Last week I could catch a HP67 for 10 Euro. Very dirty.
The Battery Cabinet looked even more dirty especially „Grünspan“. One contact spring was corroded very heavy, so the contct area is only 50%, but enough.
After cleaning outside with soap water the device looked shiny again, also the battery cabinet. The next steps, inclusive opening are described here and the link given there. Again there was a lot of picky dirt and Grünspan. Here I used alcohol 96% to avoid water. Some PCB tracks were nearly interrupted and also some wires inside were so corroded, that they broke immediately touching it, but this I experienced at my 1st repair. The contact area from the slide switches was cleaned and wetted with Vaseline or an appropriate contact grease.
The gummy wheel I replaced as described in the link above. After reassembly the card transport worked perfect. The display shows always „1“ at the 3rd position from right, but this is not very disturbing. Time used about 8 hours. Only one failure remained:
The ZERO button makes no contact. A method to solve this is described here. I hope it will help. I used the HP67 intensively developing Videorecorders from N1700 to V2000 and later VHS. I’m proud to have one HP35 and now a 2nd HP67.