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[b]Help! how to substitute?????[/b]

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hr2000

New Member
Hello, i´m new to this stuff and i´m building an fm micro-transmiter but i can´t found a capacitor, i woul like to know how to substitute this varible capacitos, it shoul ve 2.7 pf, capacitor but, can´t find any, this is the circuit, so u can give me some ideas, i hope so.
Thanks alot.
the circuit is in spanish, but the capacitor of 2.7 the instruccions said that should be variable to be able to chang the radio station where i want to hear
 

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Sebi

Active Member
You can also tuning with coil, need only one time.But what happen when the phone ringing? (100V AC!) Definitely need some protection on DC side of bridge (zener) and a condenser.
 

Russlk

New Member
The LED will act like a 1.5 or 2 volt zener, so that is OK. You can connect two caps in series to reduce the value, Ct=(C1*C2)/(C1+C2). One of the caps can be the variable one.
 

DigiTan

New Member
Say, this isn't a Lunch Box is it? Anyway, there are alot of online Capacitor Calculators (Need Java) that help a little for series/parallel capacitor networks. Typically, I try to avoid very large capacitor combinations where precision is needed because that increases your error. (It also decreases reliability, but this a much smaller concern).

Also--if it helps any-- alot of crystal radio kits contain variable caps that range from about 16pf to 365pf, depending on which band it's designed for.
 

Crofty

New Member
Try 2 short pieces of thin plastic covered wire. Solder both in where the 2.7pF cap should be. Then twist the 2 together, more twists = more capacitance, an old idea, but works, when the desired freq is found, a drop of glue or similar holds it together. A trimmer is still better though.

Steve
 
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