"Eric F. Richards" wrote:

Greetings...

I don't know where to ask this, so I'm shooting for multiple groups.

I purchased an HP 140T mainframe plus spectrum analyzer plugins,
including the 8554B mentioned in the subject, at a hamfest. It turns
out that it is working but is quite "deaf." From asking around I have
been told that this is a common problem for this plugin and the
culprit is the first mixer.

Well... the first mixer is an upconverter to 3 GHz, and one of those
magical microwave "cans" that make no sense to those of us used to
thinking in lower frequencies. The only semiconductors in the can are
a pair of diodes, back to back, with the designation "385" on them.
HP weenies have suggested that it could be short for 1901-0385, but
that part number turns up very few references, and none with the
nature of the diodes and their operating characteristics.


These *might* be microwave mixer diodes, like the 1N416D, for example. The thing
to look for is noise figure. I have a 1L20 spectrum analyzer plug-in for my Tek
scope, from the same era as your SA (but much much cruder). I have never seen
your SA, but all I can say is that the first mixer in my 1L20 contains a single
diode.
Do yours look like this?
http://www.dfpresource.org/1n416d.jpg
If so, you might try a kludge like this:
http://www.dfpresource.org/modded1n5711.jpg
Worked for me.