Re: ISB Receivers
The use of Collins filters certainly sounds plausible for a design aimed at the American market. The 1.4 MHz filters in the RA1772 might have been Cathodeon or Plessey. Both companies issued a comprehensive 1.4 MHz filter range circa 1970 in anticipation of the needs of marine receivers that were compliant with the incoming SSB requirements. Although there was some prior use of 1.4 MHz, Redifon was probably the first to use it in a “modern” receiver, and it suited the marine application being outside of any of the marine bands. And it looks to be the case that once 1.4 MHz filters became a standard industry item, that IF was also used for general-purpose receivers, at least in the UK and Europe.
This is pertinent to the core thread theme, in that as ISB receivers migrated from specialist to variations on the single-box general-purpose concept, the hitherto commonly used 100 kHz final IF often moved up to 1.4 MHz. Presumably suitably narrow 1.4 MHz filters were available for carrier extraction, but then the solid state era also brought with it PLL-type carrier recovery systems, which may have eased the carrier filter requirements.
Cheers,
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