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 diagram.
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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