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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