Mark Aren wrote:
Hi,

Wonder if anyone can help.

Q1. Fresh off of eBay, this SA is having terrible problems with input RF
leaking straight through to the IF on the lowest RF range. I assume
that the input mixer is shot (441-0671-03).

That is a design feature of this spectrum analyzer. Because the 491 has
an unbalanced mixer, the IF feed through is actually several dB stronger
than the desired mixed signal.

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



If I remove the LO feed to this mixer, the expected signal goes away,
but the crud remains. I assume that the IF on the lowest RF range is
close to one of the broadcast bands (TV or FM).

The mixer assy has two diodes which look like they should be easy to
replace. Does anyone have a schematic for this assy or maybe just tell
me what these devices are and some idea where I might get
replacements.

Q2. I feed in 144 Mhz and crank the SA centre frequency lower (130MHz) and
the trace shifts left (I rather expected it to shift right). I assume
that this could be connected with the busted mixer ?

Q3. This one is definatly operator error. The mid range 'B' has two scales
270 - 900 and 800 - 2000MHz. How do I switch between these two ranges
(!)

Thanks in advance.

Mark.