On 04/12/2013 10:28 AM, Jim W7RY wrote:
Perhaps you have an antenna system problem?
The problem is occurring with several different antennas. However,
there is on thing I have not yet taken apart for inspection: an
antenna switch.
What makes me think oscillation is:
Suppose I have the amp tuned for maximum output at 28 MHz. Indicated
antenna SWR is low, output is 1500 watts.
Now assume that for the following procedures I maintain RF drive at
all times.
Now I vary the amp loading slightly. As I decrease loading
capacitance just a bit, I see a slow, gradual drop in output of
maybe 50 watts, then all of a sudden a drop of at least 500 watts,
and at that same instant, indicated SWR jumps up wildly. This is
absolutely repeatable on several bands with different antennas.
Once the sudden "jump" has occurred, returning the load cap to the
previous setting does not return things to normal. Going well past
the proper spot, toward higher capacitance, does at some point
produce a sudden decrease in SWR and increase in output power.
73,
Paul
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