modulation problem regarding low audio-frequencies

[xplane]

In the combination of Alinco DX-SR8 with RRC 1258 I found that the modulation prefers only the higher audio-frequencies, low audio-frequencies are missing totally!?  So I have been reported in many QSOs. 

I have tested this with different band-frequencies and with different modulation-modes. Even through several changes in the RRC-configuration no improvement could be achieved.   

Any suggestions?
Any help is appreciated!

73, Dirk

[xplane]

... Even through several changes in the RRC-configuration no improvement could be achieved. ...

Additional Information:
Beside some other changes of the standard configuration -- I have tried the value 82 in parameter "Codec inp HPF Hz" (Default is 163).

[sm2o]

The audio transfer trough the Remoterig system is extremly flat so the problem is not in the audio transfer.
I have never heard about any problem with the DX-SR8 audio before but I will configure up a system here and check if we get the same feeling.

73 de mike

[xplane]

The audio transfer trough the Remoterig system is extremly flat so the problem is not in the audio transfer. ...

Hmmm... That wording suggests, that the bandwidth of the voice transmission via RemoteRig is actually very narrow?

[sm2o]

flat = flat frequency response, the audio level trough the system is constant independent of the frequency within the useful frequency range for speech.

/mike

[xplane]

... I will configure up a system here and check if we get the same feeling.
73 de mike

Hello Mike,

are there any new insights?

73,
Dirk

[sm2o]

I have tested the modulation today, and you have right the low frequencys are missing. There is a capacitor C7 in the Alinco Control panel which forms a high pass filter in combination with Remoterig. I soldered a 1uF capacitor in parallell with C7 and then the modulation sounds normal. It's done in 5 minutes just take away the back of the frontpanel and find IC2. C7 is connected to pin 1. Then solder a capacitor in parallell like on my photo. Service manuals can be downloaded from the Alinco website but I don't think you need it.

73 de mike

[xplane]

Hello Mike,

thank you very much for your investigation!   
Again very good service!

Do I need a special soldering iron? Or can I do it with a standard electronic soldering iron?   

Does this modification work regardless of the connected microphone?

73,
Dirk

[sm2o]

Yes you can do it with a standard solder iron if the tip is not to big. And the capacitor is after the Mic amplifier so it works independent of microphone

73 de mike

[xplane]

@sm2o:
Unfortunately forum guests cannot see your photo of the necessary modification.
Are any attachments only visible for registered users?

[xplane]

Here follow some pictures of my implementation.
73, Dirk

( Sorry, but the attached pictures are visible only for registered users  )

[xplane]

Hello Mike,

would a 10uf capacitor increase the positive effect of this modification?

73,
Dirk

[sm2o]

I don't think 10u make any difference, I think 1u is big enough

/mike

[xplane]

I was told, that the modulation now sounds better, but still is not optimal.

73, Dirk

[xplane]

I would like to try it with a 10uf electrolytic capacitor.
Is that possible in principle and in what direction (related to the polarity ...) should I connect the capacitor with the circuit?