Recently I have bought one of the Velleman KIT-s, and this was FM oscillator. Unfortunately it does not function. I have checked all components a few times, I have proved circuit against short-circuits and I was not able to locate the problem. Symptom is following: As fast as I connect 9V Battery to the power supply pads of the board, R3 (22E) gets hot in a second. Temperature of this resistor rises over 100 Celsius degrees. I am continuously having 22E between +9V and Ground. I am using an Electret microphone, connected in series with 27kE resistor and 100nF capacitator.
I have checked once again the circuit. Because all of the elements have been correctly assembled, I have removed them and checked the printed circuit board under the microscope.
I have found that there was a thin coat of copper between drain and source pad of the transistor T1. It seems that something went wrong due to the process of etching. Because the board protection is colored green, was this connection between drain and source not visible quite well. Therefore I wasn’t able to locate problem at the first time.
Finally I have removed this “short-connection” between drain and source, replaced the elements and the transmitter has started to function.
You should review the design of this printed circuit. Conductive lines near T1 can be placed so that they would prevent such short circuits. If such short-circuit happens to a not experienced human, it can led to serious problems. In my case was current on R3 about 430mA, what has caused undesired heating of R3 to more than 100 degrees. Enough to start fire. Luckily has everything ended O.K…
I am continuously having 22E between +9V and Ground. I am using an Electret microphone, connected in series with 27kE resistor and 100nF capacitator.