After several years of using the Yaesu VX-6r portable transceiver, it developed an unpleasant drawback - quiet modulation in the 144 MHz range. Symptoms - at the repeater’s “round table” the presenter could not make out the call sign, and when answering a regular call, the correspondents ran away in fear with the words “sorry, I can’t receive you, see you later.” 😏 The reason for this is the reduced modulation level, which appears as a quiet and unintelligible signal.

What is the reason for the decrease in modulation level? In menu 27 HLF.DEV-OFF ​​is turned off, 37 - MCGAIM 9 at maximum.

I believe there is not one reason, but several:

• the setting was made at the factory for one volume level, but over time the microphone parameters “gone”
• possibly affected by the protective membrane in front of the microphone, which could have moved and covered the microphone more tightly (stuck)
• The microphone contacts have oxidized a little (since it is a capsule type and is not soldered).

One solution is to replace the microphone (there are video) did not suit me - although I have experience in disassembling electronics (laptops), and soldering is not needed, violating the security of the VX-6r is undesirable. Besides, you don’t want to look for a replacement microphone from an outdated cell phone.

As a result, I found a simple way to return the modulation volume - using the service menu. This modification is described on the website mods.dk:

https://www.mods.dk/view.php?ArticleKey=15825aee15eb335cc13f9b559f166ee8&i=1-yaesu-VX-6R-Fix-for-low-audio-on-70cm

To enter the engineering menu of the VX-6r transceiver you need to:

• set the VFO frequency to 440.000 and turn off the device.
• Then, hold down the Band, Internet and 0 keys, without releasing them, press the Power button.

Menu navigation is done:

• by turning the knob of the valcoder (encoder) - to navigate through menu items
• pressing the keyboard button V/M - to change the parameter
• rotation of the valcoder (encoder) - to change (increase or decrease) the parameter value
• press V/M again and move to the next menu item with the encoder - to save the value
• Band button - to move to setting the next band (51, 144, 222, 432 MHz)
• Hm/Rv (Home/Reverse) button - to exit the engineering menu.

Microphone gain (modulation) adjustment is made in the "deU". The default value for the 145 MHz band was 62.
For 52MHz it was 90, for 440 it was 120.

Я уincreased the “deU” setting value in all ranges to 135. (Adjustment limits are from 1 to 255).

After setting up, correspondents began to answer the first time, noted a pleasant, intelligible modulation and had long conversations, which had not happened before. 😄

A more correct way - instrument control is not available to me. You can only observe on the WebSDR waterfall or on the RTL-SDR receiver so that the modulation does not go beyond the 25 kHz wide channel (Wide). 🔥

What should the signal width be after adjusting the modulation? For simplex channels and N-FM modulation used in the VHF 144 and 432 MHz bands, 18 kHz was mentioned. (This is the average between Narrow 12.5 kHz and Wide 25 kHz). Although in Russia, on repeaters, in my opinion, louder modulation is adopted, 20-25 kHz - this is how most operators work. From my point of view, we should focus on standards.

An official document indicating the FM modulation width for the amateur radio service in Russia:
DECISION dated July 15, 2010 N 10-07-01. On the allocation of radio frequency bands for radio-electronic equipment of the amateur and amateur satellite services

Permissible classes of radiation in the ranges of 144 and 432 MHz:

• 2K70J3E - single-sideband suppressed carrier telephony (USB), with a maximum bandwidth of 2.7 kHz
• 6K00A3E - two-way amplitude telephony with a bandwidth of 6 kHz.
• 11K0F3E - frequency modulation with a bandwidth of 11 kHz (narrow modulation, or Narrow) for channels with a step of 12.5 kHz
• 16K0F3E - frequency modulation with a bandwidth of 16 kHz (for channels with a step of 25 kHz)
• 20K0F3E - frequency modulation with a bandwidth of 20 kHz (for channels with a step of 25 kHz)

We immediately dismiss the 11 kHz modulation - it’s quiet. Used only for satellites (with a corresponding receiver tuned to the “narrow” version of narrowband FM).
In addition, quiet modulation can be obtained if you enable menu item 27 - HLF.DEV (Half Deviation - the “narrow” deviation is half of the usual one, i.e. the volume level is reduced by 3 dB).

There remain two “loud” options for FM modulation width - 16 kHz and 20 kHz.

With modulation wider than 20 kHz at peak speech volumes 🔊 an overmodulation effect is possible (wheezing, distortion, a dense signal that is not pleasant to the ear).
Therefore, I choose medium-width modulation, with a band of about 16-18 kHz - it has less distortion, more distinct speech, with clean high-frequency notes 🎶.

Sources:
https://www.ruqrz.com/oboznachenie-klassov-radioizlucheniya/
https://digital.gov.ru/uploaded/files/prilozhenie-k-resheniyu-gkrch-18-45-02.pdf
*http://www.mods.dk/view.php?ArticleKey=15825aee15eb335chk13fav559f166ee8&i=1-yasu-VH-BR-Fix-for-lov-audio-on-70km

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