Hi Frank,

I've done something just as easy - I unscrewed the LO input to the first
mixer but left it close enough to couple in some LO. Problem fixed, but
gain - of course - well down. Then I reinstated the first mixer LO and did
the same thing with the third mixer LO - with the same result. If my brain
is working this means that AN, if not THE, overload is occurring beyond the
third mixer. I still thing the first mixer may be the area to work on,
though. Any thoughts?

KBO, as Churchill used to say!

Thanks,

John

"Frank" wrote in message
news:wPMPe.213249$HI.190788@edtnps84...

"John A" wrote in message
...
See

http://homepage.eircom.net/~ei9gq/spec2.jpg


for a screen shot.

Assume the center is a zero frequency. Looks suspiciously like LO IF
overload to me. Try a 10 dB pad in the mixer output -- should be easy to
get at. As you mentioned there may be a mixer balance problem. I thought
the HP8558B used to plug in a HP141T display, but found a pic on the web,
and it appears to be a later generation. I have occasionally seen the
results of transmitting directly into those old spectrum analyzers, and in
the 70s, it was $1,000 for the attenuator, and $1,000 for the mixer. Sure
hope you don't have that kind of problem.

Regards,

Frank