First check the PC board mounting and grounds. Might be just the right
number loose to get RF feedback and oscillation. Check the parts in the
15m low pass filter, might be some out of tolerance.
I was suspicious of a spurious output but that would be matched into a
dummy load, unless that spurious was on the high side and mismatched by
the internal low pass filter. So its time to check and tweak the last
transmit mixer balance, and maybe the spurious trap if there is one for
15 meters.
73, Jerry, K0CQ
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