We all know that the ICOM IC2820 has an squeaky TX
audio response in FM
mode with a lot of exaltation of the high frequencies.
This audio response is really not comfortable to hear.
In order to solve this problem (that isn’t really a problem….)
I decided to check the schematic diagram and open the bottom cover of the radio and find where to put the fingers.... or better where to put the soldering iron.
There are three HPF “high pass filter” stages on the MIC path, so I tought to eliminate one stage and hear the difference.
I bypassed the first HPF stage right behind the MIC connector on the front panel with a 10 uF 16 V capacitor.
I put the capacitor with the positive lead on PIN 3 of the MIC connector and the negative lead on the joint between C624 and R545.
In order to have the right effect on the audio response you have to remove the 1 uF C624 capacitor from the PCB.
The following drawing and pictures well explain the modification.
In this last picture there are all the information to perform the TX
audio modification on the 2820.
The modified radio has been checked in DV mode too to verify that the now broaden audio frequency response doesn’t affect the audio quality in GMSK modulation.
Now the question is…….Why the engineering staff from ICOM put this third HPF stage on the AF path?
I really don’t know but by now it doesn’t matter!
Now enjoy your NEW IC2820 but pay attention to set the audio setting as follow:
MIC sensitivity: HIGH
With the above settings the transmitted audio will be more powerful and comfortable.