2019-01-15 2nd FM Microphone Kit Assembled

I ordered some other FM Microphone kits, and assembled one last week. This is a better one, labeled EEQKIT RF-02FM VER170525. It has three transistors, one of which is a microphone preamp and it should perform much better. The kit also has a jack that will allow you to plug in a stereo signal from a CD or other audio source and broadcast a mono signal. The link and schematic are below.

I assembled it with no problems, the parts were labeled on the PC board. The “7 turn” is the coil with the plastic and the tuning slug. One thing I like is they used a 10k for the LED current limiting resistor so the LED draws only a fraction of a milliamp, which saves the battery. The red LED is still bright enough.

I powered it with a 6 volt Ni-MH battery pack, the board is marked DC 3 – 6V. I monitored its output with an FM radio. I tried to find my ‘station’ on the FM band but I didn’t hear anything. I got my Gooit frequency counter and put it next to the antenna and it was saying 320 MHz! That’s way too high! So I found a tiny screwdriver and gently and slowly turned the black tuning slug in until I got the frequency down to the bottom of the FM band around 90 MHz.

I tuned in the radio and blew into the microphone but the audio was very weak. That should not happen because the kit has a microphone preamp. So I measured the collector of Q1 and it was less than 0.8 volts. It should be about half the supply voltage or about 3 volts. I checked the 9014 datasheet and saw that it is a high gain preamplifier transistor. The schematic shows that its base is biased by R4, which was 100k. Most low level preamps use a resistor much higher, from 470k to 1 megohm or more. So I replaced the 100k with a 470k and the collector measured about 1.8 volts, which is much closer to half the supply voltage. I powered it up and the microphone was much more sensitive, I don’t have to yell in it to be heard on the radio. So the conclusion is that the kit should have a much higher value for R4, 470k should be okay.

A link to the kit instructions (.PDF)


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