Any way of knowing what sort of waveform to expect here?

[Chris Wilson]

I am trying to trace the source of intermittent CRT display noise spikes in a 1980#'s HP spectrum analyser.

http://www.gatesgarth.com/noise.avi

Is there any way someone could give an informed opinion as to whether scoping the  Address Strobe pin on the 4.** MHZ CPU might be useful, and what sort of waveform an frequency might be expected? The pin is designated LAS. The schematic is huge, so it's only practical to post a bit of it. It's at http://www.gatesgarth.com/schematic.jpg  The point I am thinking of probing is TP2-3

Thanks.

[SuzyC]

The noise you are seeing is either generated internally or else externally. It is in the "grass", the dirty noise floor, the lowest amplitude part of the display and could be due to a damaged front end/pre-amp in the spectrum analyzer.
I doubt if it is a digital problem but some analog problem caused by a defective analog component or power supply noise problem.

In udder words, said the cow to the dog, "you might be barking up the wrong tree."

[Chris Wilson]

Thank you SuzyC. I have gone through many of the analogue boards and not got to the root of the issue. Most of the time it works fine, then, seemingly randomly, hot or cold, at turn on, or after running for hours, it will show the display noise. I have checked the supplies for noise and ripple and they all seem fine. I have now arrived at the digital stuff, but I have next to no experience of `scoping digital circuits, but i am guessing square waves?

[SuzyC]

When you see the glitches on the display,  are you sure that you cannot see this noise also at any point in the analog modules?
On a spectrum analyzer, the analog components are those that are most exposed to damaging voltages.

I am very sure the glitches you are seeing are analog noise, not digital.

If your spectrum analyzer is using a CRT, any HV arcing, like around  the HV anode connector or in the HV power supply components could cause noise to be introduced into the analog circuitry.

[tggzzz]

Is there any way someone could give an informed opinion as to whether scoping the  Address Strobe pin on the 4.** MHZ CPU might be useful, and what sort of waveform an frequency might be expected? The pin is designated LAS. The schematic is huge, so it's only practical to post a bit of it. It's at http://www.gatesgarth.com/schematic.jpg  The point I am thinking of probing is TP2-3

That will be determined solely by the type of processor and the program that is running. Can you tell us what the processor is? The frequency wouldn't be relevant, per se. If the analyser is fundamentally working, then I wouldn't expect to find any problem on such a digital line.

Given the vintage, I would expect you to be able to see everything interesting with a 100MHz scope and proper probing technique (i.e. not a 6" ground lead!). Interesting => voltage levels and transients on the edges.
 
It might be worth (1) re-seating any connectors to remove accumulated crap, (2) having a look at the stability or otherwise of PSU lines - which might indicate failing capacitors.

If you can't get it working, I'll nip up and take it off your hands