HP 8595E Attenuator problem

[gopman]

Working on a HP 8595E with an attenuator problem.  Only the 40dB attenuation seems wrong.  All of the other attenuation settings appear correct.  I don't understand how that could happen since (as I understand it), the 40dB attenuator is enabled with 40, 50, 60, and 70 dB attenuation.

I checked the voltages at the ribbon cable with the attenuator in place, and they match the table in the service manual.

Here is a video showing the attenuation steps from 10-70 and back again.

https://drive.google.com/file/d/1sG4pwMffKFmfUx6XFGPa_2zrX-nayi_I/view?usp=sharing

Thanks for any help!

Rich

[MarkL]

Try turning off corrections and look at the levels again.  If amplitude calibration was run with the attenuator broken, you can't trust anything you're seeing.

[gopman]

Thank you!  Looks great now.  Hopefully that's a mistake you only make once...

[MarkL]

You're welcome!  It happened to my 8595E too and I was ready to start digging into the ADC circuitry before figuring it out.  The sharp discontinuity around the skirt of the signal at some of the attenuator settings in your video was a dead giveaway.

If you looked at the "AUX VIDEO OUTPUT" signal on the back, it also would have behaved sanely since correction factors are not applied to that output.  Corrections are done by the processor after the signal is digitized.  Examining that output is what made me realize it was not an RF issue (after fixing the attenuator, of course).

[gopman]

That's good to know.  There is a lot to learn about these SA's.