Hi Ok One thing at a time, let’s look at the low UHF output.
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If J3006 is low then the fault is not on the RF PA board
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Go through to 3-2 3-11, it looks like to me something is out of lock or need peaking, you need to get J3006 o/p in the volts! I doubt it is a faulty component
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The next FT847 I get in for repair I will measure this