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.

Transmit Power Output

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 around 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.

Low Power 25 mW US Model

In the U.S.-legal configuration the GAL5 (U6) drives the output directly through the 0.001 µF coupling capacitor and into the PI-network low-pass filter. The Ramsey FM100B starts generating and radiating the unmodulated carrier (or audio-modulated signal if inputs are connected) immediately upon power-up, as soon as you flip the switch and the GAL5 IC stabilizes (usually within 1–2 seconds). It is important to ensure a proper antenna load is attached at all times. Always connect a 50 Ω dummy load or tuned antenna before applying power and turning it on. Even a brief power-up into an open jack can reflect high voltage back through the PI network and fry the GAL5's output stage, dropping your power to near-zero.

Stupid eBay Sellers

Watch out for stupid eBay sellers that advise they powered on the unit to see if the display worked correctly and the unit came on, but did no further testing. These sellers most likely turned the power on immediately putting the unit into transmit without a proper antenna load attached. Their uneducated testing just destroyed the unit's transmitter which is either the GAL5 (U6) on the low power version or the MRF557 on the higher power export version, or both can be destroyed in seconds.

The core safety warning applies for all variants, always connect a 50 Ω dummy load or tuned antenna before applying power and turning it on.

When you power on the Ramsey transmitter into an open antenna jack:

• The PI network reflects extremely high voltage back toward the GAL5 output pin (can exceed 50–70 V peak-to-peak).
• The GAL5 is not designed to survive that; its internal output stage breaks down and the chip either dies instantly or degrades and gives very low or distorted output from then on.

On the 1-watt (export or FM100BEX-modified) version, running without an antenna can destroy both the GAL5 and the MRF557, and it usually does. Over 70% of “dead” 1-watt FM100Bs that were powered up without an antenna have both a blown MRF557 and a dead GAL5. The rest have only the MRF557 blown (the GAL5 sometimes survives by luck).

FM100B to FM100BWT Upgrade Kit

Ramsey FM100B units for export were named Ramsey FM100BEX and included the MRF557 final transistor or what some refer to as "the pill." Official Ramsey documentation referred to the upgrade kit as "88 - 108Mhz 1 Watt Power Booster" which was included in the kit when you ordered the FM100BEX.

Their manual stated: "'Give your FM100B the power to crush the competition! This 1 Watt booster configuration especially designed for the FM100B brings the standard output of 25 mW up to more than 1 Watt! That's enough power to drive a professional power amplifier or to cover the neighborhood with a solid Hi-Fidelity signal.'""

"'At Ramsey Electronics we are constantly bombarded with the age old question “How do I get MORE POWER out of my FM100B”? For those loyal customers who want to obtain a greater coverage area, the FM100BEX modification does it all. This high quality booster takes the normally low-level RF signal (about 25 mW) from the FM100B and boosts it up to over one watt! Of course since the FCC (in the US) does not tolerate this sort of RF level being transmitted without the appropriate licenses, this kit is only legally used as an export item.'"

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.

Semiconductors

• 1 MRF557 NPN UHF Transistor (4-pin flat pack) (Q6)

Resistors

• 1 220 ohm resistor (red-red-brown) (R50)
• 1 270 ohm resistor (red-violet-brown) (R64)

Inductors

• 1 LK200 ferrite core (L2)
• 1 1.5 uH inductor (L6) (looks like a resistor, brown-green-gold with green body)

Miscellaneous

• 1 6” piece of #28 AWG Buss wire to wind L2
• 1 Copper bar heat sink (HS1)

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.