve went as well. Supposedly the magnetic rings on the yokes on these tubes were meant to be set and never adjusted...How many of you did try I wonder. I used to do minor adjustments on an ageing tube where the heat over the years had deteriorated the glue that fixed the rings in position. Quite often improving convergence! Sadly those days are gone and with then the fun of seeing what did what. You could always tell though where the rings original position was. A faint line of, what I assume, was marker pen ink left by the technician that had set them during manufacture.

[FERNSEH]

All Mullard ion trap CRTs employed what is known as a bent gun assembly.
If the ion trap magnet has an arrow in it this should point towards the screen.
Line up the ion trap with pin 3. The KB might have a Brimar C14FM CRT which will have a different type of gun assembly.
I've refitted the ion trap magnet on the Mazda CRM141 which is installed in the Ekco T221. The magnet lines up with pin 8. Mazda employed a different type of gun assembly, it has the appearance of a straight gun.

DFWB.

[RT 1006]

Thank you all very much for your replies above. After a bit of experimentation I returned the ion trap to its original position as it was found that this is where best results were obtained. Kev