Quick update - I have resolved the problem with the LPF module in my homebrew sBitx. The observed problem was that the transistors that control the relays sometimes were conducting when they were not supposed to resulting in more than one filter being switched in at a time. The solution turned out to be adding a cap to ground at the low side of the relay coil where it connects to the transistor. The hypothesis was that RF was being induced onto the inexpensive non-RF-rated relay coils. I also swapped the NPN transistors for 2N7000 MOSFETS - the 3V threshold voltage guarantees that they won't turn on from minor fluctuations on the Raspberry Pi GPIO lines. Reliable informaion on the high and low logic values for the Pi GPIO have been difficult to find. Data sheets for the Pi 1.0 suggest logic "low" could be as high a 800 mV which could turn-on an NPN transistor, so the MOSFET seemed a safer bet.
Here is the final circuit:
Thanks to everyone for their suggestions and ideas. The wisdom and entrhusiasm of the crowd really helps.
73 from Great Falls,
Dean
KK4DAS
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