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Output Spike on Icom-706

Subject: Output Spike on Icom-706
From: K0RC - Robert Chudek <K0RC@pclink.com> (K0RC - Robert Chudek)

   I've experienced this same problem (and ETO is aware of it too) with
my Alpha 87A when driven by the Icom 781 transceiver.  Depending upon
where I had the transceiver drive controls set, the initial pulse would
light the Alpha like a Christmas tree!
   I judicially adjusted the RF drive and another control (sold the Icom
so I don't remember its name) to reduce the problem.  The control is
similar to the Carrier Gain control on my Kenwood TS-950SDX.  It sets the
ALC level within the transceiver.  If I set it too high, even the
Kenwood tends to pump the amplifier on the first dot.
   I was able to adjust the IC-781 drive controls to eliminate the problem,
but if I remember correctly, I'd have to readjust the controls each time I
switched bands.  This was unacceptable to me during contesting so I sold
the Icom and stuck with the Kenwood.
   I'm not familiar with the IC-706 so I can't offer help here, but maybe
you can use this information in the quest for a solution.

   73 de Bob - K0RC

At 11:38 AM 2/28/96 -0500, you wrote:

>In preparing for an upcoming CONTEST expedition for WPX, I recently set out
>to test my new Icom 706 with my Alpha 87A.  I know this is a bizarre
>combination, but I'm looking at the 706 as a backup rig.  (Incidentally, my
>apologies to all who were in the Sunday morning 40/20m pileup on KH6CC during
>ARRL-CW and couldn't understand why I didn't answer them and just kept on
>CQing.  It turns out the receive oscillator on my main rig had suddenly taken
>a sideways step. We finally found the pile-up by using a second rig and I
>finished out the contest with a whole lot of RIT dialed in!)
>Anyway, when I try to drive the 87A with the 706 I see some very unexpected
>behavior.  No matter how low I set the drive power on the 706, the very first
>dit I send causes the power output lights on the Alpha to flash full scale.
> Succeeding dits which are closely spaced will give the expected power
>reading based on the amount of drive supplied.  In SSB, a similar situation
>holds.  I can pull down PTT so that all the T/R switching is complete but
>then the first bit of audio supplied causes a momentary full-scale reading on
>the 87A's power meter.  After that momentary flash, the reading is expectable
>in the same way as it is in CW.  
>In the course of making these tests, I noticed one other interesting piece of
>data.  If I wait a half second or so after the first dit before sending a
>second dit, then the second dit will cause a power reading which is still
>excessive but less than full scale and reduced by an amount proportional to
>the time elapsed since the first dit.  To give an example of the pattern, a 1
>second delay might show 1KW out, a 2 second delay 800w, and so forth.
>I first contacted Icom Customer Service about this and they had no
>explanation at all.  Their representative mentioned K1KP's article in the Nov
>'95 issue of QST on using a optocoupler for T/R switching, but even he
>acknowledged that the problem I was seeing didn't sound like a timing problem
>with T/R switching.  This morning I contacted ETO Customer Service (Ray
>Heaton), and he gave me what I think is a totally plausable explanation.
> This is that the 706 is actually putting out a momentary spike when it first
>starts transmitting that is full-scale (or worse) output.  Some time delay is
>required for the ALC circuitry in the 706 to kick in and cause the output
>power to be cut back to the proper amount.  This time delay is in
>milliseconds, of course, so a peak-reading wattmeter is not fast enough to be
>able to record it.  The LEDs on the Alpha do show it, though, and Ray
>suggested that I was risking damage to the PIN diodes in the Alpha if I
>operated it with this initial spike coming down the line from the 706. He
>suggested that what I really need to do is use a scope to measure the size
>and duration of that leading spike and then go back to Icom with the
>This sounds fine, but I don't have access to a scope these days.  So, for
>those of you with the perseverance to read this far, I wonder if there's
>anybody out there with a 706 and a scope who would be willing to take a look
>at this for me?  Alternatively, I'd eagerly be willing to go anywhere in the
>SFO bay area to set up a test with my 706 if you have a scope we can use.
>Bruce Sawyer, AA6KX

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