Hi Rey
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Looking at the circuit diagram R520 is a 49.9 ohm resistor, replacing this with a 5.1k would certainly affect the SWR readings.
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You might get away with using a 47 or 51 ohm through hole resistor. If you have a batch of 47 and 51 ohm through hole resistors measure each one and see if you have one at the edge of its tolerance range.
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Using the intended 49.9 ohm smd component would be ideal...even if you have to repair the PCB trace/pad with wire.
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Good luck
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Steve G4EDG?
Steve G4EDG