Problem with TEK THS720

[mmoss111]

I think that the measured input resistance to channel 1 should be 1 megohm with no probe connected.  I get 50K to 200K ohms and can not calibrate the probe.
I have heard that a coax cable from the connector to the board developes some kind of problem perhaps a partial short or some kind of leakage.  Does this cable often go bad like this?  Can it be replaced with some RG174U or mike cable or some other type of shielded cable?