Ramsey FM100B FM Transmitter

Operation

To change the broadcast frequency on your Ramsey FM100B:

1. Press and hold both FREQ UP and FREQ DOWN buttons simultaneously (this enters frequency setup mode).
2. Release the buttons.
3. Use FREQ UP to increase the frequency or FREQ DOWN to decrease it (in 100 kHz steps, 88.0–108.0 MHz range).
4. Tune a nearby FM radio to the desired frequency and adjust until you hear the carrier (quieting signal; no audio input needed).
5. Press FREQ UP and FREQ DOWN together again to exit and lock the frequency.

MRF557

This power transistor is not installed on the U.S. model. It would be at(Q6) as part of its final amplifier stage for the export model. The GAL5 IC (U6) functions as the exciter/driver, generating the modulated VHF signal and delivering a low-level output (a few milliwatts) through a 0.001 µF capacitor to the base of the MRF557. Since the MRF557 is not present on the domestic U.S. model the output is taken directly from the GAL5 IC (U6) through the 0.001 µF capacitor (C65) and fed into the PI network/filter for buffering and impedance matching, achieving the Part 15-compliant ~25 mW without a discrete final transistor.

Based on the official Ramsey FM100B manual (pages 28–29 of the schematics), the @ symbol next to Q6 (MRF557) explicitly notes: "Note: these parts are only used on the export version." This confirms that in the standard U.S. low-power (25 mW) FM100B kit, the MRF557 is not installed.

FM100B to FM100BWT Upgrade Kit

The MRF557 is present only in the export/high-power variants (like the FM100BWT or pre-mod FM100BEX setups), where it's populated at Q6 for the 1W+ output.

If you purchased the high power version of the FM100B as an export model for overseas operation, a predrilled hole has been provided for Q6 to mount on the circuit board. If you are upgrading your FM100B that you already built, you’ll need to drill out the center hole labeled Q6 with a #2 drill bit (0.221”). The rest of the parts provided with this modification kit have pre-designated positions on the main board of the FM100B already.

• 1 × MRF557 transistor (Q6 replacement – even though the stock one is the same part number, Ramsey always included a fresh one)
• 1 × 220 Ω ¼ W resistor (red-red-brown) → becomes R50
• 1 × 270 Ω ¼ W resistor (red-violet-brown) → becomes R64
• 1 × 100 pF 500 V silver-mica or ceramic disc capacitor → becomes C65 (replaces the stock 0.001 µF coupling cap)
• 1 × 15-turn molded choke, 1.5 µH (green body with brown-green-gold bands) → becomes L6
• 1 × Ferrite bead or small toroidal core + 6 inches of #28 enameled wire → to wind L2 (10–12 turns on the supplied core)
• 1 × Aluminum heat sink bar with two mounting holes (approximately 1.5" × 0.5" × 0.25") → HS1
• 1 × Small tube of heat-sink compound (silicone grease)
• 1 × FM100BEX instruction sheet (4 pages)

That is the complete, official contents of every FM100BEX kit Ramsey ever sold. No additional low-pass filter parts were needed because the stock FM100B board already has the harmonic filter built in. The ~30 dB gain jump comes from the new bias network (R50/R64), stronger drive through the 100 pF C65, and properly loading the MRF557 collector with the new L2/L6 network so it finally runs at its rated 1–1.5 W instead of the stock 25 mW.

The real Ramsey FM100BEX kit (1999–2006) never included:A big vertical copper-clad bar or large black “pill” transistor with four visible legs on top or huge finned heatsink that stands 1–2 inches tall.

The original Ramsey FM100BEX only contained: A slightly larger aluminum heatsink bar (still flat, still glued on the board like stock), A handful of resistors, a 100 pF capacitor, a 1.5 µH choke, and sometimes a new MRF557 to replace the stock one and everything soldered in the existing Q6 area; nothing tall or vertical.

Chinese High Power Modification Kit

The “pill transistor + copper bar + giant heatsink” modification you see everywhere now uses a different transistor entirely usually an RD01MUS1, RD06HHF1, or 2SC1971/2SC2078 and is not what Ramsey ever sold or documented. This kit is a knockoff designed to bolt onto the FM100B (or similar low-power FM exciters) to boost output from ~25 mW to claimed 1W+. It's a generic VHF amplifier PCB using off-the-shelf RF components. It's compatible with the FM100B by connecting to the GAL5 exciter output (pin 11 via a coupling cap), but installation requires soldering skills and risks damaging your board if misaligned.

It can deliver 0.8–1.5W carrier (up to 3–4W PEP with modulation) at 12–13.5V DC input on 88–108 MHz FM band, a ~20–30 dB gain over the stock 25 mW. This extends range from ~100–300 ft (stock) to 1–3 miles line-of-sight, depending on antenna/terrain. The RD01MUS1 is well-suited for VHF (rated 520 MHz, 1W CW at 7.2–12.5V), and user tests on ham radio forums (e.g., QRP Labs) confirm clean amplification with low distortion if biased correctly.

Broadband design works across the FM band without retuning, but efficiency is ~50–60% (draws 200–400 mA). Harmonics are moderate; pair with the FM100B's built-in low-pass filter for cleaner output. Real-world effectiveness is good for hobby/low-power broadcasting, but expect some spurs if not tuned with a VNA/SWR meter.

The RD01MUS1 is robust (RoHS-compliant, handles 1W continuous with cooling), but poor soldering can cause shorts, overheating (transistor junction temp >150°C fries it), or oscillation (leading to splatter/interference). Always use a 50Ω dummy load during testing—running without one spikes VSWR and destroys the MOSFET in seconds, just like the stock MRF557. Input voltage must be regulated (12V/1A min); overvoltage (>15V) causes immediate failure.