TR4131 Spectrum Analyser Problem

[Tony_T]

Hi All..


I bought a TR4131 some time back as a non-worker and only recently had the time to check it out.


It passes the Start-Up self test and boots fine. The default config settings appear onscreen but there is no noise floor trace.
All controls appear to work and change settings as they should do.

I have re-seated all boards and checked for any obvious component problems. There is a strange anomaly when the attenuator and reference are adjusted to the same ratio where they produce some kind of trace which is very strange.. erratic.. and off the screen in the Y plane.

At first I could not get any signal to show onscreen but after correcting a bad rf connection from the 2nd IF Out I now do get a signal showing (of sorts) but still no noise floor is visible.

To me I think the SA is working correctly except for the trace being over-scanned.
I thought perhaps the crt has a mechanical problem with the yolk being misplaced but have not yet opened up the crt area as it is closed like Fort Knox and looks like a big job.

Have done some testing of the attenuator.. mixers.. local oscillators.. log output.. and reference for expected frequencies and power levels and I am 90% happy with the results. The other 10% is because I do not have any information or schematics for the TR4131 to know if certain power levels are correct. However nothing jumps out as being badly amiss.

Because the graticule and settings appear onscreen in the correct manner I am thinking that it is not a problem with the display logic but I could be wrong as my field is radio & satellite comms and although I understand how a Spectrum Analyser works I am not used to working on them.

I've made 2 videos and uploaded them to show the problem in detail and they can be viewed at the below links.. In them I have detailed many things which would take some time to write here. So I think the videos will explain better.
I hope it is ok to put the links..




So guys.. any help.. ideas where to check next.. schematics are all welcome ;-)

73

Tony

G7WHK