Chuck,

Thanks. Everything you said makes sense so I can stop pulling my hair out.

Any recommendations for an SA on the second hand market
100kHz to 5GHz in a single sweep
70dB dynamic range
Doesn't use harmonic mixers
Option to attach a tracking generator is a huge plus
Doesn't cost a fortune.

The last item be being negotiable (pun intended)

Regards, Mark

When Tektronix made this design, they had no idea how to build a decent
spectrum analyzer. They basically copied the design of the 1L20, and
stuffed it into a 453 oscilloscope frame.

The scheme used in this unit is different from that used in more competent
analyzers. What they do is use a swept IF that tunes 200 MHz from the
FM broadcast band through the VHF TV band. There is nothing to keep
the swept IF band from coming through the mixer when you are in the
lowest RF range. In the higher ranges, there is a high pass filter that
will filter out the IF.

Your unit is working correctly. 491's are all but unusable instruments.

-Chuck Harris