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Thread: Motorola PMMN4099A RSM (and possibly others) audio mod

  1. #1
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    Default Motorola PMMN4099A RSM (and possibly others) audio mod

    So, I bought a PMMN4099A speaker-mic to use with my XPR7550. Although this RSM is designed for the Mackinaw (APX) radios, it works fine with both lines.

    This RSM has a 3.5mm audio jack, volume rocker switch (attenuator), emergency button and function button. It also features the large speaker which sounds better than the smaller speaker-mics.

    PMMN4099A.jpg

    The first thing I noticed was the rocker attenuator didn't seem to attenuate the audio very much. Minimal difference between normal/attenuate. Performance was the same on my APX7000 vs. my XPR7550. Whomever designed this function, is clearly a mouth breather. Public-safety personnel use this feature in the field to quickly reduce volume -- sometimes, for officer safety reasons, or because someone with gloves can't easily access the volume knob. The switch should reduce volume significantly, but still allow transmissions to be heard.

    I opened the mic and traced the circuit out. I determined there are two 91-ohm resistors placed in parallel (for 45.5 ohms), which serve as the "attenuator" when the switch is engaged. I don't quite understand why they used two resistors in parallel, other than for heat dissipation concerns at full volume.

    mic-1.jpg

    I removed the two resistors and replaced them with a single 150-ohm resistor. Much better performance. It's the way it should've been designed in the first place. If you have one of these mics or similar design using the same padding (45.5 ohms), it is worth your time to replace them with two 300 ohm resistors (331), or a single 150 ohm (151).

    mic-2.jpg

    Motorola, if you're reading this, fix your engineering FFS. Thanks.


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    Now that's a cool mod! I used this style of RSM a lot in the past (but rather the IP57 version without the 3.5mm audio jack - PMMN4065A). I always did hate the attenuation on this RSM and thought it was too low. But that issue was overshadowed by two other major flaws in this RSM (in my opinion):

    1) I found the PTT button to be too sensitive. It kept keying up with the slightest of touch.
    2) the cord is simply too long (at least for my taste) and I've never found a shorter alternative

    I found the XP RSM (NMN6271/NMN6274) was a better RSM. It's volume toggle is an "up/down" and not "hi-lo", has a more tactile PTT button, a flatter design, comes with a D-ring loop OEM, and (most of all) has a shorter cord which is easily replaceable.

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    Quote Originally Posted by sjxts3000 View Post
    I found the XP RSM (NMN6271/NMN6274) was a better RSM. It's volume toggle is an "up/down" and not "hi-lo", has a more tactile PTT button, a flatter design, comes with a D-ring loop OEM, and (most of all) has a shorter cord which is easily replaceable.
    Not to mention the TX audio quality is VASTLY superior. I've tested almost all of the first-gen APX speaker mics and they all sound like garbage compared to an XE500 or XP mic.
    "The Girl Scouts found several hungry REACT members at the finish line..."

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    The worst mic is the Industrial Noise Cancelling complete flop product IMO

    The switch on the INC is just a regular volume adjustment however.

    With the INC, you could train users for a week straight how to talk into it at just the right angle, and it sill won't work right.

    XP is the way to go.

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    Just wanted to clarify something: The volume attenuation switch does NOT affect the output at the 3.5mm plug. That volume level is set via the knob on the radio. The switch has zero effect.

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