UNIBUS backplane finger contact repair

Brent Hilpert hilpert at cs.ubc.ca
Wed May 13 12:16:49 CDT 2015

On 2015-May-13, at 7:24 AM, Noel Chiappa wrote:
> I have a very strong suspicion that the answer to this is going to be 'no',
> but I figured I'd do due diligence and ask...
> I have a backplane (one of the rodent barn group) which has a couple of finger
> contacts in the backplane missing. (Two came out while I was cleaning it - and
> I wasn't being hard on them, they must have been eaten away by the same kind
> of chemical action that lifted traces off the boards.)
> Is there any way to replace broken/missing finger contacts? I would _guess_
> 'no', since it looks like the backplane blocks (the things cast out of dark
> green plastic, which looks like Bakelite - anyone know exactly what it is,
> BTW?) are cast around them (since there's a wire-wrap pin on one side, and a
> finger contact on the other).
> If the answer _is_ 'no', no biggie - all except one are on one SPC slot, and
> it's a 9-slot backplane, so we'd lose one SPC slot - not the end of the world.
> The other pin is in the UNIBUS in/out area (1/9AB), _but_ the antique hardware
> gods are smiling on us, it's a ground pin (of which the UNIBUS connector
> pinout has multiple, so we can probably withstand the loss of one).

I don't know about DEC backplane connectors specifically - haven't dealt with them closely enough for those purposes, but speaking generally:

To my recollection, of the wide variety of edge connectors that I've seen, 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. It can be a pressed dimple in the pin, a slight crimp, or a twist. I'm looking at a wire-wrap edge connector right now in which the pins were twisted 90 degrees after being inserted into the housing, right where the pin comes through the housing. It can be quite subtle and one might need a magnifying glass to recognise what's going on. It is sometimes possible to do a reasonable job of replacing a pin (been there).

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. In the type of connector I'm looking at, if the pin broke at the 'right' place, it would be possible to pull one half out from the plug-in board side and pull the remainder out through the backplane PCB, and then insert a good pin from the plug-in board side, but I can't speak for whether that would be possible in the case of a DEC edge connector.

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