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neutronic 15th Dec 2019 1:57 pm

Magic Stick
 
2 Attachment(s)
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.

Radio Wrangler 15th Dec 2019 2:06 pm

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

kalee20 15th Dec 2019 3:45 pm

Re: Magic Stick
 
And I suspect that a slug of aluminium (or copper or silver) might also be better than brass!

G6Tanuki 15th Dec 2019 5:38 pm

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.

Biggles 20th Dec 2019 10:03 pm

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 I was convinced when I originally made it that it would be in constant use but sadly no.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.


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