roger@yuba.wrs.com (Roger Rohrbach) (01/13/90)
I am building a Theremin (an electronic musical instrument) from a schematic published in a very old (1967) issue of "Popular Electronics". The schematic calls for "50-300 uH RF coils". I am told that this is a very wide spread for adjustable coils by a mail-order house that carries a very limited selection of same. Furthermore, I can't find anyone locally who stocks ANY coils whatsoever. Does anyone know of a source, or know enough about the topic to suggest a substitution (I have to tune two RF oscillators so that their difference frequency is audible)? While I'm at it, the design also used "polystyrene capacitors", rated in picofarads. Can I substitute polypropylene, for which I have found a source? Any other substitutions? (My, but there are a lot of capacitor types out there; ceramic, electrolytic, tantalum, Mylar... what's the difference between them besides voltage rating?) E-mail is appropriate. Many thanks from an amateur. Roger Rohrbach sun!wrs!roger roger@wrs.com - Eddie sez: ----------------------------------------------- (c) 1986, 1990 -. | {o >o | | \ ^) "I'm afraid of widths." |
john@frog.UUCP (John Woods) (01/16/90)
In article <827@wrs.wrs.com>, roger@yuba.wrs.com (Roger Rohrbach) writes: > I am building a Theremin (an electronic musical instrument) from a > schematic published in a very old (1967) issue of "Popular Electronics". > The schematic calls for "50-300 uH RF coils". That *is* a very wide spread; I've usually seen ranges of like 2:1. Considering the purpose, I'd guess that the article really meant "two variable RF coils, maxing out somewhere between 50-300 uH"; you're not trying to tune a particular wide range, you're trying to get two oscillators to match. (Hmm, unless you're going to use a portable radio as a detector rather than building a mixer, in which case you still don't need that wide a range) > Does anyone know of a source, or know > enough about the topic to suggest a substitution (I have to tune two RF > oscillators so that their difference frequency is audible)? Is the detector part of the circuit, or do you use a separate radio? If so, what frequency, and how much capacitance is in the tuned circuit? > While I'm at it, the design also used "polystyrene capacitors", rated in > picofarads. Can I substitute polypropylene, for which I have found a > source? Maybe yes, maybe no. Polystyrene is fairly temperature stable, so as the room warms and cools you don't have to re-tweak the coils. Polypropylene isn't (generally) stable, so the oscillators are going to drift a little, and probably not together. Other stable capacitor types are NP0 ceramic (not just any ceramic, NP0 is a special type) and silver mica. DigiKey carries some NP0 ceramic disks. -- John Woods, Charles River Data Systems, Framingham MA, (508) 626-1101 ...!decvax!frog!john, john@frog.UUCP, ...!mit-eddie!jfw, jfw@eddie.mit.edu Happiness is Planet Earth in your rear-view mirror. - Sam Hurt