Here are the news: using Bill's pictures, I've found a bridge between the pickup controls and removed it. I still haven't found the difference but I'm sure it was added by the previous owner.
In fact, when I use the bridge pickup, there's a tin-like sound that's very disgusting. In a studio record, a few days ago, it was possible to hear all my switch or knobs movements and any little tap on the bass. A great problem for me and for the sound engineer... Will it be possible to have a connection drawing from 1978 electronics? Bill pictures are good but I can't go on specific connections. Thank you all!
 
Landry

Email me your mailing address, Landry. I'm afraid that my father's schematics from that time were in ultra-fine pencil and quite faint, they do not scan or fax well, but I will be glad to mail you a copy of the wiring harness schematic and the printed wiring board schematic.
 
It's the first on the list for the new schematics, that will hopefully start rolling out in about 3 months.