Hi-Fi Stereo FM Transmitter

[billbo911]

OK, status update time. Short and sweet, it's finished and I'm going to run cables instead.

The circuit works, barely. I guess for a 10mW transmitter I shouldn't expect too much. No matter how accurately I tune it, all I can get is a mono signal. The "stereo" indicator never lights on any receiver, and I've tried several. I can get good "tuned" indication, just not stereo.

Here are a couple pictures of the project and it's modifications.



This is the circuit. The blue 50K pot replaces two resisters and allows adjustment of the MPX Mix. Originally there were two 27K resisters in place.
The yellow wire going off to the left is a half wave length antenna.

Here is a picture of the entire circuit.


The small board with the two pots is both a 2.7 vdc. zener regulator and level adjustment circuit. (Thanks for the suggestion Al.)
As you can see, this can be powered by Micro USB +5 vdc. source.

All in all, it was good practice, but I am not satisfied with the result. So, either I drop a couple hundred $$ and by a better grade 25mW transmitter or I run cables. That is, unless someone here can tell me what the problem with this little beastie might be.

[MrForbes]

Play with a Belkin Tunecast.....they don't cost much

[Al Skierkiewicz]

Bill,
Does the transmitter light a stereo light on a receiver very close to the transmitter? If not, this is usually an indication that the 19 kHz pilot carrier is not functioning. If the receiver does not see the pilot it does not try to decode subcarrier. If the close in receiver does come on in stereo, the pilot injection may be too low. All of this is to prevent unwanted noise in receive when the signal to noise on the subcarrier is low. Thoughts were, a quiet mono signal was better than a noisy stereo. It may indicate that the 38 kHz crystal is defective.

[DonRotolo]

Try a half-wave counterpoise: If your antenna goes 38" in one direction, connect a wire to Ground (right where it first hits the board) and run it 38" in the the other direction. The best signal will be at a right angle to that "dipole".

It's cheap and quick, and could double your signal strength or better.

But try what Al said: If you can't get stereo from 6" away, there's a fault. I have built $25 FM kits which worked at 150 feet, no problem.

[billbo911]

Originally Posted by Al Skierkiewicz

Bill,
Does the transmitter light a stereo light on a receiver very close to the transmitter? If not, this is usually an indication that the 19 kHz pilot carrier is not functioning..... It may indicate that the 38 kHz crystal is defective.

Originally Posted by Don Rotolo

Try a half-wave counterpoise: If your antenna goes 38" in one direction, connect a wire to Ground (right where it first hits the board) and run it 38" in the the other direction. The best signal will be at a right angle to that "dipole".

It's cheap and quick, and could double your signal strength or better.

But try what Al said: If you can't get stereo from 6" away, there's a fault. I have built $25 FM kits which worked at 150 feet, no problem.

It looks like I will be dragging the scope out of storage and actually using it for the first time in over a year. I know the calibration is suspect on it, but that won't matter for this troubleshooting exercise. If the signal from the crystal isn't there, then I will know the cause.

Don, I was thinking the same thing. In fact, I have a dipole antenna from a stereo receiver I was going to try just for kicks.

I'll let you know what I figure out.