[Icom] IC 746 tuner

Tue, 17 Dec 2002 17:34:30 -0800

Guillermo,

This behaviour is caused by the ATU phase comparator picking up rapid phase
fluctuations in components of the RF signal during voice SSB operation.

The ATU is controlled by a combination reflectometer/phase comparator at its
RF input. This circuit sends a zero error signal to the ATU controller when
two conditions are achieved: (1) minimum reflected power, and (2) 180deg.
phase shift between tuner input and output. The phase comparator detects the
rapid phase fluctuations referred to above; the resulting rapidly-changing
error signal can cause the ATU motors to hunt. In extreme cases, this can
initiate a re-tuning cycle or a "high SWR" indication. Realignment of the
tuner, as described in the service manual, may reduce the problem.

The AT-500 autotuner is a well-known example of this "hunting" phenomenon on
voice peaks.

Best 73,
Adam, VA7OJ/AB4OJ
North Vancouver, BC, Canada
http://www.qsl.net/ab4oj/
Icom FAQ Site:
http://www.qsl.net/icom/

-----Original Message-----
From: [email protected] [mailto:[email protected]]On
Behalf Of Guillermo
Sent: December 17, 2002 17:20
To: Icom
Subject: [Icom] IC 746 tuner

	Hello, a friend of mine owns a 746; he uses internal tuner to feed a
multiband dipole. In a few bands, we noted that even if SWR (as seen in
746's SWR meter) is not more than 1.2 (full carrier out, RTTY mode),
when he speaks in SSB mode, TUNE label starts to blink, and after a few
seconds, tuner is disengaged. It is like that at a higher peak output
(which?, because at RTTY we have full power out, peak SSB can't exceed
this) tuner still detects standing waves and try to re-tune, with no
success.
Later we found that also at RTTY, tuner has the same behavior after a
few seconds key down.
Any hint?

Thank you in advance
Best regards
Guillermo - LU8EYW.
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