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#1
03-05-2011, 10:33 AM
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Focus coil construction
Hello, I would like to ask a strange question, if you don't mind:
I'm in the process of doing some TV experimentations, a monitor with a 7BP4 round CRT, I have already found or made all the relevant bits, except one, the focus coil! Finding one in my location is near impossible, as the focus coils were used only for a year or two in pre-mass production sets, these are rare to find and a shame to harvest for parts My somewhat stupid question is: is it possible to build a bobbin, wind some thousand turns of enamelled wire (no problem for me doing this, I'm fully equipped with more than one professional winding machine), use aluminum as "body" of the so called focus coil and call it a day? Of course, I would like to avoid ferromagnetic material if possible at all, as it is not so simple to shape. The "no ferromagnetic material" part derives from some CRT datasheet like this one, especially page 6 If the thing is feasible, a 7BP4 requires a RETMA 106 focussing coil, I don't know the magnetic strenght this coil is capable of, so please does someone have an idea of how many turns I need to wind on the presspahn coil form, in order to have the job done? If I'm thinking something vaguely reasonable, roughly 7000 turns should do it. My circuit requires around 110 / 115 milliamps in the B+ line and the EHT is shy of 9Kv, a bit "hot" for this CRT, I know If further data are required, just let me know and I'll try to find 'em Thanks in advance for all the help! Giulio Maiocco 
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#3
03-08-2011, 03:56 AM
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Thank you very much John!
So, it appears I wasn't too far off, good to know! What worried me the most was the need of a ferromagnetic core and, if this is not needed, but what is required is "only" a big fat winding, then no problems at all.. It appears that a very kind member over at ARF named Genoo solved my problem, as he sold me lots of TV goodies for a song. In this package there are some focus coils, so I should be ready to go building the monitor  Thanks a lot again for your help! Best regards Giulio Maiocco
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