Modifications for the Yaesu
FRG-7700
19-07-1998 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.