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So I'm building the FM receiver right now and have hooked it up according to the schematic. In order to debug the system, I attached a capacitor in series with a microphone at the output of pin 5 on U1.
For an ideal FM receiver, one that has sufficient gain, you would expect to hear noise when not receiving any signal. This noise would be characterized as "pink noise" or you might call it a hissing sound, with no distinquishable tones or buzzing sounds. So, perhaps your receiver is close to functioning. You mentioned connecting a line from R1 to inbetween Q2 and C8. I wonder why you mention this as this connection is clearly shown in the schematic and you already said you hooked it up according to the schematic. Anyways, that connection is necessary to provide DC bias to Q2, and is essential to the receiver.
I'm a bit puzzled why you would attach a microphone to the output of U1 at pin 5. So perhaps you meant to say you attached headphones to that point?
As I mentioned, a receiver with sufficient gain will output noise when not receiving a signal. So, let's assume your receiver is functioning. Next you would tune it by varying C1 to see if you can pick up your transmitter signal. I am guessing that your receiver is not too sensitive, so be sure to have the transmitter somewhat close to the receiver (on the same table top for example).
Congratulations! Its not many that would have got this far. At least it sorta works.
Have you tried moving both transmitter and receiver to another frequency to see if the two tones follow?
Obviously, the receiver is picking up the transmitter via paths other than RF. I suspect your receiver is very sensitive to audio interference, and perhaps the transmitter is generating something that is being picked up. We have to suspect coupling through the power supply first, so you need to isolate those to test this theory.
I guess you are still building on those plug boards. That is part of the problem and maybe its time to learn how to build on a hand cut circuit board in the style sometimes called "dead bug". Just google (image) dead bug circuit board for images of examples. There will be a lot less coupling with this kind of construction, if done properly.
This sort of thing will get you half way there. There is less capacitive coupling in this type of board, but you still have a grid of many unused plated holes that will couple signals from one to the other relatively easily. So, yes it is a step in the right direction but not as effective as starting with a blank double sided board (solid copper both sides) and just cutting your necessary pads with a knife or dremel tool on one side. In the dead bug approach that this is, the big benefit is that you naturally get a very good ground plane on the backside (assuming you start with board clad with copper on two sides, not just one). This groundplane is very helpful at RF frequencies and also for fast logic at lower frequencies. It establishes a quieter ground reference for your circuits.
Wanted to mention also that your receiver should be able to pick up FM radio stations if it is working. This assumes that where you live there are at least one or two very strong FM stations. You might need a good wire antenna for this, though. This might allow you to further debug and develop the receiver without using your transmitter, which may be helpful.
If you receive nothing using the battery, is it possible that all of the signal is coming through the power supply rather than through the RF path? Or perhaps the wiring to the power supply is picking up a slight magnetic field from the transmitter nearby? I'm not sure what to suggest but when I have to debug a problem, I like to separate the various elements of a system and make sure each works on its own before bringing them together.
An FM radio has at least 6 tuned LC circuits. Your circuit has only one.
An FM radio has an FM detector and ignores AM. Yours has an AM detector that oscillates all the time.
An FM radio works fine.
Your circuit is too simple to be an FM radio.
Looks OK. Why coat it? Just leave it open or if you are worried about shorting things out, tape a piece of plastic to the back. Coating will just get in the way when you have to modify it.
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