Nope.? While the scope functions with the cap removed, putting a good replacement back in reproduces the failure.? I suspect that the cap is part of an attenuator/filter.? Removing it is raising the test signal high enough for it to get detected during the alignment procedure.? Running without the cap, I notice that the noise floor is higher than it should be, and that changes to levels of the input signal do cause corresponding changes in display on the spectrum analyzer.? This is telling me that the gain is still too high in the IF stage.

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My current theory is that the front end is blown out and I will need to replace some component in the 1^st mixer or amplifier.?? I suspect that the 50MHz test signal is present, but too low for me to measure with current equipment.? Which component(s) are blown at this point is the question.? Next steps are to probe the 1^st IF, which I currently don’t have any suitable equipment to measure.

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I have ordered a LimeSDR which is supposedly capable of measuring up to 3 GHz to use as a stand-in spectrum analyzer.? I know it’s not a calibrated spectrum analyzer, but I am hoping it should give me enough to check relative levels and presence of the IF signals.? I will also be ordering a 54701a active probe today.?

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This whole process has been a humbling experience.? I’ve got lots of experience with lower frequency gear – oscilloscopes, DVms, radios, etc.? These spectrum analyzers are a very different beast and require very different troubleshooting and probing techniques.

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-- Mike

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Mike - The cap (C448) was not the problem? Rats.