Network Anaysis - a possible fault uncovered
 

I'm not sure if this faut I describe below accounts for the curious
problems I've been experiencing, but it might be relevant, possibly...
When I'm displaying (in rectangular mode as opposed to polar) a sweep
from 4Mhz to 1.3Ghz I get a pretty flat horizontal line right across
the screen as perhaps you might expect (with no load attached to the
test bridge and the port left open. Now if I stick my finger inside
this port and probe around; I get very little response from the trace.
Likewise if I now short the port, there's very little variation in the
trace. I'd have thought making major changes to the electrical
characteristics at the test port would give rise to some pretty extreme
waveform changes; but this isn't the case. It seems like it's missing a
large amount of senstitivity and I can only think maybe the y-amplifier
might have gone low-gain. Does that make any kind of sense in terms of
the problem I'm describing? Can anyone suggest a test to confirm or
eliminate this possibility?
Thanks,
Paul

Network Anaysis - a possible fault uncovered
 

Hose the test signal output directly into the, um, I think it's A and B
ports on that instrument, one at a time, and see if you get responses
when they are selected for display. You can just put a filter in-line
between the generator and the receiver channel you're testing and see
at least roughly the response you're getting. Check the receiver gain
settings and display scaling to make sure they are working properly,
assuming you get something reasonable displayed. I'm assuming you can
display A and B independently and not just as a ratio to the
R(eference) input, but I don't recall the details of that instrument
off the top of my head. Clearly it wouldn't be the amplifier that
drives the display y-axis, as you're getting deflection in both
directions with the polar display.

Divide and conquer: is the analyzer OK? If it is, then look into the
T/R test set. If it isn't, look into the analyzer...

Network Anaysis - a possible fault uncovered
 

Thanks, Tom. I'm working through the test procedures in the (rather
*large*) manual and am noting down the tests the machine doesn't pass.
I just assume the y-magnifier circuits are seperate (at *some* point)
between the polar and rectanguar trace display circuitry and that might
help to pin the problem area down somewhat. I'm very good at
troubleshooting (given a decent schematic) but have no experience with
this particular instrument.