EMI test receiver HP 85422E sweeps wrong frequency (100MHz = 73MHz)

[KlausKragelund]

Hi

I recently bought a  EMI test receiver HP 85422E at an auction.

I have tested it, directing a single frequency into the RF input.

The gain is spot on, at most 0.5dB off. But the frequency is wrong.
If I enter 50MHz, I get 20MHz peak. If I then add 100MHz, it only increased the peak by 73MHz. It's consistent, every time I increase by 100MHz, only 73MHz is seen (generator has been checked on another spectrum analyzer)

I guess it works by sweeping a local oscillator, and generating an IF frequency.

What could be wrong?

Regards

Klaus