In addition to my K3, I own a Yaesu FT-7800 mobile FM rig,which I have equipped with Yaesu's MH-48 DTMF microphone. I'm considering building a simple adapter that would allow me to connect the MH-48 to the front panel mike jack, which would allow me to use the keypad on the mic to access remote nodes on repeater networks, using the DTMF keys on the MH-48 mic. The connections are straightforward.
However, I do have a question to which I need an answer before I proceed:
The Elecraft manual states that Pin 6 of the front panel mic jack provides +8 VDC (which is the voltage required by the MH-48), but with a maximum draw of 10 ma. I am unable to find any reference online that states the maximum current draw of the MH-48. Given that the mike unit contains nine transistors connected to the 5 x 4 keypad matrtix (two of which will draw current with the pressing of any DTMF key) plus a couple of keypad-illuminating LEDs and a "transmit" LED. I'm concerned that the mic will draw more current than the K3 mic jack is engineered to supply, particularly given that an LED typically draws 20 ma or so. Does anyone know what the power requirement is for the MH-48? Even if I turn off the keypad backlights, will it stay under 10 ma?
Or, to put it all more simply, has anyone tried mating the MH-48 to the K3, and succeeded?
One alternative, of course, is to build a small power supply into the interface box; input 12VDC, output 8 VDC to the mic, ignoring the input from Pin 6 of the mic jack. Unless I can definitively establish that the mic draws less than 10 ma, I expect to follow this route.
All of which leads to a "feature request" for the K3: could we make the keypad useable as a DTMF generator (turned on with press of some key, of course).
73
Lew K6LMP
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