Hi
I don't think your problem is with the first mixer.I just fixed my 8554b it was quite deaf two.I was told the problem was in the first mixer circuit.I check ed out and that was not the problem at all. The problem was R1-10 ohm 1/4 w. on the (Bandwidth switch assembly) resistor was open this resistor supply the -12.6 also check the two fuses on the back of the 8552b f1-1 amp -10 volts supply and f2 1 amp -12.6 volt supply.I'm also sending you a picture of the bandwidth switch assy it will help you locate R1 10 ohms resistor.I'm sure this will fix your problem with your 8554b RF unit.Please e-mail me back and let me no if it worked or not.
Allan
73
N8ZHC


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.

So I'm wondering if anyone out there knows what type of diodes these
are likely to be and what kind of characteristics I should be looking
for in replacement parts.

Does anyone know anything about this dinosaur? Any help would be
appreciated.

Oh, BTW, the rest of the analyzer and frame work fine. I'm using a
borrowed 8553B plugin with it now and getting the results I'd hoped
for. While I don't plan on doing any critical work with it, the 8554B
is a more capable plugin and I do have to return the borrowed one...

Thanks much!

--
Eric F. Richards,
"Nature abhors a vacuum tube." -- J. R. Pierce, Bell Labs, c. 1940[/quote]