Repairing 7000-series plug-in sockets

A common problem with 7000-series scopes, especially the early models, 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 become brittle and crack, causing loss of pressure on the connector fingers and resulting in bad electrical connections. The symptoms are erratic failures that sometimes come and go when the plug-in is pushed or pulled slightly.

Repair strategies include adding mechanical support to the covers, or transplanting modern connectors.

Adding mechanical support

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 (FR-2), 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.

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Replacing the connectors with modern equivalents

While the original connectors are custom parts and no direct modern replacement is known to be available, it is possible to remove the old connector and replace it with a standard 76-pin, 0.1" pitch card-edge connector.

(NB: This procedure has not yet been tested with 79xx high-speed instruments, and will quite likely cause unacceptable reflections in a 7104, which uses special controlled-impedance sockets.)

To extract the old connector, remove the thin plastic side covers, then de-solder the contact fingers individually. Pliers, a vacuum soldering gun, and applying extra flux are recommended.

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  7503 backplane shown with one connector replaced, one removed, one original

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  Four-layer backplane PCB detail

The old connector can be cut apart to insert a modern card-edge connector between the side standoffs.

Leave a shoulder that can take the pressure of the plug-in being inserted.

Ensure the bottom of the modern connector's card slot is at the correct distance to the old connector's top plane - in other words, don't mount the new connector flush with the old connector's top, or the plug-in will likely not insert all the way. In the example shown, the new connector needed to be recessed by about 2.5 mm.

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  Core of original connector cut apart, shown with modern connector to be fitted

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  Left connector replaced, middle connector mounted but not yet soldered

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  Detail of new connector epoxied into supports cut off the old connector body

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  Mounting detail

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  Mounting detail

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  Mounting detail

It might be possible to use a connector with wrap pins if available, however, bending and inserting the pins could be a challenge.

The example shows a connector with solder lugs, extended by short wires. This will fit easily but takes quite some time to make the 76 connections.

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  Detail of new connector soldered

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  Detail of new connector mated with a 7A17 plug-in