HP 8753B VNA with 85046A test set issue

[numbersix]

Hello everyone. I recently got hold of an HP 8753B VNA and an 85046 S-parameter test set. I already had an 8753B with a single port test set and really just wanted to get the two port version, but they never come up for sale in the UK on their own. The very first time I tried it I was able to put a 50ohm load onto the S1 port and get a decent reading, then calibrated it with load, open and short and I got a couple of phase cal errors, but it didn't seem to be too bad. Port S2 worked flawlessly. I then went back to port S1 and my trouble really started.

On Smith chart mode using S1 there is just a dot in the centre of the chart that never moves. I have tried changing out the NType cables between the VNA and the test set, I've tried my other VNA, but the result is still the same, just a single dot in the middle of the Smith chart for S1, S2 works fine.

Does anyone have any suggestions for what it might be? Are these things at all repairable by mere mortals, or should I just get my money back and keep hunting for another one?

Any input is appreciated.