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Drifting FT-847 transmitter at 435 MHz

I have noticed that the transmit frequency of my FT-847 drifts noticeably when I make a long transmission on 435 MHz.  The drift is proportional to the TX power.  At max power, the TX frequency drifts down about 500 Hz during a 30 second transmission.  After receiving for one minute the TX frequency will resume at the original starting point, but it again steadily drifts down 500 Hz during a 30 second transmission.  It is very repeatable.

Is this a generic problem to the FT-847?  Is there an easy fix?  It's obviously a thermal effect.  I'm pretty sure the TX fan is working properly.  At least it sounds like it's working properly.

Wayne Estes W9AE
Mundelein, IL, USA
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