Nikon SB-28 fires only at the second frame

[-jeff]

That is correct. You need three conductors. I typically use Belden 8451 for flash cables. It has a UV stabilized outer sheath and remains somewhat flexible in cold temperatures.

[lynxlynx.ch]

Dear Jeff,

Thank you very much for this valuable information ! - I have just checked with my local dealer and he can provide me this cable :-).
I also found some 3.5 mm Jack ports in my "spare parts box". I guess i could replace the PC Sync Port with these and make some customised 3.5 jack cable with your suggested Belden 8451 ?

3.5 mm jack port - s.jpg