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Nuvistor
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Different CRT’s can require different focus strengths, a Tetrode gun requires about 20% less field than a Triode. If the focus magnet is an electro magnet then the current can be reduced, if it’s a permanent magnet then shunts may be required.

Different electron guns designs can also affect the required focus magnet strength.

Information from the * Mullard Data book circa 1955-56

* available in the data library

Frank

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Posted : 23/05/2020 10:02 pm
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PYE625
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Thanks Frank, certainly something to bear in mind.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Posted : 23/05/2020 10:46 pm
Nuvistor
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@pye625
In the Data library there is a chapter on CRT’s, not long but a good read about various aspects.

Its in the section CRT among the data for various CRT types.

 

Frank

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Posted : 23/05/2020 11:46 pm
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PYE625
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It just goes to show how you can't just plonk any alternative CRT in and expect miracles without some modification. Even though the main criteria like physical size and gun type are very similar, there are less obvious differences to consider. The same heater current is important of course too, the voltage slightly less so in a series heater string.

Aside from the base and EHT connection differences, which I forgot to mention previously, I think I was very lucky to have available a suitable focus magnet assembly to make the new CRT useable in this set. I think it would have been easier if an electrostatic focus CRT was available, but beggars can hardly be choosy. Long gone are the days when the exact replacement type can simply be ordered !

I also consider myself very fortunate to be in the position to actually have a CRT that can be used anyway.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Posted : 24/05/2020 10:33 pm
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PYE625
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A little quiz for you....

If you go back to page one and look at my second post second photo (see here), you will notice an inductor mounted directly below the neck of the old CRT. It is L20 and it controls the line linearity. It has a red shaft that contains a ferrite core that can be slid in and out to, oddly enough, adjust the linearity. So what?, you might well ask.

Well, the device is a saturated reactance control that, now it is divorced from the original focus magnet assembly where it was affixed, fails to effectively adjust the line linearity. It simply alters the width instead.

So, why should this now be the case?

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Posted : 25/05/2020 1:37 pm
Nuvistor
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Being a saturated reactor it requires a permanent magnet, I do not see one in the photo on page one so is it relying on being fixed to the focus magnet to saturate?

See Patchet vol 2 pages 89-90 for a short description of saturated reactors for line lin, available in the data library.

Frank

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Posted : 25/05/2020 2:48 pm
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PYE625
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Frank, you are as ever sharp as a pin and spot on. 👍 

Yes, L20 relies upon the focus magnet field and I must think of some way to mount it in close proximity to the new magnet. Holding it close makes it work as it should, so I have an idea of suitable positions.

It would be interesting in the event of using an electrostatic focused CRT of what would be done to provide a magnetic field strong enough for L20, or simply do without it and accept the non-linear scan. Probably only applicable to this model, but other set's may be similar.

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Posted : 25/05/2020 3:38 pm
PYE625
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The simplest method of keeping the inductor in close proximity to the focus magnet was found by making a hole in a thin piece of paxolin of the same shape of hole in the old focus magnet, using araldite to glue the base of the former into the hole, then cutting the paxolin to shape and finally using a convenient place on the replacement magnet to screw the paxolin to. It works rather well and I am pleased with the bodge....erm, "modification".

IMG 5676 50

To understand the black art of electronics is to understand witchcraft.
Andrew.

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Posted : 25/05/2020 7:59 pm
Nuvistor and ntscuser liked
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