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I am a beginner electronics hobbyist trying to follow an old schematic / instructions to build a theremin.

I have all of the pieces except one. This piece (Toko 154ANS-T1019Z) calls for a "68 µH, hi-Q, variable inductor". However I cannot find a 68 uh variable inductor at all (See link, all options are discontinued pieces which cannot be purchased).

Note: Octopart shows the part may be available when it is not. The websites Octopart uses show the part is unavailable. In other words I cannot find this part or any variable inductor with 68 uh inductance

See the potion of the schematic below: enter image description here

enter image description here

How could I substitute this piece?

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    \$\begingroup\$ In general when building with obsolete parts you either redesign the circuit, build your own parts (Amadon has adjustable coil forms, if you want to go on that adventure), scratch for new-old-stock parts (which it looks like you've found, so pounce on it), or find used parts. \$\endgroup\$ – TimWescott Jan 1 '20 at 17:37
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    \$\begingroup\$ @AndrewMorton, I checked the datasheet of that 82 uH part (assuming you mean the Toko one on Digikey) and the tunability is only +/- 3%. \$\endgroup\$ – The Photon Jan 1 '20 at 17:45
  • \$\begingroup\$ None are in stock, Octopart may list them as in stock, but trying to purchase, or visiting the sites Octopart links shows they are not available \$\endgroup\$ – dustytrash Jan 1 '20 at 17:47
  • \$\begingroup\$ Searching for new-old-stock parts sounds like my best option as a beginner, but I'm not sure how to find one or what would be an appropriate substitute \$\endgroup\$ – dustytrash Jan 1 '20 at 17:52
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82 uH is small enough that you will have no problem winding it on a coil former with tuning slug.
Suppliers such as Mouser may have formers with inductance per turn_squared specified.

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You can take a tuneable inductor with higher inductance and remove wire to obtain the inductance that you want. As inductance is proportional to N^2 (turns squared) then
L1/N1^2 = L2 / N2^2 so N2 = N1 x (L2/L1)^0.5

So eg a 1 mH inductor with 100 turns would give ABOUT 82 uH using
N2 = 100 x (82/1000)^0.5
= 29 turns.

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As ThePhoton notes, tuneable range of the part you cited is small. You should try to ascertain what the required tuning range is for the original part.

Also, why is it tuned? - it may be possible to alter some other circuit parameter.

Can you supply a circuit or link?

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  • \$\begingroup\$ There is a circuit in the pdf linked: cs.nmsu.edu/~rth/EMTheremin.pdf \$\endgroup\$ – dustytrash Jan 1 '20 at 21:55
  • \$\begingroup\$ "You should try to ascertain what the required tuning range is for the original part." The datasheet says Inductance range (uH) is 10.0+10%, Q (min) 60 freq 2.52. Does that answer the tuning range? \$\endgroup\$ – dustytrash Jan 1 '20 at 22:10

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