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The loop lowpass filter of my FT-007, purchased in mid-2003, has a second R-C branch in its feedback network that is not shown on the schematic dated in 2001 from the FCC’s Equipment Authorization pages. It appears that an additional “zero” was thus added to the filter’s transfer function around 500Hz, most likely to improve loop stability or resolve a production yield problem related to loop stability.
An Equipment Authorization search shows that a “Class II Permissive Change” was requested in 2003 under conditions of confidentiality, as is the applicant’s right. There is thus no newer public schematic nor explanation of what was changed or why. A “Class II” change implies that the certification test results were affected and new reports had to be filed before it was put into production, but it apparently didn’t require changing the model number or the FCC ID.
I found the link to the 2001 schematic posted on another forum. I was surprised to find it since confidentiality is almost always requested on schematics for competitive reasons.