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

[hr2000]

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

 

[Sebi]

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]

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]

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]

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