The Corrugator, this is a great Instructible!



I used a different method to boost the output power, which will work not only for the Belkin Tunecast, but for ANY transmitter which uses a BH1415F, or even any other Rohm FM transmitter chip. Instead of trying to figure out what components were involved in the filter/attenuation network, I used a soldering iron and a toothpick to lift and disconnect BH1415F pin #11 (RF output) from the board. Then I soldered a quarter-wave wire antenna directly to the pin, which gives direct access to the maximum possible output power. Make sure the wire is secured so an accidental tug won't rip the pin off the IC; I used a generous application of super glue to attach the wire to the board near the connection. If a transmitter uses a different Rohm FM transmitter chip than the BH1415F, look up the datasheet to find out which pin is RF output.



The real gold in this Instructable is the auto power-off disable. I've been using this transmitter to listen to music from my computer anywhere in the house, and have been looking for a good way to disable the auto power-off. Up until now, I've been doing it by running a separate copy of Winamp that plays a subsonic or ultrasonic burst for two seconds, every 30 seconds in a loop. It worked, but on some radios you could hear a slight distortion when the burst played. This may still be a useful option for people who aren't comfortable soldering, or who have a different transmitter.



I just performed your auto power-off disable mod, and it works great! Though I did make one change. The pad you're getting power from appears to be active only once the transmitter is powered up, which still requires an initial audio input. So instead, I connected the transistor pin through a 1M resistor directly to the positive pin of the DC power jack. If you're looking at the board from the LCD side, with the power connector on the bottom, it's the second pin from the left between the large SMD capacitor and the LCD (verify it with a voltmeter if you're unsure you have the right one). The transmitter now turns on immediately when power is applied, and stays on indefinitely; regardless of audio input.



Finally, one more tip. Loop the power and audio input cables a few times through a ferrite toroid filters out hum and hiss. You can use one toroid, but it's easier to use two, one for each wire. The more loops, the tighter the windings, and the closer to the transmitter, the better it works. A clamp-on style ferrite should work almost as well, though I haven't tried. Obviously, don't loop your antenna through a ferrite, as this will attenuate the signal. If you haven't added your own separate wire antenna, skip the toroid for the audio cable, as it contains the antenna.



With these changes, I'm very happy with the performance of this transmitter; and its range and quality rival any consumer or kit transmitter I've tried for less than $100.