[Elecraft] CW rise time mod

[AJSOENKE at aol.com]

It's not that 3ms is a lot of time in terms of human scale. But, it is the  
rise time of an electronic pulse. This can have a lot of impact on the 
transient  waveform that results in an audio demodulator - i.e. receiver. The 
difference is  noticeable enough to make the difference in a crowded band weak signal 
situation  when the receiving station is differentiating what he hears. That's 
why a banjo  sounds different than a guitar  or violin.
 
Al WA6VNN
 
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 
In a message dated 3/29/2008 5:34:22 P.M. Pacific Daylight Time,  
reiserj at optonline.net writes:
People,

8 ms.- 5 ms.= 3 ms., does it  matter in any practical sense?  I would really 
like to know who cares,  and why?  Can anyone hear the difference? 
Three-thousanths of a  second?  Not my old brain.

73,
John, W2GW
K3  #384


----- Original Message ----- 
From: "Lyle Johnson"  <kk7p at wavecable.com>
To: "Elecraft Reflector"  <elecraft at mailman.qth.net>
Sent: Saturday, March 29, 2008 5:21  PM
Subject: Re: [Elecraft] CW rise time mod


>> Or do Rev A  RF boards have a lower value for C222 than that shown on the 
>>  schematic?
>
> Early production K3 RF boards have a 1 uF capacitor  instead of a 0.1 uF 
> capacitor installed at C222.  Yes, the  published schematics show a 0.1 uF 
> capacitor.  The effect of the  larger capacitor is to increase the TX 
> waveform rise time to about 8 ms  instead of 5 ms.
>
> Rev B RF boards have the correct 0.1 uF value  installed.  Sometime during 
> Rev A RF board production, the value  installed on the board was changed 
> from 1 uF to 0.1 uF.
>
>  Surface mount ceramic capacitors are not marked with a value, so you 
>  cannot tell which you have by visual inspection.
>
> You can  determine if you have a 1 uF rather than a 0.1 uF by:
>
> 1)  Measuring the capacitance if you have a capacitance meter.
>
> 2)  Looking at the Tx output RF envelope on an oscilloscope or "station 
>  monitor" scope.  If the fall time and the rise time look very similar in  
> duration, you have the 0.1 uF cap. If the rise time is about 50% longer  
> than the fall time, you have the 1 uF capacitor.  You don't need  an 
> oscilloscope with an accurate time base to make this comparative  
> measurement.  If your oscilloscope has a low bandwidth (2 to 10  MHz), use 
> the 160 meter band.
>
> 3) If you are concerned  that your unit may have the 1 uF capacitor and you 
> have no way to  determine it otherwise, you can just replace it with the 
> 0.1 uF part  and sleep better at night :-)
>
> If you don't change it, you will  not damage anything.  Your K3 will just 
> have slightly softer  keying and an upcoming firmware adjustment of the 
> keying time will be  less accurate.
>
> 73,
>
> Lyle KK7P
>
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