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Yaesu M-90 Microphone with Flex 6400

marwalk Member ✭✭

I have an M-90 microphone that I'd really like to use with my 6400. I'd heard that the Yaesu mic cables are wired the same as that for Flex, and based on that I connected my M-90 to my 6400 using the cable that came with the Flex stock FHM-3 mic (which works successfully with that cable). The result with the M-90 is no audio, and the PTT only works when the Lock button is pushed down (not the main PTT on the M-90).

I've tried with SmartSDR Bias off and Bias on, and got the same results. I'm not sure why a dynamic mic would need power (certainly not "phantom" power), and am baffled to read that the M-90 requires 5v +/- 10% to function properly (possibly required by an internal pre-amp?). The 6400 only provides 3v Bias, which is outside of the stated tolerance for the M-90, and that might be one theory as to why neither the M-90 nor the Yaesu stock MH-31A8J provide audio to the 6400.

What is needed further to get the M-90 mic to work with the 6400? A different cable? The external Power Supply Kit ABB19X001 for the M-90, and if so what cable to use with that? It seems from reading various posts that some people have successfully used an M-90 with the 6400, and I'd appreciate your advice from your experiences.

Thanks, Mark, AC3EW


  • VE7ATJ_Don
    VE7ATJ_Don Member ✭✭✭
    edited October 2023

    Hi Mark.... I don't have your mic or a 6400, but from a cursory look, it appears the M-90 is wired the same as other Yaesu mics (for the 991, 891, etc.) using an RJ-45 connector. Is that right in your case? If that's so, here's the pinout: (Note this is view looking at the jack at the front of the Yaesu radio)

    and here's the pinout from the Yaesu mh-31 (which should be roughly the same as the m-90, I would suppose, or at least close). It would appear the cable is just straight through pin-to-pin.

    But, I can't guarantee this is the same for Flex's FHM-30 :-)

    From what I see in the 6400 manual, the 6400 is expecting two plugs -- a 3.5mm phone plug for mic input (which would be connected to pins 4 and 5 at the Yaesu end), and a phono plug for PTT (which would be connected to pins 6 and 7 at the Yaesu end). I'm pretty sure you don't need to worry about the +5V, DOWN, UP, or FAST connections in the case of the M-90.

    Now... unfortunately, this pinout may not be the same at the actual microphone itself. (i.e. the fhm-3 microphone cable may be switching pins between each end.)

    Hope this helps.

  • marwalk
    marwalk Member ✭✭

    @VE7ATJ_Don Hi, Don. Thanks for the insights. It looks like there's a lot of commonality among the Yaesu radio models. I'm thinking I misheard the idea about Yaesu and Flex mic cables being wired the same.

    Here is the pinout I have for the FT-450D mic connector (seems to be the same as your Yaesu radio), which I'd annotated in an attempt to identify which pins would work with the Flex.

    There seems to be no way for the Flex cable to provide any voltage to the M-90 microphone from the Flex if the mic is expecting that on a line that is not on the mic audio feed. If Bias (SmartSDR setting) is applied to the mic, the only way I can see to provide the power from the Flex (3v in the case of the 6400) is for it to go through the mic plug tip (not sure if there's another connection), as tip and ring are all there is for the mic from the Flex other than the PTT (which by its nature must be an off and on switch like a CW key). If so, it seems Flex mics must be able to handle the DC power over the mic audio line (which wouldn't be a problem with a proper internal mic configuration). I'm not so sure about that method with the Yaesu M-90 mic.

    The challenge then (as I see it at this point) is to determine what are the pinouts for the Flex microphones, as I've been unsuccessful in several web searches in finding a pinout diagram for them. Although it might take a while for me to set up a test bench configuration, my next planned step is to take the cable that is with the FHM-3 mic, and (without altering that cable in any way) determine via ohmmeter which pins on the RJ-45 jack correspond to tip, ring, and sleeve on the 3.5mm connector, and which to the pin and outer conductor on the PTT phono plug. I have some old Ethernet cables I can sacrifice for this, stripping and separating the wires on one end, and using a double female RJ-45 connector to hook it up to the Flex mic cable. Once I have the pinouts from both to compare, I then can see about fabricating my own crossover cable (of sorts) by terminating the stripped end of the Ethernet cable with an RJ-45 connector wired to get the M-90 to see what it needs.

    The only other kink in this (that I can see at this time) is the 5v (vice 3v from the 6400) needed for the M-90. The 5v could be supplied by the laptop from one of its USB ports (or other USB power source), and patched into the Ethernet crossover cable prior to the M-90 end connector; the ground line off the USB cable could also connect to the ground line for the mic, which would (it seems) provide an additional common chassis ground between the laptop and the radio.

    Does that sound plausible? Any danger of equipment damage in this plan? Advice appreciated!

    Thanks, Mark, AC3EW

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