M-90D TO FT-2980 Impedance Matching and Gain Circuit (Using BC547C) PUBLIC

Small circuit to impedance match a Yaesu M-90D microphone to a Yaesu FT-2980. This circuit has a preamp based on BC547C set for adequate gain. (Use "mic gain" 4-6 on the FT-2980)

The M-90D desk microphone will not directly interface into the FT-2980 radio primarily because of an impedance mismatch. The M-90D is a 600 ohm microphone, while the FT-2980 expects a 2200 ohm load. (Some reference 2000 ohm). Although Yeasu does offer the SCU-67 8 pin to 6 pin adapter, from what we can determine this only translates the pinouts from the 8 pin to the 6 pin connectors, but fails to address the impedance mismatch, nor does it address the bias voltage of 8V on most mobile radios, although the specifications of the M-90D calls for 5V.

This transformer/preamp properly corrects the impedance mismatch, provides the proper bias voltage, and adequate gain for the dynamic microphone to replace the factory electret microphone.

The FT-2980 puts out an 8v bias voltage, while the M-90D requires 5v (+/- 10%). To address this, the bias voltage from the FT-2980 is ignored and replaced with 5v from a 78L05 voltage regulator.

The pinouts are included for both the RJ13 on the FT-2980 side, and the RJ45 on the M-90D side. There are very few components, and most of those are there primarily for noise/transient suppression as a precaution. The diode and resistor on the PTT line is another, potentially overkill, precaution.

Both the microphone and the radio are unbalanced wiring. The transformer does have a center tap that possibly could be attached to the center-point ground/ shielding if necessary to eliminate hum or buzz. (although this appears unnecessary, and may create ground loops). There is also a provision for a groud lift switch, should a ground loop occur. It will be tested without the ground lift switch, but it may be beneficial for certain operations, such as field use, but the circuit with single point grounding works properly without any ground lift.

The BC547C circuit provides adequate gain for the FT-2980, and the circuit provided can provide increased gain if required (this requires changing resistors). The circuitry is overkill, but has redundant protections against transients, and is especially designed to prevent RF interference. This circuit was tested under high levels of VHF RF, and is mounted inside an aluminum box with faraday tape sealing all potential gaps and entrance locations. The end result is a very quiet amp (equivalent to, or less noise than, local repeaters). This should be a completely trouble-free circuit when used under normal circumstances.