Modifications for the Yaesu FRG-7700
Better FM audio FRG-7700
The FM post detector filtering in the FRG-7700 receiver makes the audio
difficult to understand. It has much too many lows and hardly any highs in
the audio band.
Surprisingly, most of the problem is caused by the combination 2200 Ohm
series and 220 nF to ground; R11 and C13 in the upper right corner of the
Cutting away C13 is easy: it is a small blue tantalum cap in the middle of
the FM board. THe FM board itself is mounted on the right of the back panel.
You could replace C13 with a 10 nF capacitor to get rid of some of the
455 kHz energy that it was supposed to filter out. I did this.
While I was at it, I changed C24 and C25 both from 1 uF to 220 nF to get rid
of the spectrum below 300 Hz.
And finally I changed C26 from 10 nF to 6.8 nF, and C28 from 4.7 nF to 3.3 nF
in order to have the response flat up to about 3.5 kHz.
But really, removing C13 is all that is really necessary to enjoy a vast
improvement in sound. Also, FM packet and FM fax/sstv reception is now
greatly improved. The Hamcom modem is easily sensitive enough to use on the
REC output, once you made this very easy modification.
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