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Re: [Amps] 87a will tell you, too

To: <amps@contesting.com>
Subject: Re: [Amps] 87a will tell you, too
From: "Paul Christensen" <w9ac@arrl.net>
Date: Wed, 15 Aug 2012 10:36:40 -0400
List-post: <amps@contesting.com">mailto:amps@contesting.com>
>> "I own one FT-9000d, one IC-7800, and one K3/factory assembled.  The only 
>> one where I can get "over shoot" or a full-power brief spike of RF is the 
>> K3.   The rig is set for, say, 40 watts output and on its first key down, 
>> voice or CW, it jumps to 100 watts for a very brief moment and strangely 
>> does not repeat this spike as that one key-down session continues, but 
>> repeats again after a longer stand-by period.   This is a common and very 
>> well known problem with the K3, but not often discussed due to the 
>> powerful love people have for their K3 radios."

That's not right.  It's an occurrence only when the menu-driven TX Gain is 
not calibrated at the 5W/50W set points.  The easiest remedy is to use the 
K3 software utility.  In the utility, select the tab under "Calibrate," then 
select "Calibrate TX Gain."  The wizard will then take you through gain 
calibration which sets ALC on a band-by-band basis.  If after calibration a 
spike is seen or detected by an amplifier, then there's a more pervasive 
problem with your K3.  With TX Gain set correctly, the envelope should look 
like the one displayed on pages 11 and 12 of DL9NC's paper:

http://www.ov-n15.de/joomla/images/zoom/DL9NC/pz_32.pdf

I use a Tektronix TM test set with SC504 scope while transmitting CW.  A 
spare output from a microHAM CW Keyer triggers the scope such that the 
leading and falling edges are accurately displayed and precisely triggered 
from the keyer.  This allows for the keyed RF waveform to remain stable 
without having to chase it by changing either CW speed or the scope's time 
base.

As a test, I just set power to 40W on 20m and looked at the keyed envelope, 
allowing for various settling times in between transmissions up to 30 
seconds.  At no point does a power spike occur.  In fact, the envelope never 
reaches a maximum, then some minimum point as is usually the case after ALC 
stabilization with other transceivers.

Referring back to the DL9NC paper, power spikes from transceivers other than 
the K3 show a period of about 1 ms or so.  That's under the sampling period 
required by commonly available sampling wattmeters.

Paul, W9AC

 

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