There are a total of four mechanical filters in the Yaesu FRDX400 receiver for CW, SSB, AM, and FM.
Two are made by Kokusai and two are by Toko. The Kokusai filters are serviceable but the Toko units appear to be potted.


One of the Kokusai filter has the labelling scrubbed off but a test shows it is likely the same filter repaired above but with slightly narrower bandwidth and more loss most likely due to some degradation. The MF-455-03AZ27 is a 600 Hz filter. It shows the most degradation with more than 20 dB loss. It was disassembled and repaired the same as above. Below is a photo taken during the clean up process. It was very similar in condition to the above filter in that the foam was crumbly but for the most part intact. Again the center elements of the filter were stuck to the plastic base.

Instead of a photo of the nanoVNA, the software nanoVNASaver version 1.4 was used. The software has the advantage that it allows more types of output for S11. The capture shows S11 as an r + jx output while S21 is log-magnitude.

There may be some issues with the nanoVNA or the software in narrow sweeps like that above. The step size says 20 Hz but the display appears to be 200 Hz steps. The frequency scale does not read out properly
Version 0.1.4 of nanoVNASaver was used because the two newer versions 0.1.5 and 0.2.0 had issues on my computer with markers. With 0.2.0 an attempt to use or enter a marker frequency crashed the program. With 0.1.5, I could enter the marker frequency but the marker would default to the left side of the screen and could not be moved.
The filter itself appears to be now in good working order. The 6 dB bandwidth is about 600 Hz and the insertion loss in the above capture of 34 dB is about the same as the filter repaired earlier with 680 ohm resistors in series with the source and load.
Working on these filters was pretty enjoyable. They are much bigger than SMD parts and can take a fair amount of handling without damage.