[1000mp] Still struggling with my AGC problem

[Ralph S. Turk]

Have you tried a good dummy load.  If so with what results.
I used to have a similar problem that was caused by two problems
I was using a 160 meter 1/2wave dipole fed with a transformer type balum.
My agc readings were all over the place.  Hi, Low, it didn't matter if there
were signals on the RX or not.
Also had strange changing SWR.
Turned out to be a bad balum and local broadcast stations 550-1700 Khz were
overloading the rx with rf on the antenna.
Changed the 160 meter dipole to a 80 meter dipole and new balum and problem
went away.

Ralph

-----Original Message-----
From: 1000mp-bounces at mailman.qth.net [mailto:1000mp-bounces at mailman.qth.net]
On Behalf Of PA5CA Frans
Sent: Sunday, February 18, 2007 11:42 AM
To: 1000mp at mailman.qth.net
Subject: [1000mp] Still struggling with my AGC problem 

2 Icom 756 Pro Radios to be given away to QSL/QTH .NET supporters!  One on
Dec 31 and one on Jan 31.
See http://mailman.qth.net for details.
______________________________________________________________

I'm still struggling with my FT1000MP mark V Field and the AGC.
Did some more tests and found the following. perhaps someone recognizes this
phenomenon.
My initial feeling was that RF came in to the receiver and therefore the AGC
came on.
But had a little more time to spend.
Did the following.
- I directly connected a antenna to the RX (external input) input.
  Made TX (SSB) and noticed the same behaviour, As soon you release the PTT
the S-meter comes from S9 and
 gradually it settles S0, time approx 2 seconds. (AGC setting Auto or slow),
switching to fast or AGC off makes a
 difference, mainly it goes to S0 spot on. (Sub receiver shows exact the
same)
 So I thought perhaps an RF power change may show different, but that became
an illusion. There is no difference in
 behaviour between 3 Watt or max 95 Watt.
- So I tried different modes. AM, FM okay. AGC seems to responds normal.
  RTTY. here I see some AGC behaviour.
  So the next was, it may sound odd but I loop back the monitor audio in to
the microphone (give that nice high tone)
  and noticed that the pitch makes the AGC vary in behaviour.
  Lower the pitch (tone) causes more AGC behaviour after releasing the PTT.
  I tried this several times and I'm almost certain that the pitch influence
the AGC behaviour!
- So the problem occurs in SSB (LSB/USB) mode.
- There is no relation with the TX RF output power.
- But tone pitch has impact on the AGC behaviour.

RX sensitivity looks normal.
ALC behaviour for what I can observe seems to be normal.
The radio shows no abnormalities except for the AGC behaviour while
releasing the PTT.
So could the ALC causing the above phenomenon??

So help or hints are most appreciated on this mater. It drives me
crazy.....#$%*(&^$))(*%

best 73,
Looking forward to some replies.

Frans (pa5ca)

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