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Old 7th Nov 2019, 10:25 am   #1
line sync
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Default Ferguson 953T focus assembly

I`ve been restoring one of these sets recently and had a problem where I couldn't centre the picture , also the focus range was right at the end of it`s travel.
I had a scrap chassis so removed the focus assembly and fitted this to my restored chassis which made a big improvement.
The focus assembly from the scrap chassis did not look as nice as the original unit so I thought I`d be clever and swap the magnets over, big mistake.
The focus is now way off and on fitting the magnets to there original assembly`s the focus is still way out.
These units are quite simple and no mistakes were made on reassembly , I even marked the top of the magnet to make sure it was fitted in the right position.
I have played about trying to remagnetize the magnet and demagnetizing the focus sleeve all to no avail.
I now have two unusable focus units.
I really don't know what has gone wrong here and would be grateful for some advice to fix this problem.

Robin
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Old 7th Nov 2019, 10:38 am   #2
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Default Re: Ferguson 953T focus assembly

Hello Robin. SNAP! Only this time it was a PYE B18T that a mate had done exactly the same with. Once you take them apart they never focus again no matter how hard you try.
There are warnings about this but not all service manuals mention it. I can't understand how it can loose magnetism simply by taking it apart and refitting it.
You will need either another replacement unit or any magnet suitable for a tetrode tube.
Frustrating and very annoying! It would be interesting to discover if anyone else has experienced the same phenomenon. Regards, John.
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Old 8th Nov 2019, 10:15 am   #3
mark pirate
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Default Re: Ferguson 953T focus assembly

I also have had the same problem with a TV22 a few years back, I stripped down the assembly due to it being seized solid and corroded.

Although I was very careful to note exactly how it came apart, after doing nothing more than cleaning & reassembling, it would not focus properly

Luckily I had a parts set to rob, fitting this assembly cured the problem, the set now focused fine in the centre of the lever.

I then just cleaned up the exterior of the replacement unit with no problems.
I could not work out why the original unit had failed, it was again dismantled to check I had not made a mistake somewhere , but to no avail.
Lesson learnt!

Quote:
Once you take them apart they never focus again no matter how hard you try.
There are warnings about this but not all service manuals mention it. I can't understand how it can loose magnetism simply by taking it apart and refitting it.
I also am at a loss to understand why this happens, but I certainly won't be stripping one again!


Mark
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Old 8th Nov 2019, 10:40 am   #4
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Default Re: Ferguson 953T focus assembly

Hi all,
I've been able to do this with later ferrite magnet focus units, but not with earlier Alnico magnet types (which the shaded pole ones often are).
Dismantling requires magnets to be removed by prising them off with brass wedges (not a steel screw driver etc.). Sliding the magnets off is a no no.
All steal parts of the focus unit then need to be degaussed and the magnets put back on again in the same way they came off.
Some later ferrite focus units have an aluminium frame, which means you can slide magnets on and off without worry.
Also magnets must not touch each other. In shaded pole types the focussing sleave should not fully come into contact with the other end of the gap (on a TV22 for example, with the focus fully sound in there is still a gap). These things can be prevented by fittind card board rings etc. to prevent parts that should not touch from getting stuck together whilst dismantling.

Anyway, that's rather a lot of drivel from me.
On another note though is that I might have a spare focus unit for a 941T (will check over Christmas). If not I do have other units available.

Cheers
Andy
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Old 8th Nov 2019, 11:33 am   #5
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Default Re: Ferguson 953T focus assembly

Thanks for that Andy. That explains why the ferrite focus magnets employed in the Ferguson 204T series didn't suffer with this problem. They used to unstick themselves from the aluminium mounting resulting in the picture vanishing up into one of the CRT corners.
It was a simple matter to reglue them without any problems. John.
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Old 8th Nov 2019, 11:35 am   #6
Lloyd 1985
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Default Re: Ferguson 953T focus assembly

Quote:
Originally Posted by beery View Post

Anyway, that's rather a lot of drivel from me.

Cheers
Andy
Not at all, very useful information! It could explain why my TV22 has lousy focus, as I did strip the focus unit and clean it up and relube it long before it could display a picture!

Regards
Lloyd
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Old 10th Nov 2019, 10:34 am   #7
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Default Re: Ferguson 953T focus assembly

Thanks guys for all your reply`s.
I was thinking of buying a neodymium magnet which you use to re-magnetize weak magnets but round magnets as used in tv focus assembly`s are more difficult and also there is a danger of making the magnet to strong.
I can easily see me playing around with this for weeks and getting nowhere so
i think I`ll put this one down to experience.
Andy , thank you for the possible offer of another focus assembly , i may have another one lined up , i`ll let you know.

Robin
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Old 10th Nov 2019, 11:21 am   #8
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Default Re: Ferguson 953T focus assembly

Quote:
Originally Posted by line sync View Post
I really don't know what has gone wrong here and would be grateful for some advice to fix this problem.

Robin
I can explain exactly what has gone wrong and how to fix the problem. I have been through it myself and worked out a solution. (Neodymium magnets will not help you much) I will explain what is going on and how to fix it:

The mechanical assembly which holds the magnets acts as a "keeper", which brings the magnetic lines of force in fairly close proximity across the mechanically adjustable annular gap that controls the focus field.

When it was disassembled it grossly opened the gap & resulted in a weakening of the magnetic field from the magnets, which becomes obvious when its reassembled.

The only way to repair this (actually easy in this case) is to re-magnetise the magnets in their assembled state

One face of the assembly that abuts the magnets is a north pole, the other a south pole. You can check with a compass, but beware about north poles being north seeking poles etc, but see below for a quick check.

You need to wind a coil around the circumference of the focus assembly (out of the TV of course and away from everything else). 1.5 to 2mm diameter enameled copper wire about 20 to 30 turns will do it.

Look up the rules for the expected magnetic polarity of the field generated by the coil for a given turn direction and polarity. Or you can "cheat" by simply running a small current through the coil, connect it across a 12V battery (away from the focus coil assembly initially) with a 47R 5W WW resistor in series to limit the current, and check with the compass that the field generated by it (the pole) on one side of the coil, matches that of your focus assembly when you rewind or place the coil on the focus assembly.

Once you have the coil on the focus assembly and you are confident of the polarity aiding (not opposing ) the permanent magnet focus coil assembly, then put on some safety glasses and connect the coil assembly for a second or less across the terminals of the 12V car battery. The massive current will re-magnetize your weakened focus magnets. There will be some sparks, don't connect it too long or the coil will overheat.

You may then find the re-magnetized assembly is too strong, in that case to calibrate it downwards in field intensity, just part the assembly to progressively weaken the field in steps, so that in the end, your mechanical focus setting is in the center of its range for proper crt focus.

(ps: The first time I did it was around 1976 on an English Philco TV in NZ with an MW31-74 CRT in it where the focus magnet had been disassembled and lost its field intensity, it seemed like a straightforward problem and I had only just done high school Physics with simple magnetic theory)

Last edited by Argus25; 10th Nov 2019 at 11:37 am. Reason: typos as usual
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Old 10th Nov 2019, 11:41 am   #9
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Default Re: Ferguson 953T focus assembly

Hi Argus , thanks for your reply.
I think I`ll give this a go but am I right in thinking that when you slide the coil of wire over the magnet the north of the coil faces the south of the magnet ?

Robin
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Old 10th Nov 2019, 12:11 pm   #10
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Default Re: Ferguson 953T focus assembly

No,

North must be associated with north and south with south, you are trying to reinforce the existing field from a location outside the magnetic gap.

You wind the coil around the circumference of the assembly, not spilling over the edges of it, maybe 1/2 to an inch wide, but no wider than the focus magnet assembly or the plates that secure them.

You just need to be 100% sure that the applied voltage polarity and wire turns direction results in a magnetic field from the coil that aids the polarity of the N & S poles generated by the focus magnet, not opposing it or sticking to it, but repelling it, or you will end up trying to reverse magnetize them and grossly weaken them.

It seems counter intuitive because N & N and S & S poles repel each other. But it makes more sense if you think of the polarity of a focus coil which achieves the same effect as the magnet and will magnetize a magnet, with that polarity, think of the magnet as having a memory for the applied field from the coil.

So unless you are confident that you can 1) identify the poles correctly on your focus magnet, and 2) calculate the expected field from the coil, it is better to set up a dummy coil with some turns direction and a known applied polarity from the battery (with a current limiting resistor) and check with the compass that the poles of the coil and magnet assembly match before you either wind the wire around, or slip the coil over the magnetic assembly and apply the high current.

I wound the wire tightly over the assembly, with the high magnetic repulsion forces when the current is high, the coil could jump around/off the assembly if not properly secured.

(Another way to weaken or calibrate the field downwards could be with the same coil, with stepwise lower pulses of current in reverse, rather than having to weaken the magnets with mechanical disassembly)

PS: Of note there are other devices you might work on where disassembly of components weakens magnets. One is the DC brush motor, when the armature is removed, but if you are wise to it you can slide in a keeper. (A classic for this is the Capstan motor in the Nagra MK2 professional tape recorder) Likely I think, these devices, in many cases , like the PM focus magnet assembly for CRT's, are magnetized after they are assembled at the factory, which explains why the fields are weakened after dis-assembly and reassembly.

Last edited by Argus25; 10th Nov 2019 at 12:40 pm.
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Old 12th Nov 2019, 11:21 am   #11
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Default Re: Ferguson 953T focus assembly

Quote:
Originally Posted by line sync View Post
Hi Argus , thanks for your reply.
I think I`ll give this a go but am I right in thinking that when you slide the coil of wire over the magnet the north of the coil faces the south of the magnet ?

Robin
I'm interested to see how you get on with Argus's idea.

Quote:
Originally Posted by Argus25 View Post
PS: Of note there are other devices you might work on where disassembly of components weakens magnets. One is the DC brush motor, when the armature is removed, but if you are wise to it you can slide in a keeper.
This is a common problem with the motors on Hornby 3 rail doublo locos. Of course neodymium magnets work in that case (in place of the original magnet), I suppose because it is a simple rectangular magnet with a north pole on one side and a south pole on the other. The focus magnet is a doughnut shape of course, so trying to add magnets at the side would no doubt result in uneven focus.

Cheers
Andy
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Old 12th Nov 2019, 6:50 pm   #12
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Default Re: Ferguson 953T focus assembly

Hi Andy
I`ve managed to get some thick enamelled copper wire of a friend today so will have a go at this tomorrow.

Robin
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Old 14th Nov 2019, 8:33 pm   #13
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Smile Re: Ferguson 953T focus assembly

It worked
The focus assembly has been saved and is now fully fitted back on the chassis.
The focusing is good and the picture centres nicely.
It took me a while to establish the north and south poles of the copper wire coil because with the 47 R resistor in series there is limited current in the coil but if you hold a compass very close to the coil you will see the pointer move.
I wound just over 20 turns around the magnet and it got quite warm after 3 or 4 flashes across a car battery.
Thank you very much argus for telling me about this.

Robin
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Old 14th Nov 2019, 8:59 pm   #14
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Default Re: Ferguson 953T focus assembly

Here is a picture of the magnet with the copper wire wound around it.
The wire was very lively like a coiled spring so I used 2 holes in the metal frame to pass the wire threw.
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Old 14th Nov 2019, 10:26 pm   #15
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Default Re: Ferguson 953T focus assembly

Quote:
Originally Posted by line sync View Post
It worked
The focus assembly has been saved and is now fully fitted back on the chassis.

Thank you very much argus for telling me about this.

Robin
Robin,

You are very welcome. And thank you in return for being receptive to the idea.

Hugo.
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Old 15th Nov 2019, 10:13 am   #16
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Default Re: Ferguson 953T focus assembly

That's a great result Robin.

Good call Argus.


Cheers
Andy
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Old 15th Nov 2019, 11:10 am   #17
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Default Re: Ferguson 953T focus assembly

Great result! I won't forget that one. Thanks Hugo! Regards, John.
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Old 15th Nov 2019, 12:10 pm   #18
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Default Re: Ferguson 953T focus assembly

Glad to hear this worked, Robin, and that the wire I supplied came in useful.
Best Regards, Mike.
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Old 15th Nov 2019, 9:42 pm   #19
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Default Re: Ferguson 953T focus assembly

Good result, will have to try it on my spare TV22 focus magnet


Mark
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