[sdiy] Oberheim OB-Xa internal ribbon cables

[Tom Wiltshire]

> On 9 Nov 2017, at 16:39, Eric Frampton <eric at ericframpton.com> wrote:
> 
>> Am 09.11.2017 um 16:45 schrieb Eric Frampton:
>>> It’s actually the ribbon that connects the mainboard with the voice boards, not the front panel. You can hold down a chord, wiggle the ribbon, and the pitches change drastically.
>> You mean the one that can be seen here...
>> http://www.florian-anwander.de/oberheim_obxa/images/IMG_4340.jpg
>> ...at the left.
> 
> That’s the one.
> 
>> I think the cable itself is not really the problem, but its seating in the connector or the press-in contacts of the connector.
> 
> I’ll double-check that. Seems like it’s in the cable itself, but I could very well be mistaken.
> 
> Also, the black cap that holds the cable to that pinching assembly (whatever it’s properly called) has lost its snaps on the sides, so it’s useless as far as holding the cable tension to the connector. I’m sure that isn’t helping things.
> 
> What are those connectors called? While I’m at it I might as well replace the cap, if they still exist.

The connectors are known as “insulation displacement connectors”, and you’d probably search for “IDC headers”. I can’t tell from the photo, but the most typical are the 0.1”spaced, double row type. So “2 x 8 16-way IDC header” or “2 x 10 20-way IDC header”. These use the typical ribbon cable which actually has a 0.05” pitch in order to have the same number of connectors in the same width as the plugs - this is definitely the cable in the photo.

There are also IDC dual inline headers for terminating ribbon cables to chip sockets. These have their uses, but aren’t very reliable in my experience. They do turn up on old gear though. As I said, I can’t quite see, but it looks a bit like the connector on the voice tray end might be one of those.

The “cap” part on these connectors is usually a cable strip relief. You clamp the cable into the plug in a vise, and then fold it back over itself and clip the strain relief on top to keep it there. It’s possible to use the plugs without this extra bit. I don’t much like the strain relief because I always get confused about which way the plug will finish up pointing when I’m done and then get it wrong. And IDC connectors are pretty much a “one time only” thing - if you screw it up, you’ll be lucky to get the cable out again without damaging the connector. Better to cut it off and do it again with a new one. Perhaps you can get it right, but I never seem to.

HTH,
Tom