(See also JGW-G1909605)
Today I soldered some parts on RF PD circuits boards to modify them and use them as 2f/3f RFPDs.
For 2f PDs, the modification is trivial; some notch parts and resonance parts are swapped with respect to the original 1f ones.
What is tricky is 3f PD. See the pictures. (In the first picture C31 is still there, but I removed it afterwards. I had just forgotten to remove it)
I glued a piece of universal circuit board on the PD board to make a room for additional notches.

Operation tests and tuning of them should be done soon.