The cruise control ampli, indeed I checked mine visually and could not detect a cracked joint but went ahead and removed the varnish layer, resoldered all contacts and replaced 3 critical capacitors :
- the two 47 uF capacitors, 'in the middle of the board' which have a 16V rating
- the 0.1 uF capacitor connected to the FET and the relay on the top side of the board (which holds the reference voltage). Although the caps looked OK, (no leakage), replacing the made both of my units work again !
I was triggered by targa80 (
https://forums.pelicanparts.com/porsche-911-technical-forum/1022088-fixed-vdo-461-cruise-control-module.html ) and continued on to make the schematics for the '85 unit as they seemed different.
I now have a '78 and an '85 box that are working. Although they have the same part numers, they are different inside (different schematics).
The draft schematics for the '85 are here :
https://forums.pelicanparts.com/porsche-911-technical-forum/1018643-vdo-cruise-control.html
The final clean version is available on request (pdf file) as I cannot upload pdfs here