Hi All,

My HP8558B has developed a fault. Could anyone advise?

Up to about 4MHz the sweep consists of a distorted 0MHz peak then a sloping,
noise-free, baseline. Above 4MHz the analyser works fine At a Reference
Level switch setting of -30dB there is about one division of noise nad
signal appear as expected

The distortion of the 0MHz peak and the slope of the 0 to 4MHz section of
the sweep varies with the setting of the Reference Level Switch but the
actual gain settings seem to be correct, once you are above 4MHz. Also odd
single spikes appear in this region at different Ref Level settings

Anything else you need to know?

TIA

John

This is a dumb question, but I have to ask......................does this
only show up at the wider I.F. bandwidth settings? If so, this is normal. Do
you have the sweep set to automatic or manual mode? If you have your sweep
set to manual mode and your sweep is set too fast, this can occur.

Pete

"John A" wrote in message
...
Hi All,

My HP8558B has developed a fault. Could anyone advise?

Up to about 4MHz the sweep consists of a distorted 0MHz peak then a
sloping,
noise-free, baseline. Above 4MHz the analyser works fine At a Reference
Level switch setting of -30dB there is about one division of noise nad
signal appear as expected

The distortion of the 0MHz peak and the slope of the 0 to 4MHz section of
the sweep varies with the setting of the Reference Level Switch but the
actual gain settings seem to be correct, once you are above 4MHz. Also odd
single spikes appear in this region at different Ref Level settings

Anything else you need to know?

TIA

John

Hi Pete,

1. The sweep is in automatic.

2. The phenomena is as described - 0MHz peak then flat, noiseless, sloping
sections - at all bandwidth settings. I see where you're coming from - but
I'm not that wet behind the ears! In fact, when you centre the 0MHz peak on
the screen, the slope of the noiseless sections either side are the same -
i.e., from left to right, the sweep is noise into -4MHz, then a slope
DOWNWARDS to just before the 0MHz peak, then the true, quite narrow, 0MHz
peak, then a further sloping downwards section to +4MHz - where normal
behaviour is resumed. A bit like this (typed, and intended to be views in
Plain Text)

| |
_______ | |_______
######## --------______
--------______############
__
Note that this Spectrum Analyser worked well both where I used to work AND
for two to three years after I rescued it from the firm's scrap pile and
brought it home to my shack - i.e. what it is doing now is definitely NOT
what I have been used to it doing!

My guess was that the problem might be overloading due to mixer
mis-balance - if all of the mixers are supposed to be balanced, that is.
Uncoupling either input to the mixer which converts down from 304 to 21.4MHz
gets rid of the 0MHz peak, which I take to be proof that that mixer is
balanced, but I've got no test gear covering above 50MHz and so I can't
debug the front end of the Analyser.

Does this further info help? I've been told that there are loads of Spectrum
Analyser and/or HP experts in this group.

John



"Pete KE9OA" wrote in message
...
This is a dumb question, but I have to ask......................does this
only show up at the wider I.F. bandwidth settings? If so, this is normal.
Do
you have the sweep set to automatic or manual mode? If you have your sweep
set to manual mode and your sweep is set too fast, this can occur.

Pete

"John A" wrote in message
...
Hi All,

My HP8558B has developed a fault. Could anyone advise?

Up to about 4MHz the sweep consists of a distorted 0MHz peak then a
sloping,
noise-free, baseline. Above 4MHz the analyser works fine At a Reference
Level switch setting of -30dB there is about one division of noise nad
signal appear as expected

The distortion of the 0MHz peak and the slope of the 0 to 4MHz section
of
the sweep varies with the setting of the Reference Level Switch but the
actual gain settings seem to be correct, once you are above 4MHz. Also
odd
single spikes appear in this region at different Ref Level settings

Anything else you need to know?

TIA

John




"Pete KE9OA" wrote in message
...
This is a dumb question, but I have to ask......................does this
only show up at the wider I.F. bandwidth settings? If so, this is normal.
Do
you have the sweep set to automatic or manual mode? If you have your sweep
set to manual mode and your sweep is set too fast, this can occur.

Pete

"John A" wrote in message
...
Hi All,

My HP8558B has developed a fault. Could anyone advise?

Up to about 4MHz the sweep consists of a distorted 0MHz peak then a
sloping,
noise-free, baseline. Above 4MHz the analyser works fine At a Reference
Level switch setting of -30dB there is about one division of noise nad
signal appear as expected

The distortion of the 0MHz peak and the slope of the 0 to 4MHz section
of
the sweep varies with the setting of the Reference Level Switch but the
actual gain settings seem to be correct, once you are above 4MHz. Also
odd
single spikes appear in this region at different Ref Level settings

Anything else you need to know?

TIA

John

See

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


for a screen shot.



John

"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

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