Chuck: I’m somewhat surprised that no one responded to your post.

The short answer is that filters cannot be interchanged between the TR7 and the R4C. But this isn’t entirely true because the crystal filter center frequency in the TR7 is 5645 kHz regardless of mode but the crystal filter center frequency in the R4C varies by mode.

For example, when the R4C is in the AM mode, the AM filter center frequency is 5645 kHz.

However, when the R4C is in the SSB or CW mode, the filter center frequency is 5695 kHz. This difference in crystal filter center frequency comes about because the IF output from the 1^st mixer (6EJ7, V2) is 5645 kHz, and in the AM mode there’s no need for a BFO, so the 5645 kHz IF is maintained through the 1st IF filter/amplifier.

The SSB and CW modes require a BFO, so the crystal filter center frequencies were moved up 50 kHz in these modes to accommodate a 50 kHz BFO.

The bottom line is that it’s not possible to use TR7 filters in the R4C with the exception of the AM filter.

In terms of the filter input/output impedance, the INRAD data sheets for their TR7 and R4C filters indicate that all were designed around a 50 ohm input/output impedance. Presumably the impedance of the original filters were in this range as well.

I hope this helps.

73,

Scott

N7NB