Ramsey FM100B FM Transmitter
Operation
To change the broadcast frequency on your Ramsey FM100B:
- Press and hold both FREQ UP and FREQ DOWN buttons simultaneously (this enters frequency setup mode).
- Release the buttons.
- Use FREQ UP to increase the frequency or FREQ DOWN to decrease it (in 100 kHz steps, 88.0–108.0 MHz range).
- Tune a nearby FM radio to the desired frequency and adjust until you hear the carrier (quieting signal; no audio input needed).
- Press FREQ UP and FREQ DOWN together again to exit and lock the frequency.
MRF557
Final output transistor (Q6) responsible for amplifying the signal from the driver amplifier (U6, a GAL5 chip). In the standard FM100B model, the MRF557 is omitted, and the output is derived directly from the GAL5 driver chip, which is designed to deliver up to 25 milliwatts of power. However, when the 1-watt upgrade is installed, the MRF557 is added to the circuit to provide the higher output.
The MRF557 is known to be sensitive to high standing wave ratio (SWR) and can be easily damaged if the transmitter is operated without an antenna connected or with a mismatched load