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Thread: Differences between RMN5050A and RMN5070A

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    Default Differences between RMN5050A and RMN5070A

    RMN5050A = TRBO desk mic
    RMN5070A = XTL/APX desk mic

    Both of these bastards look the same, but the XTL/APX mic sounds like *** on the XPR radio. The audio is distorted and overmodulated. Adjusting the pot on the back of the mic doesn't do anything to resolve the distortion -- it just changes the amplitude of it.

    Does anyone have the capability/desire to take a decent photo of the mainboard of both mics, so we can figure out the component differences? There's a single LM358 op amp in this mic. I suspect differences in the input/output/coupling stages. It could be something as simple as a SMD resistor that needs to be changed.

    FWIW, I injected a 1 kHz tone via a probe, into the + side of the mic element. Sounds like total ***. Nothing like a sine wave -- this is regardless of gain settings on the pot. If I inject the tone at other locations within the circuit, including on the output of the audio amp, it's perfect/clean and sounds excellent. I'm quite confident we can identify the differences and fix this issue.

    Please post pics/findings and let's get this issue sorted, so we can do 5050/5070 conversions going forward.


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    Could it possibly be TDMA interference getting into the XTL mic, whereas the Trbo mic has additional filtering to account for the extra EMI? That's the only thing I could think of. Similar to how third-party accessories on the Trbo radios can sound like trash if they get the TDMA signal on their lines?

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    It has nothing to do with TDMA. Please try using an RMN5070A mic on a TRBO radio and report back with your observations. I'm not a ******* idiot and I wouldn't post about TDMA interference. Give me a break, man.

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    What about voltage drop on the supply? Might be a higher impedance supply on 'TRBO radio mic supply voltage?
    It is a fine thing to be honest, but it is also very important to be right

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    Mars, you want to /rant about what you found on your cloned Chinese counterfeit microphone?
    Apparently NOT a radio professional.

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    With an op amp type circuit the ratio of input resistor to feedback resistor sets the gain of the stage. If you suspect it's got too much gain, changing the ratio (1 resistor) will change the gain. Usually, the input is applied through a series resistor to the (+) input to the amp, and the output is fed back through another resistor to the (-) input. Usually, the (+) and (-) inputs will have the same value series resistor, or a divider stage to ground through 2 resistors to cut the level down if needed. Try changing the feedback resistor.

    Another idea is take the good mic, and inject a tone like you did before, but measure the point where turning the level up any higher causes distortion on it. Write that level down. Then with the same signal applied, measure the output pin level of the amp. Write that down. Then, put in the bad mic, apply the same signal to the mic high side and measure the op amp output. Change feedback resistor until you get the same level as the good mic.

    Or, just change all the resistors in the bad mic to match the good mic, and that should do it...

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    Mars (Aug 10, 2019)

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    I will update soon, guys. No time today.

    com501 and I worked on this for a little bit yesterday (comparing mics, thanks bro -- appreciate it) and what I have here is a very good CHINKY CLONE. Their build quality is good, but they copied the RMN5070A design and sell it as the RMN5050A.

    I have seen the issue I'm describing with legit 5070As being used on a XPR (TRBO) radio. It sounds like ***, as the gain is too hot/distortion/etc.

    Once I figure this out, I'll post an update for anyone else who gets burned in the future.

    Also, I shared a pic yesterday with some of you. I suggested the tantalum caps were installed backwards. I was wrong. Unlike every other type of cap, tantalum caps have their positive side marked with the dark band. So they were indeed installed correctly. I learned something yesterday.

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    I don't know where the magic happens, but I know that when you change mic styles on the Trbo radios, you have to reboot the unit with the new mic or things don't work correctly. Might just be the keypad mics but who knows??
    Apparently NOT a radio professional.

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    I spent more time on this today.

    OK, so the mic design is complete shit. The element they're using is crap. The op amp is configured improperly. The frequency response is not suited to IMBE or AMBE+2. It's missing parts. The monitor switch on the top side of the mic is not hooked up (but it is on the bottom side). I can't even begin to describe what junk this thing is. It is 100% counterfeit chinese shit. I took an ASTRO Spectra desk mic and tested it (swapped harness) and it works 100% perfect.

    I'm done wasting time on this. I've spent about 8 hours on a $68 mic. You know how that goes.
    Last edited by triptolemus; Aug 12, 2019 at 08:04 AM.

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    com501 (Aug 11, 2019),mss-dave (Aug 12, 2019)