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Note the A, B, R and RF in on a 75 ohm 85046B test set are all 50 ohm. It is only the test ports, which are 50 or 75 ohm.

I did actually convert a 75 ohm 85046B to 50 ohm by replacing the bridges from a 50 ohm 8503A 1.3 GHz test set. This site gave me the idea



At the time, the combined cost of the 1.3 GHz test set plus a 75 ohm 85046B test set, were much cheaper than an 85046A 50 ohm test set. Since I have a 20 GHz microwave VNA, I was not bothered about limiting the performance to 1.3 GHz. In fact, the 1.3 GHz bridges actually meet the spec of the 85046A to 2 GHz, and is quite usable at 3 GHz.

If the A and B ports of an S-parameter test set are missing, then its really anyone's guesses what has been done to it. Obviously, someone has picked it apart for some reason - perhaps would of the bridges are blown up, and it became a parts doner.