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15th Dec 2019, 1:57 pm | #1 |
Pentode
Join Date: Oct 2009
Location: Bergen, North Holland, The Netherlands
Posts: 180
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Magic Stick
For the alignment of coil L3 in the Barlow Wadley XCR-30 I had to compress and expand the windings.
To be sure not to overdo things some kind of tool should be a help . The recollection of an article in the book Elements of Radio Servicing by Marcus and Levy (1955, page 437) gave me the idea to copy their example of a tuning stick. Just a rod with a a brass and steel bolt at the ends and it worked remarkable well. But I am still a little surprised that differences in two metals would give such a reaction to an electrical field. Jard N. |
15th Dec 2019, 2:06 pm | #2 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,899
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Re: Magic Stick
The brass end acts like a shorted turn partially coupled to the coil of interest, reducing its inductance. The steel adds a ferromagnetic effect, increasing L but also increasing losses. A bit of dust-iron core is a better choice than steel.
David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
15th Dec 2019, 3:45 pm | #3 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,087
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Re: Magic Stick
And I suspect that a slug of aluminium (or copper or silver) might also be better than brass!
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15th Dec 2019, 5:38 pm | #4 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,007
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Re: Magic Stick
Those were always known as "tuning wands" in my day. Brass one end {to act as a shorted turn and lower the inductance), ferrite the other end (to increase the inductance).
You don't need much of either to get a noticeable effect! Reminds me of another 'tweak' I came across some years back - to increase the inductance of a coil, wind a bit of recording-tape round it! This has more than once been used here to add a smidgin of needed extra ""L" to bring to resonance an IF coil whose slug has been cracked/wedged by the Phantom Twiddler. |
20th Dec 2019, 10:03 pm | #5 |
Rest in Peace
Join Date: Feb 2004
Location: Hexham, Northumberland, UK.
Posts: 2,234
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Re: Magic Stick
I made a tuning wand many years ago but the only time it moves is when I rediscover it every once in a while among my other "tuning sticks" of various vintage. I was convinced when I originally made it that it would be in constant use but sadly no. The wand is made from an RS cotton bud stick, with a hollow ferrite tuning slug jammed on one end and a small brass stand off pillar jammed on the other. It does work well though, should I need it one day.
Alan. |