Hobby transmitter help for total newbie

[housecat358]

Trying to build this: **broken link removed**

Alterations: slightly smaller pcb, mostly SMD caps and resistors, no audio stage yet (replaced by an old mp3 player), pn2222's instead of bc-whatevers, air core coils, resistor instead of inductor in buffer stage.

I've built several of these, and the only one I got to work somewhat properly was a 72Mhz NBFM version (24mhz crystal, first oscillator stage 24mhz as well, the second 72). Also tried to build several WBFM versions like in the original schematic (11/33/66, 11/33/99, 11/55/110, 14.318/42.954/85.908 etc etc etc), but none of these could be adjusted to its proper frequency. NO modulation whatsoever on center frequency, poor signals with modulation 1-2 mhz off to either sides... and the "side" frequencies drift, in fact you could (temporarily) adjust these in at least 10mhz range, it seems as if the crystal isn't really doing anything at all in there... Very little audio power is required for this WB modulation (2 on scale of 30 on the audio player, the NB variety needs about 25). Cranking the volume up won't produce audio on center frequency, the signal just becomes intermittent... Any help appreciated..

 

[chuckey]

It sounds as though your circuit is unstable and oscillating at some unknown frequency. So make sure there is shielding between stages and adequate Vcc line decoupling.
To de-bug it try putting a 10K resistor right on the end of your live DVM lead. Measure the DC voltage on the oscillator base, then put your finger across the crystal - the DC should change if it is oscillating. The 10K is to stop the meter upsetting the RF conditions too much, repeat measuring the other transistor leads. Now you should have an idea as to the state of the oscillator. If while measuring the voltages, moving a hand across the rest of the circuit is causing changes, then the whole board is oscillating.
Frank