Agilent/HP 54845A

[Tony_G]

I suspect the AUX out section as all chips in this part of the PCB have been replaced before i bought the unit. I would be eternally grateful if somebody could please check the rise time and amplitude of the AUX out signal set to “probe comp” and wired directly to a channel with a short BNC. I have about 560ps rise time and have tried quite some cables as I though my cable hat to much capacitance. There is an old video on YouTube from Roger Stenbock that does a demo of a unit and has 350ps.

It would be much help to me to even know where to start. I can also do some compare measurements if one of you needs some info.

Here is mine:



Based on my experience doing this I suspect that you don't have 50 ohms turned on:



TonyG

[Tony_G]

Not sure what the cross posting/dupe etiquette is but I wanted to make sure you didn't miss the other thread

Just saw this come up on an auction site and thought it might help, if the price is right of course:

http://www.go-dove.com/en/auction/view?id=12334874

No relationship with the site or seller, except that I bought pieces through the auction site before.

Apologies in advance if this isn't allowed.

[Dexter2]

Many thanks Tony_G for all information,

I was to quick and done my first measurement wrong with input set to 1Meg as you mentioned, but also realized my error in the meantime. Setting it to 50ohm I get also a rise time around 300ps and a signal of about 350mVpp what is expected. So, this is good but still my Trigger calibration does not pass  I have tried to do the Trigger system performance tests from the service manual with a signal generator (100,500,1000Mhz) and all pass. So, this is all strange. If have also measured the trigger delay of my scope AUX out set to Trigger out and it measures 25ns, can you maybe check this also?

I even recorded the whole calibration procedure. Put a low load 1.5Ghz active probe to look what goes on during calibration. I was quite surprised that quite some different waveforms come out when the scope performs the calibration. Other scope was set to 500mV/4ms per division, trigger set to -140mV and sensitive to both edges, persistence off. What is maybe strange that to the end of calibration nothing happens it’s all quiet for quite some minutes...

Video of CH2 calibration (CH1 is longer but as both fail i did onlly the video of CH2)
https://drive.google.com/open?id=0B-jLzsxL7QoAanpFQUhrWUV3ZVE

[Tony_G]

Many thanks Tony_G for all information,

No worries - Happy to help out here.

If have also measured the trigger delay of my scope AUX out set to Trigger out and it measures 25ns, can you maybe check this also?

It's early morning for me and I'm still dealing with jet lag so I'm not understanding what you're asking here - Could you expand?

Thanks,

TonyG

[Dexter2]

If have also measured the trigger delay of my scope AUX out set to Trigger out and it measures 25ns, can you maybe check this also?

It's early morning for me and I'm still dealing with jet lag so I'm not understanding what you're asking here - Could you expand?

When you go to Calibration menu you select something like "Trig Out" for the AUX output. The scope will output the same signal as for the "Probe Comp" but will trigger internally not to the actual sampled signal when you go to main screen. AUX out is connected to channel 1 and input is set to 50ohm. With markers, you can measure the delay between the displayed trigger point and when the signal is rising. Mine has a delay of about 25ns. I did not find a specification for this in the service manual, so a comparison value would be helpful.

The linked video is just the recording what comes out of my AUX out during my failed CH2 calibration, so this would also be helpful, but some time is needed to make it and you need an extra scope with a low load active probe not to interfere the procedure.

[Tony_G]

Thanks - Much clearer - Here is what I'm seeing:



You can see that Tmin/Tmax are right around 25ns. Tmin does jump to -25nS occasionally but I don't think that is a material result.

Unfortunately I don't have a high impedance active probe so I can help with the waveforms for config.

TonyG

[Dexter2]

This behavior is also the same as with mine. I thought something can be seen here as I get also a "Trigger delay error" when calibrating. But the it seams the problem is elsewhere.

Maybe you can also do the "AUX out" sniffing with a passive probe during calibration on CH2, and the same setup I tried. The calibration will fail but you don't have to save it... There is to many different waveforms with different timabases to quantify during calibration but maybe it would give me an idea if my scope does not output something it should during the whole procedure... But I understand if you will rather not do it.

This problem is really driving me nuts, if nothing I will probably have to buy another working to fix this, or leave it as is, as during normal operation it works normal. Again Thanks for all Tony_G.