W6SI Blog Page (Entry 2) |
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9/30/14 (rev. 10/16/14) |
How to Improve Transmit Audio of Yaesu FT7900 Transceiver |
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Have anyone told you that your transmission audio is low, even though the signal is full-quieting? |
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A perfect fit, right? Well... I had a problem. My transmission audio was said to be too low. How could that be? Come to think of it, a few SOARA folks do have that same problem. Let's explore... I normally use my old faithful Heil Sound Traveler headset for mobil operation. The reaction from the normal on-air buddies were less than favorable. My transmit audio was faint, and intermittent. Wow, really? Then I use the stock microphone. It's still low. Wait, it’s a brand new rig! How can it be broken? After a little soul searching, I came to a pair of conclusions.
First, the Traveler headset comes with a dynamic element of 600 ohm output. It puts out -57dB to that load. According to Yaesu spec, FT-7900 has 2Kohm input for the microphone. The stock MH48 microphone has electret element and has -40dB output into 2Kohm. A simple estimate that 600ohm microphone might have 1/3 of output feeding into 2Kohm, that’s over 20dB difference from the stock microphone. No wonder my audio level was low. All these time, I have taken for granted the most, if not all mobil gears come with 600 ohm input. What a shocker. Now, so if I am using the stock microphone that came with the transceiver, how come my audio is still weak? Then I remembered that I heard from way back US market, because compared to Japanese language, US English is spoken rather loud, which resulted in too much audio level for their early models of VHF/UHF rigs. HF rigs level can be adjusted since they typically have mic gain on the front panel, but most VHF/UHF gears do not. Not wanting another “factory adjustment”, they “plugged” the inlet path to the microphone to lower the net gain. I recall, back in late 90’s I actually drilled a hole in my mic / speaker unit for my HT to overcome a similar problem. So I searched on the net to find there are several people with the same problems out there. Let's fix that microphone! |
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Time to open up the microphone shell to remedy the problem. It’s pretty simple. The procedure requires a small screw drivers (plus and minus), no soldering required.
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That’s it. Basically remove the plastic baffle and enlarge the holes to the mic element. Just doing that improved my signal report markedly. So it solved the problem for the stock hand mic. But I like to use head set while driving, so I still aim to fix the Traveler. FT7900 is a popular mobil rig, and I don’t think I’m alone having this problem. If you are in a similar situation, try it. |
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Above is a reflection of my thought and only mine. But if you have any questions, feedback and/or suggestions, please send me an e-mail. I might reply on this page (sorry, no guarantee) |
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