From: "Kim Elmore" <[EMAIL PROTECTED]>
I want to understand why we're seeing the negative deflections under
modulation. I'd expect positive deflections, not negative ones. What is
this telling me about the transmitter's operation?
That is not at all unusual, even though theoretically the plate meter should
stand completely still during modulation. The most likely cause, if the rig
uses a common power supply for modulator and final, is that modulator plate
current is dragging down the plate voltage. Sometimes, even poor regulation
at the 117 v. mains can cause the problem. The other likely cause is
nonlinearity in the modulated stage. The final tube may be a little low on
emission. Also, with screen grid tubes, the screen has to be modulated
along with the plate. If this modulation is not 100% optimum, some
nonlinearity will result, and the most common method of screen modulation,
thru a dropping resistor off the modulated plate voltage, is rarely optimum.
Sometimes the screen bypass caps will reduce the screen modulation at
higher frequencies so that it doesn't follow exactly the modulation of the
plate.
I wouldn't worry about it unless the downward swing in plate current exceeds
5% or so of the total plate voltage.
If you have a monitor scope, look at the envelope pattern to see if
everything looks ok. Better still, try a RF trapezoid pattern (this
requires going into the transmitter and sampling audio from the modulator),
so you might not want to bother with it. The trapezoid will give the
ultimate indication if there is any nonlinearity, but the envelope pattern
is sufficient to tell if anything is wrong, such as flat-topping or severe
waveform distortion.
73, Don K4KYV
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