Hi Joseph,
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That is very interesting!
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The P04-004 is a valid Sommerkamp board number and is the IF board from the Sommerkamp TS-288A transceiver, which was the Sommerkamp version of the FT-101 with added CB channels.
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This board should be very similar to the PB-1080A and I have attached a schematic of P04-004. I have not checked that these boards are interchangeable, but I believe that they would be. I believe that the original board would have had the Yaesu SSB filter fitted. I do not know why yours has a homebrew filter. Perhaps the original filter was somehow damaged, or a previous owner wished to improve the performance? The early FT-101 and TS-288A were fitted with a 6-pole SSB filter, whereas later models of the FT-101 used an 8-pole filter.
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73,
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Peter
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G4DJB
