Oh. And PS. I modified a few of the IF boards per Roland. He’s a smart guy, has a doctorate in electronics! Way smarter than me but the IF mod didn’t work right. Had a side effect so I put all of them back to stock. I think he’s close with his theory that there is an impedance issue with the filter board
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On Feb 19, 2025, at 10:16 AM, atlasstuff via <
g4fph@...> wrote:
Digging into this a little more today.
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Using my SDR transceiver as a narrow-band, RF spectrum analyser, I checked the DSB TX IF signal going into the Filter Unit. Audio fidelity from the mic. I selected as best match to the '301 (an old Astatic MOD. 575-M6) was very good, with modulation sidebands extending to at least 4 kHz either side of the (suppressed) carrier, and with some signs of roll-off from around 3.3 kHz. So, I think the issue is likely to be the response of the second crystal filter on the NB (/ RF Processor) Unit.
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Even with the processor turned off, the (now SSB) TX IF signal still passes through this second filter, which seems like an odd decision by the Yaesu design engineer(s). I see references in the service manual that the filter is optional, but I think it is present in all three of the transceivers (one -301 and two -301D) that I own. It's not listed as an option in the English brochure that I have a PDF of.
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Later this week, I will pull a complete NB Unit from another '301 and try it instead of the one I have been using. Depending on results, I may even try removing the filter on the NB Unit to see what difference it makes. I assume it is present principally to clean up artifacts arising from the RF clipping process, so it would be nice to retain it for that. Perhaps a modification is possible...?
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I note that 'ELcon' () published a modification for the Filter Unit that better terminates the filters in order to improve the response on USB. My testing has all been on LSB so far.
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Regards,
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Mark, G4FPH.
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