Dave,
Interesting that you have that problem. I have an
8552B that exhibits the same symptoms! Or at least I
think it does. My display (in log mode - linear mode
is not often useful in a spectrum analyzer) shows
about 20dB down from the intended calibrated level.
You are to be congratulated on narrowing it down to
the final amplifier - that's good troubleshooting.
Remember that if an amplifier fails, it does not block
all signals. Instead, a single-stage amplifier may
change from a 15dB gain block (if that's what it was
intended to be) to a 15dB LOSS block - in my case,
that would explain the 30dB down symptom.
In a solid state amplifier, the first suspect is the
amplifying device itself - the transistor. If the
amplifier is common-emitter, it may have a resistor
from emitter to ground, bypassed by a capacitor. If
that capacitor fails open circuit, then the
amplifier's gain will be seriously degraded.
Particularly suspect would be any electrolytic
capacitors, at the age of these instruments.
How were you able to operate the IF section outside of
the mainframe? Or did you plug the module into a
mainframe without an RF section and able to
troubleshoot from the little bit of topside access
that would get you?
As far as the vertical positioning question, remember
that a spectrum analyzer is not intended to be
zero-centered like a scope. Instead, the bottom line
or the top line of the graticule is the reference. In
linear mode, that means zero volts at the bottom of
the screen. The analyzer uses a diode detector in
linear mode, so there's never negative voltage - just
RF envelope voltage. In log mode, the top line is the
reference. If you set the gain controls so that zero
dBm (one milliwatt into 50 ohms) is at the top of the
screen, then each division down from that is 2dB or
10dB depending on the setting of the log/lin switch.
With an oscilloscope, we move the vertical position
control around to get the signal on screen, and take
note of where zero landed. The signal goes up and
down from zero. On a spectrum analyzer, the absolute
position of the signal on screen is a part of the
measurement, so the zero volt line is at the bottom,
always, and the dB reference line is at the top,
always.
Good troubleshooting!
Dave
Date: Tue, 3 Aug 2004 15:06:34 -0700
From: "Dave Faria" <dave_faria@...>
Subject: HP 8552b Problem
Good afternoon list. I have an HP141T with the Hp
8553b and Hp 8552b
plug ins. I am new to solid state test equipment and
need some help. I
have down loaded all the manuals from the BAMA list.
The situation of
my unit is it appears to work in all aspects except it
does not have
sufficient vertical gain. It will show its
calibration signal in all
modes 2db log, 10db log, and linear but, the display
will not show the
peaks at the levels that it should(7.1mv, -30db). The
signals are abt 2/3
of what they should be on the .1uv and -60db scale. I
have done the
test indicated in the 8552b manual to test the final
amp. I injected a
3mhz signal at pin 14 of card XA4 with that card
pulled. The test
indicated the amp was not working. Where do I go from
here?? The down
loaded Hp manual is not that easy to use or read.
Another thing that I
don't understand is why would all functions continue
to work with a failed
amp. One thing that may help someone who understands
this unit is when
I change the gain(db or mv) the entire trace will
shift either up or
down on the screen, sometimes out of the range of the
vertical position
control. This problem would be corrected by a balance
control
adjustment on an old tube type scope.
Thanks for any help or comments
Dave Faria WA5TEZ
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