[Elecraft] CW rise time mod

[John Reiser]

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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