7000-series Plug-in Sockets
A common problem with 7000-series scopes are the sockets in the mainframe that accept the plug-ins. The fingers of the connector are springy metal. The fingers are supported by a flat piece of plastic on either side of connector. This piece of plastic tends to crack, which causes the connector fingers to be unsupported, which causes bad electrical connection with the plug-in, which typically results in erratic failures that sometimes come and go when the plug-in is pushed or pulled slightly.
Andreas Schubert describes his experience with a 7504 that had this problem:
Bought the nice 7504 Ebay 112036180730 recently, which after some cleaning was nearly as new from its cosmetic appearance but, as with many of these, it had the connector problem. This made the operation on all plug-in places useless with random uncorrelated faults.
Had a close inspection, because I wanted to give this to a friend. The contacts itself are not the problem, but the deterioration of the plastic covers on the sides of these connectors. If they lose their strength, the contact pressure is gone. In order to apply the procedure below, the plastic covers must still be in place and not broken. Sorry for bad pictures, I was in a hurry.
On this 7504 model and surely on (most or all) other 7000 series main connector boards you see small additional piggyback circuit boards. I found the distance of these to the connectors to be fairly constant about 5 mm.
I had boards of what we call Pertinax, synthetic resin paper, of 5 mm strength, the width of the boards was 75 mm and it may not be much more - if you cut it, it must be shorter than the connector itself, have a look at the other side at the plug-in itself. I had to cut eight pieces of 23 mm.
For the raw material see picture 0 below. It surely is possible to use a different nonconductive stiff material instead.
Six bits fit quite nicely in the six gaps between piggy back boards and connectors of a four-plugin unit. On one place the distance was only 4 mm, but the piece was easy to file down. At some points additional rasping is necessary due to wires or components, but most of the gaps are clear of these. The Pertinax is easily treated.
These make a good pressure to the long sides of the connectors and are adjusted nicely perpendicular if inserted in the gaps and pushed completely down to the main board.
On the outside, the two remaining pieces must be fixed to the connector side in a different way. I used two metal bars for this and carefully made screw holes to the edges of the connectors. See picture 1 with plug-in; here you can see also why the strips may not have the same length as the connector.
It was useful to take the main board off the instrument to perform this work, but I did not remove the fixed wiring. It took few hours to do this, but was worth the effort. The plug-ins have a tight fit now and the scope is working perfectly well.