[Elecraft] CW rise time mod

[Lyle Johnson]

> 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