"Dotted" Oscilloscope Wave

[Syllith]

Before I really dig into this thing, I was hoping somebody might have seen a phenomenon like this before. I have a Nicolet 440 oscilloscope somebody gave me and I was told it just "didn't work". When I have it a 13v sine wave, this is what I got. Any idea what might be going on? I definitely think it's hardware but since I don't have access to the machine right now I thought I'd ask and get some ideas first so I know what to start with

http://box.endurehosting.com/contents/public/Wobbly.MOV

[StillTrying]

I've been searching through the html(it's amazing how much junk is there) for the image you hadn't yet posted.

It looks just like noisey sample dots to me, here's one to compare.

[Syllith]

I've seen that video before, in fact, it's the only one I can find that has anything to do with that scope. It should look like his. I'm sure something failed on the board I just have no idea what. From what I could see it seems pretty clean.

[alsetalokin4017]

Yep, the dots are probably due to having "dot (or sample) display mode" instead of "vector" or whatever Nicolet calls it,  selected in one of the menus somewhere.

Step 1: RTFM.  I couldn't find a manual in my quick search; do you have the operator's and service manuals for the scope?

[Syllith]

Under the "display" menu there is actually a "dotted" option but it doesn't seem to change anything. I do not have a service/owner manual but have been looking. No Luck yet. Not a ton of info at all on this machine. I've played with the settings till the cows came home but nothing fixes it. This should be a perfect sine wave (its just stepped down mains ac). The saturation at the peaks of the waveform almost look like something is blown. Maybe a cap somewhere? If I try low current things, such as music from an aux jack, it barely picks up on it.

[alsetalokin4017]

Well, if it were a simple analog scope I'd say "Great, you have a display and a focussed trace so your HV section is probably all good". But this is a pretty complex digital scope, so it is going to be hard to proceed without the SM. Of course the first thing would be to check all the power supply voltages, but.... no manual, no schematic, no idea where testpoints are nor what the voltages should be. The fact that the display isn't complete is troubling, along with the "0.0 us" which is also troubling. Is there some easy way in a menu to reset the scope to "default" settings?
You might try carefully cleaning all the mechanical switch contacts with a good brand of contact cleaner (Deoxit?) but that's just a shot in the dark.

[StillTrying]

It looks very much like an open circuit mains noise waveform to me. I say the first thing to check is that a clean waveform is getting to the scope including the gnd connection. 13Vac, 36Vp-p is a bit large for a test waveform.

[Syllith]

Heres an update. I've changed the test signal to some music from an aux jack out my radio. It appears it is working, but not 100%. I had to remove the 3 additional channels this scope had because with them all plugged in I got some odd things happening with the waveform and it sometimes would give me errors. Every time I took them out and left just the one, it seemed fine. I'm fine with using just 1. I just made sure everything was cleaned up really well (inside the PSU was awful).

So I can now at least get a decent waveform on the screen but it's quite slow. I'm trying to go through the settings to fix it but haven't found anything yet. I know it's an old scope but I feel it should be a little faster than that. But at least I'm making progress.

http://box.endurehosting.com/contents/public/Goodish%20.MOV

[james_s]

Does it have any SMT electrolytic capacitors? Leaky capacitors are a common culprit on many late 80s-mid 90s devices, they can cause all kinds of havoc when the electrolyte gets out on the boards.

[Syllith]

Sorry, I just noticed I recorded that video in slomo. But you get the point. I played with it for like 4 hours straight after getting it to work, I've replaced the old button cell in there with another one so now my settings are saving and it seems like the more I used it the better it got. I'm getting half way decent speed with it but It's still slower than my cheapo $100 MHz scope from ebay. I will check for those caps and let you know what I find. As a side note I will say the board is at least very clean. No water damage or anything odd. Just some dust, which has been taken care of.