I've been trying to make an FM modulator, using reactance techniques and varactor diodes and the 566 VCO. Nothing has worked until today when I got two working 566 ICs in the mail. About a month ago I ordered a 566 from EBay and when I got it I set it up according to a book but it didn't work. Never having fussed with the chip before I didn't know if my book was at fault or the IC was bad. Today I discovered it was a bad IC by the fact that the book's implementation worked with the two ICs that just arrived in the mail. However, I'm not getting the frequency I'm supposed to be getting. The calculated fc is about 7 kHz but I'm getting 3.33 MHz with one of the new chips and 11.6 kHz with the other. Even so, I hooked up a low frequency signal to pin 5 and sure enough the fc moves back and forth on the scope just as one would expect. At 1 Hz I can watch it in slow motion. Does anyone have any idea why the actual frequency is different than the formula's prediction?