UNIBUS backplane finger contact repair

Noel Chiappa jnc at mercury.lcs.mit.edu
Sat May 16 06:48:10 CDT 2015

    >> it looks like the backplane blocks .. are cast around them

    > From: Brent Hilpert

    > all have secured the pin in the housing via some deformation of the metal
    > pin, post-insertion; as opposed to moulding the housing around the pin.

    > From: Ethan Dicks

    > these are cast blocks of plastic with embedded pins.

    > From: Jon Elson

    > On some of the connectors, the pins are pressed in from the card side.

There appears to be some question as to whether thyy are cast in place,
or inserted.

Actually, I started to wonder about my assertion shortly after I posted it,
and took another look at the backplane. The thing is that the hollows that
the finger contact pins are in are fairly complicated (since they allow the
contact pins to move back and forth, but hold them from going too far), and
I'm really wondering if it would be possible to cast that shape, and then
remove the mold, and also have the pin in there. So I'm wondering if in fact
they weren't inserted post-casting.

    > From: Brent Hilpert

    > The problem in your situation of course, is getting the remainder of
    > the pin out from between the edge connector housing and the backplane
    > PCB without having to unsolder the entire connector.

Well, I'm kind of assuming they are inserted from the edge-connector side,
and not the wira-wrap pin side (what with the contact pins being bent, etc on
the edge-connector side), If they _were_ inserted from the wire-wrap pin
side, I'm totally @&@^$#%@&^, there's no way to get that PCB off now.

So in theory, at least, I'd have to de-solder them from the PCB and then try
and push the remaining part back through (after identifying and defeating
whatever the capture mechanism is - which I might not be able to reach with
the PCB in the way). And then I'd have to find replacement pins and put them
in. Both of those sound non-trivial.

(All assuming, of course, that they are inserted post-casting, and not
cast into place... still not sure about that.)

This has assumed a little extra urgency as on closer examination, some of the
pins in the UNIBUS in-out area are clearly very weak - I imagine there's only
a tiny bit of metal holding the contact pin in place, and it could break off
any time. And I won't be so lucky as to have them all be ground pins..

So if one breaks off, the backplane is toast (unless it's the the last thing
on the UNIBUS, it could be kludged - i.e. attach the termination/ pull-up
directly to the wire-wrap pin).

Which I guess is not the end of the world - DD11's aren't _that_ rare...


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