[Icom] IC 718 and AGC

[George Sieverson]

Greetings Rem and esteemed assemblage,

I recently acquired one of these little marvels as a travel/portable radio.
However, I also found myself using that same old-timers trick to eliminate
the fatiguing AGC characteristic.  I use the rig for CW, too.  ...and the
AGC is even more obnoxious in that mode.   FWIW, I have a friend with a '706
and I noticed the same annoying action on his radio, too.

From what I can tell, the AGC suffers from two maladies....

First, the AGC release time is way too fast for SSB.  I prefer a much slower
AGC release for CW, too.

Secondly, the AGC attack time is a bit too slow.  I've noticed some
distortion when a moderately strong signal first appears.  It goes away once
the AGC loop has stabilized.  The distortion is particularly noticeable on
CW, where one expects to hear a clean note, sans overtones.  Since I prefer
to listen to a low pitched CW note of around 400 Hz, most of the harmonic
distortion products are well within the passband of the audio stages.  So
they really stand out.

So the correct solution may not be as simple as simply adding another
capacitor to slow down the AGC decay timing.  Without putting the radio on
the bench, I'm guessing I may also need to reduce the IF signal level to the
product detector or the detected AF to early audio stage in order to avoid
distortion from the overshoot.  If the overshoot is really bad, I may need
to compensate the entire AGC loop.  I won't know until I get the radio on
the bench.

In the meantime, I've ordered the service manual on CD.  I have two projects
to complete before I can fix the design in my IC-718.  I will report back to
the group once I have a solution in hand.  (ETA: mid-summer)

To those considering the purchase of this rig, I consider the AGC design to
be it's only real downside.  It's really a great little rig.   ....too bad
it was built by engineers and not radio operators.

73 'n cheers 'n beers,
George - K9GDT
************************************
http://www.qsl.net/k9gdt/index.htm
Homebrew & Vintage Radios
Last update: 1/6/02
***********************************

----- Original Message -----
From: "Rem P Roberti" <remegius@pacbell.net>
Sent: Sunday, April 28, 2002 10:19 PM


Still, it astounds me that the designer(s) of this fine little rig saw fit
to leave it without an AGC control.  My work-around for this is the
old-timer's trick of using a relatively high audio gain setting, and then
using the RF gain as a volume control.
Rem
WA6CJK