[Icom] A couple of PRO 2 questions

Mon, 17 Feb 2003 16:53:33 -0800

Hi Steve,

Regarding your first question: If the rig is stored at an ambient below 15 C
(59 F) for a few days, the TFT display backlight may be sluggish in starting
up. This may be due to condensation of the mercury vapour in the CCFL
(cold-cathode fluorescent) backlight lamp. I assume that when you go away
for a few days, you turn down the central heating; that would account for
the sluggishness. I have observed this phenomenon on my 756Pro, which is in
my lab/shop at 13 C - but not on my Pro II, which is in my shack at 18 ~ 20
C.

On your second question: I have also observed this my station. I suspect
that the apparent loss of sensitivity occurs when a very strong signal (or a
noise spike) at the instant of switching from transmit to receive develops a
large AGC voltage.  If this is the case, the duration of the apparent
desense is a function of the AGC setting. If you select Fast AGC for QSK CW,
and decrease the decay time via the menu, the symptoms may disappear.

I went to the length of connecting to the Pro antenna socket a test fixture
consisting of a directional coupler terminated in a dummy load. The reverse
coupling port was connected to a signal generator. I adjusted the generator
for an S1 reading at 14100 kHz, and then keyed the transmitter in the QSK CW
mode. Using this test setup, I was unable to reproduce the desense - this
led me to suspect that the cause was a strong signal or noise burst at the
instant of the TX-to-RX transition. I do not think you have any reason for
concern, unless the desense occurs on a regular basis.

Hope this was helpful.

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 Steve
Sent: Monday, February 17, 2003 16:20
To: [email protected]
Subject: [Icom] A couple of PRO 2 questions

I bought a new 756PROII about 6 months ago.  I notice two things about
it that I'd appreciate some help with.

First, it seems that if the unit has been off for a day or two (or
longer) the screen doesn't light up immediately.  Last week, for
instance, I was away for a few days.  When I returned and powered up the
rig, the screen did not light up although the receiver began making
audio in the typical ten seconds or so.  The screen came to life about a
minute and a half after I hit the power switch.  After it's been on, if
I turn the rig off and immediately turn it back on, the screen
immediately lights up.  Anyone else seen behavior like this?

Second question:  During the recent contest I noticed a few occasions
when the 756PROII receiver seemed to desensitize for a few seconds after
a transmission (CW, full break-in).  The audio output seemed to drop
and/or the receiver sensitivity decreased.  It recovered on its own from
this behavior within a few seconds.  This "desense" occurred perhaps a
dozen times during the course of the weekend contest on 80 through 10
meters.  Am I misadjusting something to cause this?  Or is something
failing in the rig?  I use the 756PROII with a PW1 amplifier, Johnson KW
Matchbox, open wire transmission line feeding a pair of dipoles.

Thanks!

Steve
[email protected]

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