The silver mica 220pF capacitors arrived, substituted and failed… Still no response. Only random noise (if it wasn’t random. it wouldn’t be noise, would it?). So the only thing that remains to be done is to follow the schematic and have the soundcard probing various positions on the circuit so I can see (hear) where the signal ends.
Ray Wilson from musicfromouterspace.com has a nice article on using your sound card as an oscilloscope and I use http://zeitnitz.de/Christian/Scope/Scope_en.html for display
(Ray Wilson’s article is here)