7L5 bad trigger



I've recently acquired a 7L5 spectrum analyzer which is just about working, but needs some attention; any guidance in fault finding would be appreciated.

The analyzer is OK in the frequency domain, though needs some tuning, but doesn't work when operating as a sweep/trigger in the time domain. Specifically, when the Frequency Span/Div is set to 0 and the mainframe set to display and trigger from a vertical amplifier (7a18 in left bay of 7603), the signal is displayed but cannot be triggered and the time/div doesn't look right. The trace free-runs at a fairly constant rate 10ms/div(?) irrespective of the time/div setting or trigger mode/level. The on-screen display does show the correct time/div setting, so I think this is something awry in the trigger/sweep circuitry rather than the controls.

The trigger circuitry is buried on the sweep PCB which is hard to access - I'll probably need to solder probe wires onto it to measure anything. From a visual inspection one of the coupling tantalum caps (C582) has a pimple which may indicate failure so I'll probably start by replacing these. However, I have seen others with 7L12/13 on here have problems with the Sweep Control IC (U580 / 155-0056). Is this a common fault and/or do the symptoms sound comparable? Are there any other easy checks that could isolate the problem further, given the limited access?



Richard Steedman

I realize that I'm replying to an old post but, just for the record, I thought I had exactly the same issue and it turned out to be simply the fact that I had accidentally enabled storage mode (DISPLAY A and B LEDs both lit) or the 7L5 had powered up in that mode. Couldn't get time domain mode to trigger. Switching off storage (A and B LEDs off) solved the issue.