Making a battery-free MFJ-299 desktop microphone

By Tore Aasli, LA1SSA


I use an MFJ-299 desktop mike with my Yaesu FT-950 transceiver (thanks a lot to the MFJ staff, their eminent service is impeccable!).

The problem is that I forget to switch the microphone off when I am finished using it for the evening. Since it is battery operated I have to replace the 9 V battery quite often (several times a year..). So, why not make it battery-free, running the the mike fed with 9 V from the power supply? A standard ham power supply voltage is 13.8 V, and this voltage has to be reduced to approx. 9 V.


Here's what I did to solve the battery issue.

A simple circuit with a 9.1 V zener diode and a resistor will work satisfactory.

The simple circuit is like this:


Input voltage is 13.8 V from the power supply, output voltage should be 9.1 V since this is a standard zener voltage. The goal is to determine values for the resistor R and PR, and Pzener.

1) According to MFJ specification, the maximum mike current (Imike) is in the order of 11 mA. The zener leakage current Iz is about 5 mA, the sum rounded up to 20 mA. This is the value of I as shown in my circuit diagram.

2) The zener power is calculated from Pzener = I ∙ Uzener which gives
Pzener = 0.020 ∙ 9.1 = 0,182 W, which I round up to 0.2 W.

I pick a SOD57 zener diode, 9.1 V / 3.25 W from my component box since this diode power is all I have at hand.

3) The resistor R is calculated by Ohm's law, since I know the voltage drop across R (14-9=5 Volts). Or actually 13.8 and 9.1 V for the power supply and zener voltage.

R= formula1. The closest E12 or E24 value to 250 Ω is 270 or 220 Ω. I choose a 270 Ω resistor from my component shelf, since I have 6 of them and only three 220 Ω resistors.

4) Finally, the resistor's power has to match at least P= I ∙ ∆U = 0.02 ∙ (14-9) = 0.1 W.
These resistors are luckily easy to find.

Note: If you don't have this particular resistor value, you can use any value from 180 to 390 Ω. The resistance is not critical, but the zener voltage and power is more critical. The lower the resistor value, the higher the zener power has to be.



This circuit has to be connected in series with a thin 2-lead cable, which in turn is connected to the power supply, and to the MFJ-299 mike battery power connector. Connector-zener circuit-power cable.

It's an ordinary 9 V connector needed, but remember to reverse the polarity (the large "hole" is positive but must be connected to the negative pole from the zener circuit)! There is a similar battery connector in the MFJ-299, so these two would click easily into place.

I also had one of these connectors in my component rack, and soldered it to the mike end of my thin power cable.

I also bent the negative leg (anode) on the zener diode to make it "parallel" to the resistor, giving the circuit a sleek and slim design. Next, I used a couple of pieces of electrical tape to insulate the bare component wires, and a piece of tape to cover it all. here it's up to you, use the tape or crimp. The excess bare wires from the zener diode and resistor was cut off after soldering these two components to the thin power cord and connector.

The circuit with the connector is finally placed in the battery compartment of the microphone, and ready for use! Of course I had to test it with a voltmeter first, both the polarity and voltage, and the heat from the resistor. After half an hour "burn-in", I put it all inside the mike and closed it.



The bottom line:
I never have to think of forgetting to switch off the microphone again. Hopefully. It no longer drains battery power!

Some tranceiver brands though have an 8 V output from the radio's microphone connector, which is more practical to use.