Triax 2 Lug to 3 Lug Interconnection

At a rally last year I purchased a Keithley 616 Electrometer for a very reasonable price. Unfortunately it had a serious problem as can be seen from carefully looking at the connector in the picture below.



Investigation's on the web through up this Wikipedia article (http://en.wikipedia.org/wiki/BNC_connector) that explained all. As this says:

"Triaxial (http://en.wikipedia.org/wiki/Triaxial_cable) (also known as triax) connectors are a variant on BNC which carry both a signal and guard as well as ground conductor. These are used in sensitive electronic measurement systems, particularly of Keithley manufacture. Early ones were designed with just an extra inner conductor, but later tri-axial connectors also include a three-lug arrangement to rule out an accidental forced mating with a BNC connector. Adaptors exist to allow some interconnection possibilities between tri-ax and BNC connectors."



Unfortunately the plug shown above is very expensive, and even if I had found one I would have then had to find some cable and make the connector, which if making an ordinary BNC up is anything to go by would not have been easy, and the adapters mentioned in the wikipedia article are rarer than hens teeth. I must have searched every new and surplus connector tray at the Friedrichafen show last year, and a good few English rallies and found nothing that would work.

I was at the point of giving up and soldering some wires to the outer contacts (I thought of replacing the connector with a standard BNC, but the internal construction of the Keithley 616 made this a very difficult and hazardous to the instrument) when a friend to whom I am most grateful gave me a triax to BNC adapter.

Unfortunately the adapter was a three lug adapter and as indicated in the wikipedia article it would not mate with the two pin socket (deliberately so). After a lot of measuring and staring at the connectors we concluded that the ONLY difference between the two and three pin varieties was the positioning of the lugs and that whilst not perfect a one lug socket would work well enough (at least compared to the current situation of no mating connector). So out came the Dremel, off came the lug and on went the adapter.





Now in case you are wondering, I do have a reason to want to be able to measure the astonishingly high resistance values that this instrument will measure, but more of that at a later date.

Stewart/G3YSX


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