Just to round out this thread.
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Given a good crystal filter on the NB Unit, there would seem to be no real need to modify the transceiver to bypass this when RF PROC = OFF.
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By chance, I came across a recent video on YT from Stefan, DL7MAJ. He had a Yaesu FT-101ZD on the bench, on which the processor did not appear to be working. In his video, he showed the relevant part of the circuit of the '101ZD. It looked quite similar to that of the '301D, except that it had a bypass path around the second crystal filter for when the processor was off.
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For anyone interested, the '101ZD circuitry is:
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The circuitry needed to implement such a facility of the '301D would be very similar - something like this:
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I may try this on the board with the faulty filter, as it would make the board partially usable in the transmit path. Out of curiosity, I may first try bypassing the filter entirely. I doubt this would give spectrally satisfactory results with the processor on, due to the distortions that the RF clipping process introduces. The original G3LLL RF Clipper that was used with early FT-101, the FT-101E/F and the FT-101Z all had crystal filters to 'clean-up'.
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Regards,
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Mark, G4FPH.
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